Environmental Applications

**Chapter 4**

## Biochar for Environmental Remediation

*Dinesh Chandola and Smita Rana*

#### **Abstract**

The environment is deteriorating rapidly, and it is essential to restore it as soon as possible. Biochar is a carbon-rich pyrolysis result of various organic waste feedstocks that has generated widespread attention due to its wide range of applications for removing pollutants and restoring the environment. Biochar is a recalcitrant, stable organic carbon molecule formed when biomass is heated to temperatures ranging from 300°C to 1000°C under low (ideally zero) oxygen concentrations. The raw organic feedstocks include agricultural waste, forestry waste, sewage sludge, wood chips, manure, and municipal solid waste, etc. Pyrolysis, gasification, and hydrothermal carbonization are the most frequent processes for producing biochar due to their moderate operating conditions. Slow pyrolysis is the most often used method among them. Biochar has been utilised for soil remediation and enhancement, carbon sequestration, organic solid waste composting, water and wastewater decontamination, catalyst and activator, electrode materials, and electrode modification and has significant potential in a range of engineering applications, some of which are still unclear and under investigation due to its highly varied and adjustable surface chemistry. The goal of this chapter is to look into the prospective applications of biochar as a material for environmental remediation.

**Keywords:** biochar, biochar properties, biochar reactivity, environmental remediation

#### **1. Introduction**

Biochar (biomass-derived char) is a versatile renewable source and is gaining popularity due to its diverse raw material sources, high porosity, large surface area, surface functional groups, and high treatment efficacy for a variety of contaminants [1]. Biochar is produced from three types of materials (plant residue, sewage sludge, and animal litter) that are pyrolyzed with little or no oxygen (typically below 1000°C) [2]. Biochar production not only deals with waste, but also benefit from waste, for example, pyrolysis of sewage sludge can reduce pollutants and turn it into a valuable resource [3]. Therefore, it is a great way to make biochar out of solid waste. Because of its unique properties, biochar has sparked widespread concern about its potential for use in the environment [4]. As indicated by the increase in the number of published publications regarding biochar in the last 10 years, it has gotten a lot of attention (**Figure 1**). Biochar's main technique for removing contaminants and remediating the environment is sorption. And, biochar's sorption capacity is directly related to its physiochemical features, such as surface area, pore size distribution, functional groups,

**Figure 1.** *The number of articles published in recent 10 years. (Source: [5].)*

and cation exchange capacity, which vary depending on the preparation conditions [4]. Like, biochar produced at high temperatures has a larger surface area and carbon content than biochar produced at lower temperatures, due to the rising micro-pore volume caused by the elimination of volatile organic molecules at high temperatures [4]. The yields of biochar, on the other hand, decreases as the temperature goes up [6]. Therefore, in terms of biochar yields and adsorption capacity, an ideal synthesis method is required. To increase its physiochemical characteristics, biochar can further be modified with different chemicals like acids, alkalis, oxidizing agents, and ions for various environmental processes [7]. Due to its own properties such as large surface area, recalcitrance, and catalysis, biochar has been widely used in environmental applications such as soil remediation, carbon sequestration, water treatment, and wastewater treatment. In addition, biochar's application for energy and as an agricultural amendment is not a new concept. Biochar has also found its application in climate change mitigation and as a renewable energy source [8]. Biochar's use in engineering applications has received far less attention, despite the fact that economic estimates for biochar production for direct agricultural use have been poor for some time [9]. To that aim, a summary of our current understanding of biochar's potential for use in a variety of environmental remediation applications, as well as emerging obstacles and prospects for biochar usage in environmental remediation, is discussed below.

#### **2. Biochar mechanisms for contaminants removal**

Biochar's function mostly refers to its ability to uptake (e.g., sorption) other substances. The sorption of biochar can be divided into two categories, chemical sorption and physical sorption. Moreover, in term of biochar's interaction with other substances, there are three types of interactions: sorption, catalysis, and redox as shown in **Figure 2**. Sorption is a major environmental process that has a major impact on pollutant biogeochemistry. In sorption, the surface properties of biochar, which includes surface functional groups (carboxyl, carbonyl, phenolic–OH, ester, aliphatic, aromatic, hydroxyl, amino, and azyl groups), surface charges, and free radicals,

*Biochar for Environmental Remediation DOI: http://dx.doi.org/10.5772/intechopen.105430*

**Figure 2.** *Biochar remediation mechanisms. (Source: [10]).*

are important for the behaviour of the interface between biochar and organic and inorganic pollutants, as it provides important sites for sorption and catalytic degradation of pollutants. These functional groups can form hydrogen bonds with other substances, As a result, Biochar can adsorb a variety of pollutants, including organic compounds, metals, nutrients, gases, and microbes [11, 12]. Moreover, the removal of some contaminants are also achieved by partitioning, electrostatic interaction, and pore-filling between biochar and pollutants and depends largely on biochar and pollutant characteristics [5]. Biochar also aids in the transformation of abiotic contaminants through various methods such as free radicals mediated transformation. Free radicals on the surface of biochar can react with chemicals like hydrogen peroxide and persulfate and promote the breakdown of organic pollutants [13]. Apart from that, biochar surfaces contain a variety of catalytic sites, such as quinone and phenolic functional groups, as well as persistent free radicals (PFRs), they enable biocharmediated pollutants transformation [14]. For example, surface functional groups like quinones, convert sulphide into polysulfides, which accelerates the breakdown of azo dyes by increasing electron transport [14]. PFRs on the surface of biochar have a high reactivity and act as a catalyst in pollutant breakdown [13]. Also, the dissolved fractions in biochar, which are primarily composed of aliphatic and aromatic with quinone-like structures, has been tested and found to enhance the photochemical transformation of many organic pollutants by generating reactive intermediates or reactive oxygen species (ROS) [15]. Surface redox active moieties are the main contributors to the redox of biochar even though there are only a handful of relevant reports in publication so far. The surface redox-active moieties in biochar can directly react with pollutants via non-radical pathways, as well as activate some oxidants to

form reactive radicals like OH and SO4. For example, OH generated from the activation of H2O2 in biochar reduces about 20% of *p*-nitrophenol (PNP); however, about 80% of PNP is degraded by directly interacting with reactive sites, most likely the hydroquinone in biochar. Therefore, biochar not only enhances the degradation or transformation of pollutants by facilitating the transfer of electrons as a catalyst, but it can also directly react with pollutants, which will have a significant influence on the environmental behaviour of contaminant [16]. Apart from that, In terms of element composition, the major elements that make up the matrix of biochar are C, H, O, and N, while other elements like Si, P, and S have varying mass percentages in different biochars and play a special or even major role in sorption of various other specific pollutants. For example the sorption of Pb and Al on biochar is attributed to coprecipitation with P and Si in the biochar as Pb5 (PO4)3 (OH) and KAlSi3O8, respectively. An overview of metal ion precipitation and coprecipitation is shown in **Figure 2**. Ion exchange is another crucial phenomenon in the sorption of some heavy metals by biochar [11]. Furthermore, in biochar, there are two different phases: organic and inorganic. By raising pyrolysis temperatures, which results in increased surface area, pore volume, and aromaticity, sorption mechanisms evolved from partitioningdominant to adsorption-dominant, and sorption components developed from polar-selective to porosity-selective [4, 17]. Furthermore, due to the movement of the organic components from aliphatic to aromatic, the sorption rate shows a transitional process: from fast to slow, then back to fast. In terms of inorganic components, it was discovered that ash has a catalytic effect on the formation of biochar with more orderly graphitic structures during the pyrolysis process; additionally, deashing after pyrolysis increases hydrophobic sorption sites, favouring the sorption of hydrophobic organic contaminants [10]. Therefore, the surface structure, functional groups and surface area and mechanisms of these functional groups are observed in the removal of pollutants.

#### **3. Environmental remediation by biochar**

#### **3.1 Soil remediation and amelioration**

Biochar can be used to clean up soil pollution caused by organic contaminants and heavy metals. Soil remediation using biochar is mostly accomplished by sorption and the mechanisms involved are surface complexation, hydrogen binding, electrostatic attractions, acid-base interactions, and *π*–*π* interactions as shown in **Figure 3**. For example, biochar produced from *Carya tomentosa* (a tree in the Juglandaceae or walnut family) and Pecan (*Carya illinoinensis*) (the tree is cultivated for its seed in the southern United States) can adsorb Clomazone and Bispyribac sodium (herbicides used in agriculture) in soil, and effectively reduce the leaching of clomazone and bispyribac sodium. Similarly, sawdust-derived biochar and wheat straw-derived biochar, on adding to the soil, significant reduces the polycyclic aromatic hydrocarbons (PAHs) [13]. **Table 1** shows how adding biochar to soil can help remove several forms of organic contaminants. However, there are several factors such as the types of feedstock, the applied dose, the targeted pollutants, and their concentrations all affect the removal of organic pollutants in soil by biochar. Biochar has the potential to absorb heavy metal ions as well in soil. The heavy metal adsorption mechanism on biochar includes surface complexation, precipitation, cation exchange, chemical reduction, and electrostatic attraction [29]. For example, the adsorption of Pb, Cd, Cr, Cu,

#### *Biochar for Environmental Remediation DOI: http://dx.doi.org/10.5772/intechopen.105430*

#### **Figure 3.**

*Biochar mechanisms in soil for contaminants removal. (Source: [18]).*


#### **Table 1.**

*Adsorption of organic pollutants in soil by biochar.*

and Zn by sesame straw-derived biochar demonstrates varied adsorption capacities for each among them. Pb absorption is the highest in biochar among the metals. Furthermore, when the metals are present together, Cd adsorbed on by sesame biochar is easily replaced by other metal ions. And water hyacinth-derived biochar can adsorb around 90% of As (V) whereas rice straw-derived biochar is able to adsorb Zn2+ [30, 31]. Adsorption of antibiotics like sulfamethazine on biochar increases and subsequently decreases with pH, which affects the surface charge of both biochar and sulfamethazine, and the sorption processes evolve from electron donor–acceptor interaction to negative charge-assisted H-bond. And, metal ion adsorption occurs on the biochar surface's proton-active carboxyl and phenolic hydroxyl functional groups, and adsorption increased with pH in the range of pH 7. Apart from that, ion exchange and cation bonding are also found responsible for the sorption of K<sup>+</sup> and Cd2+ by [32]. The types of feedstock and experimental conditions have a big impact on the removal efficiencies. A number of parameters affect the adsorption capacity of biochar, including pH, surface functional groups, porosity, surface charge, and mineral composition. Therefore, when biochar is used as a remediation method, optimization of various parameters should be done based on the targeted organic contaminants. **Table 2** summarizes the removal of heavy metals from soil by biochar. Tables show how different biochars remove organic pollutants and heavy metals at varying rates. As shown in the **Table 2**, the types of biochar used and heavy metals are so different, it is difficult to compare them [46, 47]. Because different biochars have distinct physiochemical properties, they have varying adsorption capacities for inorganic and organic contaminants. As a result, selecting the right feedstock is more significant


#### **Table 2.**

*Heavy metal stabilization in soil by biochar.*

#### *Biochar for Environmental Remediation DOI: http://dx.doi.org/10.5772/intechopen.105430*

for removing impurities than adjusting the pyrolysis temperature or changing the surface characteristics of biochar [19]. Additionally, modification of biochar is another option for increasing the removal capability of heavy metals. Apart from the removal of organic contaminants and heavy metal from soil, biochar can neutralize acidic soil, boost cation exchange capacity, and improve soil fertility, for example, the acidity of soil can be enhanced by 2 units after 1 month of treatment with soy bean stover-derived biochar and oak-derived biochar. Moreover, the cation exchange capacity can be increased significantly with 5% biochar. As a result, it aided maize growth and with 3% biochar [13]. The addition of biochar made from bamboo also enhances maize production and growth [8]. The addition of biochar to soil improves soil fertility due to the following reasons: (1) increased water retention capacity (2) increased soil aggregate stability; (3) reduction of soil compaction; and (4) decreased soil bulk density and increased porosity. The aforementioned factors may encourage root growth, boosting crop growth and yields even more. However, based on varied soil and feedstock, the most important reason for improving soil fertility needs to be investigated further.

#### **3.2 Carbon sequestration**

The process of storing carbon in soil organic matter and thereby removing carbon dioxide from the atmosphere is known as carbon sequestration. As part of attempts to establish climate resilient agriculture practices, the idea of using biochar to trap carbon in the soil has gotten a lot of attention in recent years. Biochar (biological charcoal) is a carbon sink that absorbs carbon from the atmosphere and stores it on agricultural grounds. Biochar is biologically inert, allowing it to retain fixed carbon in the soil for years to millennia while also absorbing net carbon from the atmosphere [20]. In addition, agriculture fixes 30 gigatons of carbon per year, but 30 gigatons of carbon return to the atmosphere as the plants die, resulting in no net change. When Biochar is combined with compost, soil, and plants, it recovers and stores a significant amount of carbon in the ground, resulting in a continuous and significant reduction in atmospheric greenhouse gas (GHG) levels. In recent years, climate change has sparked an increased interest in lowering carbon dioxide emissions into the atmosphere. Soil, being a major carbon sink, plays a critical role in the global carbon cycle, which has a direct impact on climate change. Carbon sequestration has offered as a strategy to reduce carbon dioxide emissions. Biochar has a great resistance to biodegradation due to its extremely condensed aromatic structure. As a result, biochar is thought to have a positive impact on soil carbon sequestration. Many investigations have been carried out to determine the impact of biochar on soil for carbon sequestration. However, due to the variability in carbon dioxide emissions, no consistent result can be presented. For example, adding carbon from fire to soil increased soil organic carbon turnover. However, adding biochar made of wood sawdust to soil inhibited carbon mineralization, resulting in more carbon sequestration. The mineralization of soil organic matter after the addition of biochar is shown to be higher in low-fertility soils than in high-fertility soils [21]. Carbon mineralization is also higher in soils with low organic carbon concentration than in soils with high organic carbon content. Also, the application of biochar to soil has found an increase in the rate of organic matter decomposition. This so-called "priming effect" affects carbon sequestration efforts since increased microbial activity might lead to breakdown rates exceeding carbon input rates. While the exact mechanism causing this impact has yet to be determined, it could be due to the increase of microbial activity as bacteria consume

the carbon and nitrogen in biochar. However, the carbon in biochar can be separated into two types: liable and recalcitrant carbon. When biochar is introduced to the soil, soil microbes may quickly consume available carbon, resulting in an increase in carbon mineralization at first. This explains why adding biochar to soil accelerates carbon mineralization. Moreover, recalcitrant carbon content in biochar is significantly higher than labile carbon concentration. In soil, recalcitrant carbon can persist for a long time. As a result, the carbon input generated by biochar is more than the carbon outflow induced by relevant carbon mineralization. And, shorter pyrolysis times and higher pyrolysis temperatures, according to recent research [4], result in more recalcitrant biochar (i.e., it persists for longer periods in the soil). However, these pyrolysis conditions yields less biochar per unit feedstock, there are trade-offs. The effect of biochar addition on carbon sequestration is largely unknown in general. The priming impact varies depending on the feedstock and pyrolysis conditions, suggesting that the relationship between biochar's effect and feedstock type must be investigated further. The inherent properties of biochar, as determined by feedstock and pyrolysis conditions, interact with environmental factors like precipitation and temperature to determine how long biochar carbon is held in the soil. Soil texture, as is typically the case, plays an important influence in the stability of biochar carbon. Biochar interacts with soil particles to stabilize itself in the soil.

However, numerous uncertainties remain about the efficiency of biochar in carbon sequestration. It is also crucial to investigate the link between pyrolysis conditions and biochar's carbon sequestration ability. While biochar contains a lot of carbon, it is unclear how long that carbon will stay in the soil after it has been applied. In terms of boosting soil carbon reserves and combating climate change, biochar remains a hot topic. Many uncertainties remain, however, before definitive conclusions can be drawn about what conditions allow biochar to contribute positively to soil carbon sequestration.

#### **3.3 In organic solid waste composting**

The constant increase in solid waste seems to have a negative impact on human society's long-term development, which has raised numerous concerns. Organic waste accounts for around half of all solid waste generated. The ability to effectively treat organic solid waste is critical for successful solid waste disposal. Composting has received a lot of attention as a waste treatment method because of its benefits, such as low cost. Composting is a biological process that takes place. Organic matter from raw materials is exposed to biological breakdown during the process. Biochar has a direct influence on microbes, which has an impact on composting. Many researches have been carried out to see how biochar affects the composting of organic waste. The following are the effects of biochar on microorganisms during the composition of organic solid waste: (1) providing a habitat for microorganisms; (2) providing ideal growing conditions for microorganisms; (3) enriching the microbial diversity. It is documented that biochar addition accelerated the decomposition of organic solid waste due to the favorable effect of biochar addition on composting. **Table 3** shows the impact of adding biochar to the composting process. In general, adding biochar to compost has a good impact on the process. The priming effect, on the other hand, can be overlooked in low-fertility, alkaline, temperate soil. The type of soil affects the performance of biochar in compositing [22]. Furthermore, the types and doses of biochar, as well as the soil types, have a significant impact on the composting of organic solid waste. As a result, a biochar application strategy should be developed depending

*Biochar for Environmental Remediation DOI: http://dx.doi.org/10.5772/intechopen.105430*


**Table 3.**

*Impact of adding biochar to the composting process.*

on the characteristics of organic solid waste composting and soil. Furthermore, it was discovered that bacterial consortiums combined with biochar can stimulate microbial activity to accelerate degradation, increase bacterial community richness, and change the specific selection of bacteria, providing a method for effectively improving microbial activity and enhancing organic solid waste degradation.

#### **3.4 Decontamination of water and wastewater**

Many studies have demonstrated that biochar may adsorb contaminants from water and wastewater, including both organic and inorganic pollutants. Antibiotics, for example, are becoming common organic contaminants in the environment. Sludge-derived biochar has been shown to be a cost-effective and reusable adsorbent for the elimination of antibacterial drugs. **Table 4** shows how biochar can remove organic pollutants from water via adsorption [68, 69].

The adsorption of pollutants by biochar in water depends on the physiochemical characteristics of targeted pollutants and the types of biochar. For example, the sawdust-derived biochar can remove entirely 20.3 mg/l of sulfamethoxazole while


#### **Table 4.**

*Organic pollutant removal by biochar in waste.*

wood-derived biochar demonstrates substantially lower removal effectiveness of sulfamethoxazole (20–30%). For biochar obtained from organic farm, it demonstrates the lowest removal effectiveness of sulfamethoxazole (<6%) [23]. Varying pyrolysis temperatures led in different tetracycline removal efficiencies for biochar generated with rice husk [24]. The removal efficiency of tetracycline ranged from 26% to 60% when the pyrolysis temperature was 800°C and the initial concentration of tetracycline was 200 mg/l. When the pyrolysis temperature was 500°C and the initial tetracycline concentration was 5 mg/l, the removal efficiency was around 90%. It is therefore, established that pyrolysis temperature had important effect on the adsorption capacity of biochar. Other parameters such as pyrolysis time, in addition to pyrolysis temperature, can influence the physiochemical characteristics of biochar, which in turn affects the adsorption capacity of biochar. Heavy metal contamination is a major problem that requires immediate attention. Heavy metals can be removed from the aquatic environment using adsorption as well. Biochar's ability to remove heavy metal ions is listed in **Table 5** [80]. The removal of heavy metals by biochar is dependent on the types of heavy metals and the types of feedstock, similar to the removal of organic pollutants by biochar. Biochar has a lower removal capacity for Cd2+ and As5+ than other heavy metals like Pb2+ and Zn2+ among the major heavy metals [25]. Biochar produced from corn straw, for example, had a different Cu2+ adsorption capability like 0.1 g/l of biochar can remove 1 mM of Cu2+ when the pyrolysis temperature is set at 800°C. And, when the pyrolysis temperature is set to 400°C, 20 g/l biochar can remove 20 mg/l Cu2+ [26]. Similarly, biochar produced from water hyacinths shows different adsorption capacities for Cd2+ and Pb2+, demonstrating that biochar adsorption capability varies depending on the targeted heavy metals. Zhang et al. [27] discovered that biochar prepared at high temperatures was effective in removing Cr (VI). A recent study found that sludge-derived biochar may successfully remove ammonium by monolayer chemical adsorption [59], implying

*Biochar for Environmental Remediation DOI: http://dx.doi.org/10.5772/intechopen.105430*


#### **Table 5.**

*Heavy metal uptake by biochar in water.*

that competition adsorption occurred when biochar was utilised as adsorbents for the removal of heavy metals and organic pollutants in the presence of ammonium. It should be highlighted that the adsorption capacity of the functional groups-modified biochar is clearly improved by the functional groups. The amino-modified biochar, for example, significantly increases the adsorption of Cu (II) due to strong complexation [60]. Moreover, biochar can enrich microorganisms, which can aid in the removal of organic matter, in addition to adsorption. Luo et al. [48] discovered that the proportion of Archaea was significantly greater in the presence of fruitwoodderived biochar, which relieved the stress of ammonia and acids on the microbes, raising microbial activity even more. Lu et al. [35] discovered a similar phenomenon as well. When using biochar for water and wastewater treatment, it's important to keep in mind that it can be recycled and reused. Based on the foregoing findings, biochar performs well in batch experiments in removing the contaminants of concern. However, various contaminants coexist in water and wastewater. Competitive adsorption may occur, resulting in results that differ from those obtained in the laboratory. In addition, the adsorption of contaminants by biochar may be affected by actual flow conditions. As a result, more research should be done in the lab to imitate the realworld condition and study the efficacy of biochar in the removal of contaminants.

#### **3.5 Building sector**

Biochar is a good building material for insulating buildings and managing humidity because of its low thermal conductivity and capacity to absorb water. Biochar, together with cement mortar clay and lime, can be used with sand in a 1: 1 ratio. As a result, the plaster made using this technology has excellent insulation and breathing capabilities, allowing it to sustain humidity levels of 45–70% in both summer and

winter. This prevents dry air, which can cause respiratory problems and allergies, as well as moisture caused by air condensing on the outer walls, which can lead to mould growth [27].

#### **4. Future research**

The capacity to carefully adjust the structure and chemistry of biochar at nanoscale (nm) scales allows certain aspects of the biochar to be altered to target certain environmental engineering solutions, comparable to the proposed "designer biochar" for agricultural uses. It is crucial to remember, however, that once in the field; biochar characteristics do not remain constant over time. Even at ambient temperatures, ageing, oxidation, and microbial degradation can modify surface functional groups and chemistry, affecting sorption characteristics. The list of biochar's potential engineering applications is continually growing. Due to its unique magnetic properties, magnetic biochar opens the door to facilitating removal of various contaminants from soil or other media. This broadens the scope of biochar's possible use in environmental remediation.

#### **5. Environmental concern of biochar**

Along with the widespread use of biochar, it may have some disadvantages which may lead to harmful impact on the environment. When using bio-char in the environment, one of the most crucial aspects to consider is stability. The carbon structure makes up the majority of biochar. Biochar stability refers to the stability of the carbon structure in general. Aromaticity and the degree of aromatic condensation in biochar are markers of its carbon structure. Biochar stability must be considered because different biochars have varying physiochemical properties. Due to the instability of biochar, Huang et al. [28] observed the potential dissolution of organic matter from biochar in the complexation of heavy metals, implying that dissolved organic matter from biochar can be discovered in solution. Furthermore, the aromaticity, stability, and resistivity of the dissolved organic matter may be high. When biochar is used in the treatment of water and wastewater, the carbon content of the water body may rise due to the release of carbon from the biochar. Furthermore, biochar, particularly sludge-derived biochar, includes heavy metals, which may leach out during the water and wastewater treatment process, resulting in heavy metal contamination. When biochar is used as a catalyst support, the catalyst's stability tends to deteriorate after a few uses. One reason for the lower catalyst stability could be charcoal structural degradation. As a result, biochar stability is also linked to water and wastewater treatment quality. In conclusion, the stability of biochar has a significant impact on its environmental applicability. As a result, more research is needed in the future to determine the stability of biochar. Because pyrolysis conditions can change carbon content and structure however, research into the relationship between biochar stability and pyrolysis conditions is important. Biochar's possible toxicity on microorganisms should be considered in addition to its stability. Biochar increases the enzymatic activities of soil microorganisms at low doses, according to Gong et al. [75], demonstrating that low doses of biochar had no toxicity on the bacteria. Dong et al. [79] shown that Fe3O4-modified bamboo biochar has a low cytotoxicity potential. In contrast, high doses of tobacco stem-derived biochar exhibited cytotoxic and

genotoxic effects in epithelial cells through promoting ROS production. As previously stated, biochar has a wide range of physical and chemical properties. More research into the potential toxicity of biochar to the environment is needed to support its effective application. Fish, algae, water fleas, and luminous bacteria can all be used to conduct toxicity tests.

#### **6. Conclusions and remarks**

This chapter provided an overview of biochar application and its interaction with other substances, focusing on its use in environmental remediation. Firstly, the raw material especially waste materials used for biochar production offers a treatment option for wastes that contributes to environmental sustainability. Furthermore, biochar's practical applicability is aided by its low-cost feedstock and simple preparation technique. Biochar has the ability to remediate, improve soil, and mitigate climate change, all of which contribute to environmental sustainability. However, the primary explanation for the increase in soil fertility remained unknown, and the work on the impact of biochar on carbon sequestration needs to be conducted and understood. Composting organic waste using biochar can help promote biological decomposition of organic waste. However, different doses of biochar were required for various organic wastes and biochar kinds. As a result, a biochar application strategy should be developed depending on the characteristics of organic solid waste composting and soil. Biochar can be employed as absorbents in the decontamination of water and wastewater, but its adsorption capacity and stability must be improved. Biochar can activate persulfate, which can be used to remove hazardous organic pollutants from water and wastewater, however the relationship between biochar structure and persulfate activation needs to be studied further to figure out how it works. In conclusion, biochar has a bright future in improving environmental sustainability. The majority of bio-char research is currently being done in laboratories. Biochar's environmental impact has yet to be fully understood. Furthermore, the real world is more complex than the laboratory, resulting in ambiguity about biochar's environmental impact. More in situ tests are needed to determine the true impact of biochar on the environment, such as environmental microorganisms, before it is used on a broad basis. Furthermore, the preparation conditions of biochar for industrial use must be enhanced depending on the various environmental reasons.

### **Author details**

Dinesh Chandola\* and Smita Rana Centre for Land and Water Resource Management (CLWRM), GB Pant National Institute of Himalayan Environment (GBPNIHE), India

\*Address all correspondence to: chandola.dinesh@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Biochar for Environmental Remediation DOI: http://dx.doi.org/10.5772/intechopen.105430*

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#### **Chapter 5**

## The Potential Roles of Biochar in Restoring Heavy-Metal-Polluted Tropical Soils and Plant Growth

*Abdul Kadir Salam*

#### **Abstract**

Biochar shows interesting and environmentally useful properties, among which is its relatively high cation exchange capacity (CEC). High CEC may lower the easily plant-available heavy metals in soils due to the increase in the soil adsorption capacity resulted from biochar application. Quite a lot of current researches reveal that the extracted heavy metals in tropical soils particularly Cu and Zn were significantly lowered in the presence of biochar at 5−10 Mg ha−1. Heavy metal–contaminated tropical soils planted with corn plants (*Zea mays* L.) show significant decreases in Cu and Zn concentrations at moderate- and high-level addition of heavy metal–containing waste. The growth and dry masses of roots and shoot of corn plant improved immediately as a result of biochar amendment. Planting heavy metal–polluted soils treated with biochar with thorny amaranth (*Amaranthus spinosus*) also demonstrated a similar phenomenon.

**Keywords:** biochar, heavy metals, tropical soils

#### **1. Introduction**

Heavy metal contamination and pollution in soils and environment are still of a serious concern since the presence of heavy metal may directly and indirectly endanger living things [1–12]. Reports on the occurrence of soil contamination and pollution come intensively from all over the world related to modern industries [1–4, 7, 10, 13–31]. The negative effects of heavy metals on plants, animals and human beings are also documented in the current literature [5, 6, 8, 9, 25, 26, 30, 32–35]. One important case of the negative effects currently documented was the occurrence of Minamata and Itai-itai diseases in Japan [2]. These suggest that the problem related to heavy metals in the soil environment must be more extensively studied.

Among the various chemical methods available to cope with heavy metal contamination and pollution in soils is the use of organic materials [13, 36–43]. Organic materials such as plant compost may enhance the capability of soil materials to immobilize soil mobile heavy metals. Composted organic matters may effectively lower the soil mobile heavy metals to lower their concentrations to the levels that are not harmful to plants and animals. Organic matters may consist of various functional groups such as phenolic, carboxylic and hydroxyl that may increase the soil cation

adsorption capacity [2]. Therefore, the addition of organic matter compost into heavy-metal polluted soils was reported to significantly decrease the soil mobile heavy metals [41, 42]. For example, the addition of cassava (*Manihot utilissima*) leaf compost into tropical soils amended with heavy metals containing waste significantly lowers the soil DTPA extractable Cu and Zn [41]. This phenomenon was observed in the laboratory and greenhouse experiment employing some tropical soils of Alfisols, Ultisols and Oxisols from Lampung, Indonesia. A recent report also showed that the residual Cu and Zn in industrial waste amended soils were lower in soils treated also with cassava-leaf compost [41, 42]. The effect was more significant at sampling time < 10 years amendment [42].

Some researchers [41, 42, 44] reported that the effect of organic matter compost was more significant when added simultaneously with other potential materials. The addition of organic matter compost and lime was shown to better decrease the soil mobile heavy metals [37, 41, 42, 44]. The results of research in [41, 42] showed that the lowering effect on soil heavy metals of cassava-leaf compost and CaCO3 was significantly greater than addition of organic matter or lime alone. The DTPA extracted Cd from Ultisols, Oxisols and Alfisols was significantly lowered by additions of cassava leaf compost and lime [41, 42]. The residual Cu and Zn were also lower in soils amended with cassava-leaf compost and CaCO3 than with organic compost or CaCO3 alone [42]. The presence of increasing OH− ion by the increase in soil pH [45] may have stimulated the H releases from the organic functional groups and thus widened the capability of the soil materials in adsorbing the heavy metal ions from the soil solution. The adsorption of heavy metal free ions by soil materials may stimulate the releases of heavy metals held as chelates and complexes and also soil heavy metal precipitates and thus finally lower the soil extracted heavy metals.

As shown by numerous data, organic matter compost may significantly affect the soil concentrations of heavy metals. Most reports show that various organic matter may significantly decrease the soil concentrations of heavy metals. However, several reports demonstrated that organic matter may relatively quickly decay in soil system [13, 42, 43, 46]. These observations suggest that the use of organic matters to lower the concentrations of heavy metals in soils is limited for a short duration. Their effectiveness is lower for long-time uses. The problem will be more significant in tropical regions where the soil average temperature and moisture content are relatively high. Therefore, other materials with high durability to organic decomposition are needed. Current literature suggests that biochar will be the best candidate for this purpose [38, 44, 45, 47–62]. As reported by [45, 57], biochar is produced through pyrolysis or charring, causing their structure and composition to be more stable and durable in soil system. In addition, biochar also possesses chemical properties better than ordinary organic materials in terms of cation exchange capacity, pH, specific surface area and nutrient contents.

This chapter was to evaluate the properties and effects of biochar in restoring heavy metal–contaminated or contaminated soils and their effect on the concentration of heavy metals in soils affected by heavy metal–containing materials like industrial wastes.

#### **2. Effects of high concentrations of heavy metals on plant growth**

Heavy metals are detrimental to living things, particularly at high concentrations [2]. As mentioned previously, their negative effects are reported from various sites in

#### *The Potential Roles of Biochar in Restoring Heavy-Metal-Polluted Tropical Soils and Plant… DOI: http://dx.doi.org/10.5772/intechopen.105791*

the world. Research report in [63] shows the negative effect of heavy metal–containing waste on the growth of water spinach, caisim and lettuce in 23 years old heavy metal–containing waste amended tropical soils. Clearly found that the growth of these plants was depressed at high heavy metals and the growth in control soil was the best (**Figure 1**). Lettuce was not survived at high heavy metal contents only until 2 weeks after planting (WAP). It is also obvious that water spinach grew better than the other two plants at any level of soil-heavy metals.

The data above demonstrated that high concentrations of heavy metals (in this case Cu and Zn) were detrimental to plants (**Figure 1**). Their effects are dependent on their concentrations and plant species. Higher concentration of heavy metals gave more significant effects. Water spinach was more adaptable to high concentrations of heavy metal and therefore it grew much better. It is possible to employ plants like water spinach in phytoremediation. Biomass analysis showed also that the plant uptake of Cu and Zn of water spinach was much higher than were other two plants [63].

A similar phenomenon was demonstrated by thorny amaranth. The growth of thorny amaranth was significantly retarded in 24 years old waste amended soils with high heavy metals (treated with 60 Mg waste ha−1) (**Figure 2**). The retardation occurred along the growing time from 0 to 6 WAP. Low heavy metals (treated with 15 Mg waste ha−1) only slightly lowered the growth of this plant.

The effect of heavy metals was more clearly shown by the growth of plant roots. In general, the growth of plant roots may adjust to the high concentrations of Cu and Zn and probably of other heavy metals. This environmental stress by heavy metals may stimulate plant roots to work harder and cause plant biomass to distribute more to plant roots (**Figure 3**). The root/shoot was shown to positively and linearly correlate with the soil-heavy metal concentration. The writer in [64] stated that higher root weight may cause higher root cation exchange capacity (CEC) that may retain

#### **Figure 1.**

*The growth of several plants in heavy metal contaminated soil (S1 control, S2 low heavy metals, S3 high heavy metals; lettuce dead in S3, WAP weeks after planting) (after [63] with permission).*

**Figure 2.**

*The growth of thorny amaranth in heavy-metal polluted soils (C control, LHM low heavy metal, HHM high heavy metal, WAP weeks after planting).*

**Figure 3.** *The relationship between the root/shoot and the soil DTPA extracted Cu and Zn (after [64] with permission).*

more heavy metal cations on the surface of plants' roots so that less heavy metals may move to plant shoots. Higher soil CEC may then lower the stimulation of the growth of plant roots. High concentrations of heavy metals in soils caused more biomass distribution to plant roots (**Figure 3**). Higher CEC can be attained by increasing soil pH [2, 65]. Plant roots also produce some exudates such as low molecular organic acids

*The Potential Roles of Biochar in Restoring Heavy-Metal-Polluted Tropical Soils and Plant… DOI: http://dx.doi.org/10.5772/intechopen.105791*

that may chelate heavy metal cations in soil solution and lower heavy metal effects on plants [66, 67].

#### **3. Some physical and chemical properties of biochar**

Organic compost is significantly different from biochar both in the process of production and in its properties. Organic compost was produced by a complete decomposition of plant materials in the presence of microorganisms in a wellregulated condition of O2, heat and water moisture. Urea N is usually added to accelerate the decomposition process while the soil pH is maintained high by lime addition. Microorganism is introduced through cow dung addition. Low C/N ratio is used as a measure of compost maturity. Biochar is produced by incomplete thermodecomposition of some feedstocks like woods, leaves, feces, straws, husks and manure in a limited or no oxygen supply called pyrolysis or charring [45, 57]. Therefore, biochar consists of much higher C content and consequently, it is more stable with high durability in soils. Reports of [45, 57] show that biochar also showed several better physical and chemical properties. Some of feedstocks abundantly available in Indonesia are woods, straws of corn and rice, bagasse and dairy manure. Therefore, application of biochar may provide a low-cost method of coping with environmental problems. One example of biochar is shown in **Figure 4**, which shows the production of biochar from rice husk and the physical appearance of the rice husk biochar.

Biochar shows porous surfaces so that in the soil system it may physically absorb pollutants like heavy metals. Combined with the increase in the soil adsorption capacity the biochar porosity may significantly enhance the soil retainment on heavy metal cations in biochar-treated soils. In addition to the better physical properties, biochar also shows better, interesting and useful chemical properties [45, 57]. Like organic matters in general, biochar possesses some functional groups like hydroxyl and carboxyl that may bear great amounts of negative charges. It shows a high CEC

#### **Figure 4.**

*The production of rice husk biochar in the University of Lampung experimental farm (courtesy of Sri Yusnaini with permission).*

of 28.8–327 mmol kg−1 and high pH depending on the charring temperature, higher at higher charring temperature. The pH of biochar ranges from 5.81−10.1. Biochar also shows high specific surface area (SSA) ranging from 40.99 to 189.8 m2 g−1.

The potential of biochar at increasing the soil pH may raise the soil adsorption capacity. The increase in OH-ions by biochar treatment may dehydrogenase the biochar functional groups of hydroxyl and carboxyl raising the soil adsorption capacity. Finally, through the synergic works of its high porosity, abundant functional groups and potential to increase the soil pH, biochar may significantly immobilize heavy metal cations in soils.

Therefore, the most important properties of biochar useful in the management of heavy metals in soils is its high SSA, abundant functional groups, high cation exchange capacity and potential to increase the soil pH [45, 57]. Therefore, its presence in heavy metal contaminated or polluted soils may significantly lower heavy metal contaminants. Several mechanisms may involve in the immobilization of heavy metals in soil-biochar mixtures that include physical sorption, ion exchange, chemisorption, complexation and precipitation. Biochar may eventually reduce heavy metal mobility and bioavailability [45]. Wastewater treatment with biochar is reported to immobilize up to 99% of Cd, Pb and Zn in an optimum condition [57]. The effectiveness of biochar is dependent on biomass and soil types and also on heavy metals [60].

#### **4. Improvement of soil chemical properties by biochar**

There are several forms of heavy metals in the soil environment [2]. Of which, heavy metal cation is the most directly affected by the active negative charges of soils through adsorption and desorption processes [68–72]. The adsorption of heavy metals that decrease the concentration of heavy metal cations in soil solution may, of course, stimulate the release of heavy metals of other forms such as chelates through de-chelation, complexes through decomplexation, precipitates through dissolution, and other soil chemical reactions that may altogether lower the total concentration of total soil heavy metals as shown in **Figure 5** [2].

The above interrelationship shows the importance of heavy metal cation form in the soil environment and therefore the effort to cope with the problem of heavy metals in soils must be first focused on lowering the concentration of heavy metal cations. The increase in the soil's negative surfaces was repeatedly suggested to suffice this relationship [2]. The presence of soil solid negative surfaces may electrostatically decrease the mobility of heavy metals cations through immobilization process. Heavy metal cations are strongly held by the soil materials and finally decreased the total soil heavy metals in soils as shown in **Figure 6**.

The quantity of heavy metals held by soil materials is negatively charged surfacedependent. High amounts of negative charges are attainable by enrichment with high quantity of negatively charged materials and/or negative charge stimulating materials. Previous observation shows that this condition can be attained by the addition of cassava leaf compost and/or lime materials that were reported to lower the soil concentration of Cd [41]. The cassava leaf compost may provide high amounts of negative charges to its various functional groups. The lime materials may raise the soil pH that may then stimulate the release of H ions from organic matter functional groups. The addition of organic materials and lime material may then finally widen the total negative charges and may increase the immobilization of heavy metal cations in soils.

*The Potential Roles of Biochar in Restoring Heavy-Metal-Polluted Tropical Soils and Plant… DOI: http://dx.doi.org/10.5772/intechopen.105791*

#### **Figure 5.**

The improvement of the soil negative charges by biochar application may give more significant effect on the amount of the soil negative charges since as stated previously the biochar possesses high amounts of negative charges [57, 59]. The CEC of biochar ranges from 28.8 to 327 mmol kg−1 [45, 57]. The increase in soil pH caused by biochar addition may increase the significance of biochar application. Consequently, biochar application may enhance the retainment of soluble heavy metals in soils and finally lower the total extractable heavy metals in soils. This process will provide suitable soluble heavy metal levels in soils and enable plants to grow better.

#### **5. Restoration of heavy metal–polluted soils and plant growth**

The relationship between the biochar application, the increase in the soil negative charges, and the improvement of plant growth stated in Section 4 is exemplified in **Figure 6**. The improvement of plant growth by this process is expected in soil contaminated or polluted by heavy metals. Better growth of plants may absorb heavy metals at safe levels and may lower the soil heavy metals from immobilized forms like soil precipitates or soil adsorbed heavy metals much faster. The danger of heavy metals to plants may also be alleviated since plants may absorb heavy metals at lower levels of solubility in the presence of biochar. By this means, the soil's heavy metals are lowered by plants that grow better at safe levels of heavy metals. Thereby plants may also grow better in heavy metal polluted soils.

The decrease in soil Cu and Zn levels in the presence of biochar was currently reported from 23-years old polluted tropical soils planted with corn (*Zea mays* L.) as shown in **Figure 7**. The lowering effect of biochar on the soil extracted Cu and Zn is clearly depicted. The soil concentrations of Cu and Zn decreased in the order of soil treatment with 10 < 5 < 0 Mg biochar ha−1, indicating that the presence of biochar

#### *Biochar - Productive Technologies, Properties and Applications*

lowered the soil extracted Cu and Zn. The most possible reason for this phenomenon is that the soil adsorption sites for heavy metals were enlarged by the presence of biochar. The enhancement in the soil adsorption capacity towards heavy metals was also probably associated with the significant increase in soil pH by biochar application. This synergic effect of biochar presence in soils may have finally lowered the soil concentrations of Cu and Zn in soils (**Figure 7**).

As the consequence (**Figures 5** and **6**), the growth of corn plants was significantly altered by biochar application, which was indicated by plant height (**Figure 8**) and plant biomasses (**Figure 9**). The trend in the corn plant height was clearly associated with the significant increase in the soil Cu and Zn concentration and the significant decrease in the soil Cu and Zn in the presence of biochar (**Figure 7**). The decrease in plant height was associated with the increase in the levels of amended soils that increase the soil Cu and Zn while the increase in plant height was associated with the decrease in heavy metal concentrations stimulated by the presence of biochar. A similar trend was also indicated by the changes in the plant biomasses as affected by the levels of amended waste and biochar application (**Figure 9**). The corn plant biomasses including corn roots and corn shoots were lowered by soil concentrations of heavy metals and increased in the presence of biochar associated with the decrease in the soil heavy metals (**Figure 9**).

*The Potential Roles of Biochar in Restoring Heavy-Metal-Polluted Tropical Soils and Plant… DOI: http://dx.doi.org/10.5772/intechopen.105791*

**Figure 7.** *The effect of biochar on Cu and Zn concentrations in waste-amended soil extracted by N HNO3 (after [73] with permission).*

The research result in [73] showed that the related analysis of variance (ANOVA) also indicated that the amended waste levels significantly enhanced the soil concentrations of heavy metals particularly Cu and Zn and significantly depressed the plant height and plant biomasses (roots, shoots, and the whole plant). Several previous research also showed that the waste-borne Cu and Zn in the soils depressed the growth of several other plants including caisim, corn plant, lettuce, Napier grass, and water spinach [63, 64, 73]. Elevated concentrations of heavy metals in soil system are detrimental to plants. Biochar at 5−10 Mg ha−1 was generally effective in changing plant characteristics in heavy metal–containing waste-amended tropical soils. Biochar significantly affected the soil heavy metals, organic C and pH, and also Cu accumulated in corn plant shoots as well as plant height and biomass dry-weight.

The effect of biochar in alleviating the high concentration of heavy metals particularly Cu and Zn was also reported for thorny amaranth [74]. Thorny amaranth was demonstrated to absorb quite high heavy metals from polluted soils and shown to be one of the heavy-metal bio-accumulators and therefore significantly decreased the Cu and Zn concentrations in the 23 years old waste amended tropical soils (**Figure 10**). The presence of thorny amaranth was shown to significantly lower the soil Cu from 79.3 to 60.0 mg kg−1 (24.3% decrease) and the soil Zn from 69.2 to 57.4 mg kg−1 (17.1% decrease) at the waste level of 60 Mg ha−1. The decreases were much higher or 46.0% for Cu and 24.3% for Zn at lower waste level of 15 Mg ha−1. Copper and Zn showed similar behavior in response to planting but the per cent decrease of Cu was higher than that of Zn, demonstrating that Zn was less mobile and less easily absorbed by plant roots than was Cu. It is stated in [74] that not all lost Cu and Zn was absorbed by plant roots. Some of these heavy metals may have also

shifted to more strongly adsorbed heavy metals due to the increase in soil pH caused by planting. Copper was probably more easily and strongly adsorbed by soil colloids or precipitated than was Zn.

The lowering of total heavy metals was also expected in phytoremediation. As stated in [75], at suitable levels, the absorption of heavy metals by plant roots may proceed fast enough since the presence of lower levels of heavy metals will not disturb the physics and works of plant roots during phytoremediation. The amount *The Potential Roles of Biochar in Restoring Heavy-Metal-Polluted Tropical Soils and Plant… DOI: http://dx.doi.org/10.5772/intechopen.105791*

**Figure 9.** *The improvement of corn plant biomasses in waste-amended soil by biochar (after [73] with permission).*

#### **Figure 10.**

*The effect of thorny amaranth on the concentrations of Cu and Zn in a heavy-metal-polluted tropical soil treated with biochar (after [74] with permission).*

of heavy metal removal may be higher at lower than that at higher levels of heavy metals. Therefore, the presence of biochar, which lowers the soil concentrations of heavy metals (**Figure 10**), may fasten the cleaning of heavy metals in soils by phytoremediation.

A similar trend with that in the growth of corn plants was observed in the plant root and shoot dry weights of thorny amaranth (**Figure 11**). The waste origin Cu and Zn may have disturbed the physiological functions in plant tissues and inhibited the growth of plant roots and shoots. It is clearly shown in **Figure 11** that, without biochar, waste treatments lowered the shoot dry weights by about 25.8% and 36.4% at waste treatment of 15 and 60 Mg ha−1, respectively. These values were related to the increase of 8.90 (24.5%) and 43.0 mg kg−1 (116%) in Cu or 6.9 (23.5%) and

#### **Figure 11.** *The growth of thorny amaranth in heavy-metal polluted tropical soil treated with biochar (after [74] with permission).*

*The Potential Roles of Biochar in Restoring Heavy-Metal-Polluted Tropical Soils and Plant… DOI: http://dx.doi.org/10.5772/intechopen.105791*

32.9 mg kg−1 (112%) in Zn caused by the respective waste addition. The higher the soil Cu and Zn concentrations the more effective the heavy metal effect on plant shoot growth retardation. A similar trend was observed in the same soil samples for other plant species like caisim (*Brassica chinensis*), lettuce (*Lactuca sativa*), Napier grass (*Pennisetum purpureum*), and water spinach (*Ipomoea aquatica*) [63, 64, 75]. The growth of these plants was significantly retarded by the increase in the soil extracted Cu and/or Zn caused by waste treatment.

The root dry-weight increased by waste addition at 15 Mg ha−1 (**Figure 9**), suggesting that the growth of roots was more progressive under high concentrations of Cu, Zn and other heavy metals. This pattern was also reported by [74]. The study in [64] showed high correlation between the root/shoot of Napier grass with the soil concentration of Cu and/or Zn (**Figure 3**). However, high concentrations of heavy metals were found to decrease the root weight of thorny amaranth, suggesting that these plant roots were negatively affected by the higher concentration of Cu and Zn at a waste level of 60 Mg ha−1.

Since it is reported to have high cation exchange capacity and high effect on soil pH [18, 35, 36], biochar was shown to improve the above agronomic responses of thorny amaranth (**Figures 11** and **12**). The presence of biochar may have increased the soil adsorption capacity and lowered the soil labile fractions of Cu and Zn, thereby alleviating their phytotoxicities and finally stimulating the plant growth. Numerous observations demonstrated that high soil Cu and Zn in general decreased with biochar treatment. Calculation shows that the extracted Cu at waste levels of 60 Mg ha−1 were 60.0, 59.8 and 46.1 mg kg−1 with biochar treatment of 0, 5 and 10 Mg ha−1, respectively, and those for Zn were 57.4, 54.0 and 45.5 mg kg−1, respectively. The increase in the soil adsorption capacity caused by the presence of biochar significantly decreased the soil labile Cu and Zn about 0.33 and 0.59%, respectively, at 5 Mg biochar ha−1 and 23.2 and 20.7% at 10 Mg biochar ha−1, respectively. The increase in the soil adsorption capacity towards Cu and Zn was probably to be originated from the unique characteristic of biochar that possessed high amounts of organic functional groups that may provide abundant negative charges. Copper and Zn in biochar-treated soils were transformed into less soluble forms with higher bonding energy. The amount of stabilized heavy metals was determined by the biochar-treated soil-adsorptive surfaces. Therefore, biochar 10 Mg ha−1 was more effective than 5 Mg ha−1 in decreasing heavy metals at waste level of 60 Mg ha−1 (**Figure 10**). These changes may lower the negative effect of heavy metals on the growth of thorny amaranth. Therefore, the treatment of soil with biochar may improve the growth of thorny amaranth in heavy metal polluted soils.

The increase in soil pH induced by biochar treatment may have stimulated the enlargement of the soil adsorptive sites caused by the dissociation of biochar and soil colloid functional groups. However, as pointed out previously, a biochar level of 5 Mg ha−1 was probably not sufficient to handle heavy metals at a waste level of 60 Mg ha−1, and the growth of plants at this treatment was in general not better than those without biochar (**Figure 12**). It is obvious that the effect of biochar was dependent on its level. The level of 5 Mg biochar ha−1 was effective at a waste level of 15 Mg ha−1 but not at a waste level of 60 Mg ha−1. Biochar level of 10 Mg ha−1 was effective at waste levels of 15 and 60 Mg ha−1. The improvement effect of biochar was also observed on plant shoot and root dry-weight (**Figure 12**). The improvement of shoot dry weight was clear; the effect of 5 Mg ha−1 was more effective than that of 10 Mg ha−1 as also that on root dry-weight (**Figure 12**).

#### **Figure 12.**

*The effect of biochar on the dry weights of thorny amaranth biomasses in tropical soil polluted with heavy metals (after [74] with permission).*

#### **6. Conclusions**

The increase in the soil and environmental concentrations of heavy metals are reported from all over the world. The increase in heavy metal concentration may occur stimulated by industrialization. Since they are toxic and detrimental at high concentrations, the increase in the soil's heavy metal concentrations is reported to induce plant growth retardation. The presence of biochar that possesses high amounts of negative charges and may increase the soil pH may enlarge the soil's heavy metal cation retention. Therefore, the biochar application may increase the heavy metal immobilization in soil and cause a decrease in the soil available heavy metals. By these means, biochar application may also increase the growth of plants.

The biochar application may lower the soil concentration to the level at which plants may absorb heavy metals at suitable levels so that the absorption of heavy metals and the decrease of heavy metals in soil occur faster without physical and physiological disturbance. In phytoremediation, the use of biochar may accelerate the heavy metal absorption without physical and physiological disturbance on plant roots by the presence of high concentration of heavy metals.

However, in addition to its advantages to lower the concentrations of the polluting heavy metals in the environment, the use of biochar shows drawbacks, among which is the fact that biochar is bulky. The levels used in most experiments which were 5−10 Mg ha−1 are of great amount. It will cause difficulty in its field transportation and treatment. This needs further research to utilize biochar at lower levels without decreasing its effectiveness, for example by adjusting its particle size.

*The Potential Roles of Biochar in Restoring Heavy-Metal-Polluted Tropical Soils and Plant… DOI: http://dx.doi.org/10.5772/intechopen.105791*

#### **Author details**

Abdul Kadir Salam Faculty of Agriculture, Department of Soil Science, University of Lampung, Bandar Lampung, Indonesia

\*Address all correspondence to: abdul.kadir@fp.unila.ac.id

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[73] Salam AK et al. The biocharimproved growth-characteristics of corn (*Zea mays* L.) in a 22-years old heavy-metal contaminated tropical soil. IOP Conference Series: Earth and Environmental Science (In production). 2022;**1034**:1-10. DOI: 10.1088/1755-1315/1034/012045

[74] Afrianti NA et al. The biocharenhanced phytoextraction of heavymetal-polluted tropical soils by thorny Amaranthus (*Amaranthus spinosus*). IOP Conference Series: Earth and Environmental Science, vol. (In production). 2022

[75] Rachman F, Supriatin S, Niswati A, Salam AK. Lime-enhanced phytoextraction of copper and zinc by land spinach (*Ipomoea reptans* Poir.) from tropical soils contaminated with heavy metals. AIP Conference Proceedings, vol. (In production). 2022

#### **Chapter 6**

## Biochar Application in Soil Management Systems

*Theophilus Olufemi Isimikalu*

#### **Abstract**

Due to its potential for improving soil fertility and reducing greenhouse gas emissions, biochar is frequently used as a soil amendment. This chapter presents an overview of its application and soil conditioning mechanisms as a technique for longterm carbon sequestration and lower greenhouse gas emissions, as well as an option for improving soil fertility. It focuses on biochar amendment for improved soil properties that support plant nutrient uptake and crop yield improvement, soil properties and biochar carbon sequestration dynamics, biochar degradation processes, and soil interactions and conditioning mechanisms that influence biochar carbon stability in soils. Current biochar stability assessment techniques used in academic studies are also addressed, along with their suitability for use with various goals and situations.

**Keywords:** biochar, soil, management

#### **1. Introduction**

Sustainable soil management in agriculture aims at developing economically sound and environmentally safe crop management systems that build the quality of soils while being utilized for food production. Such systems are associated with efficient management of soil organic carbon (SOC) and soil fertility, the credible measurement of which Lal [1] regarded as an indicator of soil quality and health. Biochar, which the International Biochar Initiative defined as a solid material derived from the thermochemical conversion of biomass in an oxygen-limited environment, has received wide attention in the past two decades for its documented potential to improve soil fertility and mitigate greenhouse gas emissions.

Studies report that biochar application can enhance soil fertility, reduce greenhouse gas (GHG) emissions [2], increase stable carbon forms in soil [3], improve nutrient and water retention, reduce heavy metal toxicity [4], and increase soil ability to suppress soil-borne pathogens. Woolf et al. [5] stated that the use of biochar in soil could mitigate as much as 1.8–9.5 Pg (1015 g) carbon dioxide carbon emissions annually, globally.

Biochar's soil fertility improvement mechanism is through the manipulation of soil properties such as increased soil microbial activity, soil water holding capacity, soil porosity, soil reaction (pH), soil aggregation, soil organic carbon, among others. When these soil physical and chemical properties are improved, soil nutrient retention and uptake to support plant growth improve. Many studies have reported increased agronomical crop performances following biochar amendment such as in Asai et al. [6]; likewise, others, including Butnan et al. [7], have reported none or unfavorable crop yield responses.

A suppression of greenhouse gases emission is another benefit of biochar addition to soil that has been widely proven in earlier research [8, 9]. Biochar's production in an oxygen-limited environment gives it a chemically recalcitrant carbon-rich solid property, being produced from biomass by heating in an oxygen-limited environment. Although biochar is expected to be largely resistant to biological degradation, research shows that some of its components are relatively easily biodegradable. Thus, several studies have examined its soil and crop yield improvement, and carbon sequestration potential, and widely varying responses have been reported [10, 11]. This has resulted in varying mean resident time (MRT) estimates of biochar-C, ranging from decadal to centennial scales.

The stability of biochar in soil is of high importance to its use as an organic amendment. Lehmann and Rondon [12] defined stability as the determining factor on how long C in biochar will be sequestered (remain in soil) to mitigate climate change and how long a biochar material will continue to benefit soil and plants. The wide variation in research observations has made it very hard to generalize findings on biochar-C stability in soils and thus makes it very important to study its stability in individual soils and under peculiar prevalent environmental conditions. Currently, variations in biochar effects in soils have been attributed mainly but not solely to soil properties such as soil texture and mineralogy, feedstock material, production conditions, environmental characteristics, and the interaction of these elements. Of these factors, biochar feedstock and production conditions are two factors more easily controllable in biochar use in soil.

#### **1.1 Biochar production and basic properties**

Biomass pyrolysis is generally classified according to the rate of reaction into slow, fast, and flash pyrolysis. Through the pyrolysis process, biomass can be transformed into bio-oil, syngas, and biochar (the percentage of each component depends on the pyrolysis condition). The two major thermal conversion processes widely used in biochar production, however, are slow and fast pyrolysis [13]. Slow pyrolysis is most widely used and carried out at lower temperatures (~350°C) and heating rates and longer residence times compared with fast pyrolysis (~1000°C), which optimizes biochar yields over energy production.

Lehmann [8] among other researchers found that the chemical and physical properties of biochar depend majorly on the properties of the original feedstock material and the production conditions (essentially temperature and charing time). Ogawa et al. [14] described biochar chemical structure as one containing different aromatic C structures and considered it a transitional form with intermediate properties between carbohydrate-based biomass and graphite carbon that can appear as a microcrystalline structure. The chemical structure also contains macro-, meso-, and micro-pores, which are derived from cellular fractures of plant cells. Downie et al. [15] similarly characterized biochar as having large surface area, which in addition to its chemical properties and structure gives it high sorption capacity as is the case with other organic compounds. Its composition is widely differentiated into a relatively recalcitrant C, labile (leachable C) and ash (**Figure 1**).

Schmidt and Noack [16] reported that the chemical difference between common OM sources and biochar is that it contains a higher proportion of aromatic carbon that has a fused structure, which differs from the aromatic structure seen in other

*Biochar Application in Soil Management Systems DOI: http://dx.doi.org/10.5772/intechopen.106337*

#### **Figure 1.**

*Biochar properties analyzed using proximate and ultimate analytic procedures [9].*

OM sources such as lignin. The fused aromatic structure can also vary, depending on the production temperature. Nguyen et al. [17] stated that these forms can include amorphous and turbostratic C, which occur at low and higher pyrolysis temperatures, respectively. It is this C structure that gives biochar the chemical stability that makes it hard for microorganisms to readily utilize its C, N, and possibly other nutrients it contains as energy source (**Figure 2**).

Lehmann and Joseph [19] reported that a fraction of biochar may be readily utilized or leached, and this fraction depends on the biochar type. Steiner [20] also noted that biochar may stimulate microbial activity and increase their abundance in soil due to its composition of essential macro- and micro-nutrients, which may serve as biological energy substrate. Some of the most important research applications of biochar-aiding soil functioning are as follows: (1) the improvement of soil fertility and adequate biomass production, (2) storage and cycling of carbon, and (3) alleviation of chemical toxicity and sustenance of soil biodiversity. Ever-increasing human populations and the attendant pressures on soil resources have resulted in extensive

#### **Figure 2.**

*a. Microscopic imagery of fresh wood biochar; b. imagery of the surface of aged wood biochar (image source: Joseph et al. [18]).*

use of pesticides and other intensive management techniques, which has negative climate change impact, and threatens soil quality and human survival. These factors make the aforementioned potentials of biochar highly attractive in agricultural production today.

#### **2. Biochar applications in soil management systems**

This section discusses the applications of biochar along soil fertility and crop yield improvement, carbon storage and cycling, and soil remediation potentials of biochar. **Figure 3** shows biochar processes in the environment.

**Figure 3.** *Applications of biochar in soil (image source: [21]).*

#### **2.1 Biochar application for soil fertility and crop yield improvement**

Improvement in crop yield following biochar amendment has been reported in many previous research studies such as that of Rondon et al. [22] in acidic and weathered tropical soils. Few numbers of research studies such as Husk and Major [23] have also reported positive effects in highly fertile temperate soils. In a meta-analysis, Biederman and Harpole [24] analyzed results of 371 individual studies and found that biochar amendment resulted in higher above-ground crop productivity, soil microbial biomass, K+ concentration in plant tissue, rhizobial nodulation, soil N, P, K+ , and C in comparison with control conditions. There was, however, no obvious trend in soil productivity with biochar addition, and crop productivity varied with increase in application rates. **Figure 4** shows the properties of biochar from different feedstock materials.

In addition to the neutral or negative effect of biochar recorded in some previous studies, there also appear to be an upper limit beyond which biochar addition does not result in improved crop productivity. Lehmann et al. [25] reported that crop responses to biochar addition were positive at rates up to 55 t/ha, while a reduction in growth was recorded at higher application rates. Rondon et al. [22] on the other hand reported a much higher threshold of 165 t/ha. According to them, biochar application of >165 t/ha to a poor soil in a pot experiment resulted in yield decrease that equaled to that of unamended control.

Some other authors have reported yield decreases at lower levels of application. Asai et al. [6], for example, reported the highest rice yield at 4 t/ha biochar application rate in comparison with 8 and 16 t/ha. They reported that yields dropped to the level of the control treatment at 16 t/ha application rate. Jeffery et al. [26] reported that more positive responses from biochar addition to soil have been reported in pot than in field experiments, in acidic than in neutral soils, and in sandy than in loam and silt soils. Increases in yield in comparison with controls range from <10 to >200%.

**Figure 4.** *Approximate properties of biochar derived from different feedstock materials (image source: Joseph et al. [18]).*

#### **2.2 Biochar carbon sequestration dynamics**

Woolf et al. [5] and other authors have proposed biochar use in soil as a means of long-term C sequestration and reduced GHG emission. The main mechanism of biochar-C sequestration is through its incorporation into soil as a highly stabilized C produced through pyrolysis of biomass. Because pyrolysis progresses in the absence of oxygen, the C content of feedstock material is locked in the biochar, which is then applied to soil. Although Lehmann and Rondon [12] reported up to 50% loss of biomass C in biochar production, they reported that a considerably greater fraction of the locked stable C in biochar remained in soil for longer time periods in comparison with direct biomass input in agricultural fields.

Woolf et al. [5] also suggested another potential C negative benefit of biochar as the reduction in emission of CO2 through reduced fertilizer demands to achieve crop yields. This idea is premised on the potential of biochar to improve soil water and nutrient retention capacity of soils. In addition to CO2 emission reduction, Spokas et al. [27] reported reduced N2O emission following biochar addition, and Leng et al. [28] reported that biochar addition resulted in reduced methane (CH4) emission from agricultural soils through the improvement of soil aeration and reduction.

In a meta-analysis, Wang et al. [29] showed that biochar application could stimulate soil CO2 emissions by as much as 28–32% and revealed that average biochar decomposition rate in studies lasting for <6 months was 0.023%/day. This suggests possible priming effects of biochar on SOC or other indirect interactions resulting in CO2 emission from soil following biochar addition. CO2 losses observed in previous research studies following biochar amendment vary widely, and attributed causes include variations in biochar feedstock, production conditions, duration of experiment, and environmental variables.

While Bruun et al. [30] reported cumulative C loss of 2.9 and 5.5% in a sandy loam amended with wheat straw biochar produced from slow and fast pyrolysis, respectively, some other studies such as Fang et al. [31] have reported lower biochar C mineralization rates of 0.1–3% of applied biochar-C mineralized per year. In summary, the carbon sequestration value of biochar is hung on its degradation in soil and the environmental factors that influence it.

#### **2.3 Biochar-pesticide interactions in soil environments**

Despite the fact that biochar was initially developed as a soil amendment because of its beneficial effects on carbon sequestration, greenhouse gas emissions reductions, and soil fertility improvement (Spokas et al., 2009) [32], it has recently drawn more attention for its potent ability to lower the bioavailability of pesticides [33, 34]. It has also been acknowledged that the presence of biochar in soil influences the nature of sorption mechanisms and the bioavailability of pesticide residues for living organisms in addition to improving the sorption of various pesticides [35].

By reducing the leaching of sprayed pesticides, the use of biochar in agricultural soils near bodies of water may also successfully lower the risk of pollution of subterranean water [33, 36]. Pesticide sorption ability of biochar have also been reported in previous research [37]. This is accomplished by using biochar's impacts on pesticide adsorption mechanisms and desorption behavior as a powerful tool to alter pesticide bio-accessibility and toxicological effects.

The repair of contaminated soils has been proposed using procedures such as soil washing, soil flushing, bioremediation, and soil vapor extraction. However, due to limited effectiveness, high maintenance costs, fertility loss, nutrient leaching, and soil *Biochar Application in Soil Management Systems DOI: http://dx.doi.org/10.5772/intechopen.106337*

#### **Figure 5.**

*The removal mechanism of heavy metals by biochar [41].*

erosion, among other factors, these approaches are typically inapplicable in field settings [38]. Application of biochar as an *in situ* form of amendment for contaminated soil has thus shown promise as a method that represents a financially prudent alternative to address remedial demands [19].

By (1) binding pesticides to minimize their potential motility into water supplies and living beings, and (2) supplying nutrients to encourage plant growth and drive ecological restoration, biochar is a less disruptive approach of remediating pesticide-contaminated soil [39]. Additionally, applying biochar to soil requires only a small amount of pretreatment because it is an organic substance made from biological matter [34].

Khoram et al. [40] studied the functions of biochar in fundamental processes of pesticides in the environment and summarized those roles in remediating pesticidecontaminated soils as follows: (1) enhancing pesticide adsorption capacity; (2) reducing desorption and mobility of pesticides in soil layers; (3) reducing the amount of pesticides that are bioavailable in soil pore water, which is thought to be the portion that is bioavailable to soil organisms; (4) enhancing soil microbial activity by supplying necessary nutrients; and (5) enhancing soil physicochemical characteristics such as pH, CEC, and water holding capacity. Biochar amendment has also been shown to help in the remediation of heavy metal pollution in the environment (**Figure 5**).

The mode and other application variables of biochar to soil are a group of significant parameters that affect biochar reaction and stability in soil. It is thus important to be mindful that the complete lifecycle costs of handling and using biochar at scale must be kept as low as possible in order to maintain biochar management as a carbon-negative practice.

#### **3. Mode, frequency, and rate of biochar application to soil**

Wide-varying application rates have been used in previous research, ranging from <5 t/ha to >100 t/ha. IBI (2010) in its biochar fact sheet recommended rates

**Figure 6.** *Particles of biochar derived from different feedstocks.*

of 2–22 t/ha in field trials and lower levels of 2–5 t/ha for large-scale agricultural use. Handling and application should generally determine particle size. The adsorption of ammonium and hexavalent chromium ions from aqueous solution was found in tests to be more effective with fine biochar particles [42, 43]. **Figure 6** shows biochar particles derived from different feedstock materials.

Comparative studies on soil fertility revealed that cowpea biomass production and nutrient uptake were unaffected by biochar particles with diameters of 1 or 20 mm [44] and 10 mm or less [45]. The specific surface area and the resulting accessibility of binding sites in biochar are, however, characteristics that are expected to vary depending on particle sizes and should be considered in biochar application to soil. A thorough understanding of the relationship between the properties of biochar and its applicability will allow for the establishment of appropriate process conditions to produce a biochar with the desired characteristics.

Currently, there are no standard application rates of biochar to soil for different agricultural aims due to varying responses from numerous tests. Variabilities result from biochar feedstock material and production conditions, among others factors as discussed earlier. These factors influence biochar characteristics including nutrient levels, ash content, carbon recalcitrance, etc., which all influence application rate. Due to the expected recalcitrance of biochar in soil, researchers such as Major et al. [46] suggest that a one-time application could provide positive benefits for more than one growing season.

Studies have, however, shown that unless a biochar material is derived from manure or is blended with nutrient-rich materials, it may not substitute for chemical fertilizers. Research has also shown that the level of biochar application to soil affects soil processes such as carbon dioxide (CO2) emission rates [9], which is an important aspect of biochar use for carbon sequestration in soil.

#### **4. Biochar-soil interaction and soil conditioning mechanism**

Soil response to biochar has been shown to be a complex physical, chemical, and biological interaction. Kuzyakov et al. [47] among other authors report that the type and rate of interaction between biochar and soil depend on factors such as feedstock composition, conditions of the pyrolysis process, biochar particle size, soil properties, and local environmental conditions. Also, Mukherjee et al. [48] stated that biochar surface area has aromatic and aliphatic functional groups, which facilitate direct and indirect bonds

*Biochar Application in Soil Management Systems DOI: http://dx.doi.org/10.5772/intechopen.106337*

with soil organic and mineral phases to form complexes in the inner core of biochar material. This complex formation may occur through specific bonding between biochar surface functional groups and soil mineral phase, sorption of soil OM on biocharmineral phase, or through metal-organic cation bridging. Six et al. [49] in an earlier study showed that specific bonding of soil OM and minerals can inhibit the microbial decomposition of soil organic matter (SOM) and enhance aggregate formation.

To measure the influence of production conditions on biochar-C stability, Bamminger et al. [50] applied maize silage biochars produced through pyrolysis at 600°C and hydrothermal carbonization at 220°C to a forest and an arable soil. They reported that 13–16% of the hydrothermal-produced biochar was mineralized in 8 weeks, and the char exerted a positive priming effect on native SOM. On the other hand, 1.4–3% of the pyrolysis biochar was mineralized and a negative (−24 to −38%) priming effect on native SOM was recorded.

Due to the wide variations in mineralization rates of biochar in different research, biochar-C MRT varies widely in the literature. While Keith and Singh [3] in a 3-month soil-biochar incubation experiment reported MRT of 62–248 years, Murray et al. [51] reported half-life time of between 22 and 1506 years, and Wu et al. [52] reported MRT of 617–2829 years. The majority of differences in observations were attributed to influences of biochar, soil, and environmental properties.

#### **4.1 Biochar-soil texture-soil mineralogy interactions**

Kleber et al. [53] stated that clay type, functional groups and their distribution, the concentration and composition of cations and anions, and the polarity of soil compounds are some of the important factors that determine the interactions between OC and clay mineral surfaces in soil. They further highlighted the possible mechanisms of biochar/minerals interactions in soil such as cation bridging, ligand exchange, H bonding, and direct electrostatic interactions through hydrophobic and hydrophilic interactions. Lehmann and Sohi [54] also suggested that biochar-C may be concentrated within soil microaggregates, which supports the proposal of organomineral associations to enhance biochar-C stability in soil.

Brodowski et al. [55] reported higher stabilization of biochar-C in soils of higher clay content. Fang et al. [31] also observed the lowest biochar-C mineralization in a clayey Vertisol and higher mineralization in sandy clayey loam Entisol and sandy Inceptisol. They stated that oxides and oxyhydroxide minerals in an Oxisol contributed more to biochar-C stabilization than smectic minerals in the Vertisol. Research results in contrast to these findings have also been reported. Wattel-Koekkoek et al. [56], for example, in their study reported that there was no relationship between OM content in the clay-sized soil fractions and soil clay mineralogy in six kaolinite- and smectite-dominated soils obtained from different countries.

#### **5. Biochar degradation processes**

Many research studies have shown that the mechanisms of biochar degradation in soil are similar to those of other OC sources in soil. They categorized biochar degradation mechanisms into biotic and abiotic oxidative and nonoxidative degradation, and loss due to other phenomena. The biotic degradation path involves the breakdown of biochar materials by soil microorganisms, while the abiotic path involves the surface oxidation and bulk oxidation of biochar confirmed by the fact that CO2 consumption correlates strongly with oxygen consumption during incubation experiments [57].

Another evidence of abiotic oxidative biochar degradation is the report of Bruun et al. [13], which showed that during incubation experiments, there is a lack of lag phase in the release of CO2, which would be expected if soil microorganisms are inoculated in incubation samples. Investigations on the oxidative degradation of biochar by Nguyen et al. [17] also reported a permanent increase in C loss following temperature increase from 30 to 60°C. The increased biochar mineralization despite temperatures that are unfavorable for soils microorganisms suggests the presence of an abiotic degradation pathway.

Many research studies have shown that biochar addition to soil results in an immediate increased CO2 emission that lasts for about 14 days after which it decreases exponentially. As is the case with mineral weathering, water availability, which plays a major role in soil processes such as hydration, hydrolysis, dissolution, carbonation, and decarbonation, is also expected to affect biochar weathering in soil and soil biota activities. Rates of these reactions are expected to depend on the nature of the reaction, biochar type and properties, and pedo-climatic conditions. This is demonstrated in the study by Isimikalu et al. [58], which evaluated the effect of soil moisture and temperature on biochar C degradation. In their study, they found that C mineralization declines under elevated moisture and concluded that C losses relating to soil water may be more connected to the leaching of dissolved organic carbon.

In a meta-analysis, Wang et al. [59] discovered that the amount of soil clay, the length of the experiment, the feedstock, and the temperature of the pyrolysis all significantly affect the pace of biochar breakdown. The MRTs of the labile and recalcitrant biochar C pools were calculated to be around 108 days and 556 years, respectively, with pool sizes of 3 and 97%. The findings demonstrated that only a tiny portion of biochar is accessible for degradation and that a significant portion (~97%) directly contributes to long-term carbon sequestration in soil. Additionally, they discovered that the mineralization of soil organic matter (SOM; overall mean: 3.8%, 95% CI = 8.1–0.8%) was modestly delayed by the addition of biochar in comparison with soil without the amendment.

#### **Figure 7.**

*Relationships between the decomposed amount (A) and rates of decomposition (B) using 128 observations of different feedstock biochar-derived CO2 from 24 studies with stable (13C) and radioactive (14C) carbon isotopes. The dotted line indicates the 95% confidence band (source: Wang et al., [59]).*

*Biochar Application in Soil Management Systems DOI: http://dx.doi.org/10.5772/intechopen.106337*

The C storage value of biochar materials is commonly assessed by the fraction of biochar-C that remains in soil for >100 years [60]. This proposition is based on the 100-year time horizon used in assessing the global warming potential of GHGs following IPCC [61], which is used in defining permanence in carbon offset. To determine longer-term stability from short-term measurements (research), data are extrapolated to 100 years' duration. **Figure 7** shows biochar decomposition trends in from several studies and feedstock types.

#### **6. Biochar stability testing methods**

IBI [60] categorized biochar stability testing methods into three: alpha, beta, and gamma methods based on the measuring techniques and working principles of the systems. Also, Leng et al. [62] categorized and ranked C stability testing methods into three as follows: analysis of biochar-C structure and composition, determination of biochar oxidation resistance, and evaluation of biochar persistence through incubation and modeling. This classification corresponds to alpha, gamma and beta methods, respectively, under IBI [60] categorization.

According to Leng et al. [62], only biochar persistence measurement through incubation and modeling gives the specific duration of biochar-C in soil and thus regarded it as the core method of biochar stability assessment, which serves as the basis of the other two methods. This is because analyzing biochar-C structure, composition, and oxidation resistance only shows a relative stability and not the actual persistence unless they are correlated with incubation and modeling data.

#### **6.1 Alpha biochar stability measurement methods**

Alpha methods are the cheapest and are used to execute routine estimations of biochar stability. The methods are time conserving—in the range of hours, and two of these alpha methods are mainly used in scientific research. These are the determination of volatile matter (VM) content and determination of hydrogen to OC (H/Corg) and oxygen to C (O/C ratio) molar ratio [60]. They are regarded as indirect methods of stability measurement and require calibration using a beta or gamma method.

#### **6.2 Beta biochar stability measurement methods**

Beta methods of biochar stabilization measurement currently most used in research studies are the laboratory and field-based incubations and to a lesser extent, the field-based chronosequence measurements [60]. Beta methods are applied in combination with modeling in order to estimate biochar loss and stability over a period much longer than the incubation duration. An attribute of these methods is that they directly quantify biochar loss over a certain time period. Using the knowledge gained by the beta techniques, an alpha method can be calibrated to provide a quick tool to estimate biochar stability. The time required to conduct a beta stability test is, however, much longer in comparison with an alpha test, and they consequently cost more.

#### *6.2.1 Laboratory and field incubation studies*

Incubation experiments could be executed in a laboratory environment or in natural field conditions. In laboratory incubation, soil samples are incubated in the absence of plant roots. In the field, however, CO2 emissions may represent C decomposition and root respiration. In order to separate CO2 sources in this type of studies, isotopic labeling of C is required, which both requires intensive instrumentation and costs that may not be readily available to researchers.

Zimmerman [57] showed that many studies use a simple evaluation that measures the total CO2 efflux that does not require CO2 source measurement. In such an assay, biochar-C mineralization is not separated from SOC mineralization, and the priming effect of each component on the other cannot be assessed. A common way of determining C loss from different sources in this type of trial is to deduct C loss under control treatment from losses under amended treatments.

#### *6.2.1.1 Incubation duration*

The trend of C mineralization from previous studies shows that C decomposition decreases until it reaches a constant rate 600–700 days after incubation. This, according to Chao et al. [63], possibly indicates that the biochar-C left may have a higher level of stability. Due to this phenomenon, incubation duration is seen as an important factor in biochar stability determination. The effect of this is that longer incubation time results in higher MRT owing to the lower rates of mineralization used for modeling. Generally, C mineralization experiments have lasted from 14 days to 8.5 years in the study by Kuzyakov et al. [64].

Kuzyakov et al. [64] in their 8.5 years' research reported that labile form of biochar-C was mineralized almost completely after about 3.5 years of incubation, and only about 6% of added biochar-C was mineralized in 8.5 years. Leng et al. [62] among other researchers therefore suggest that studies spanning less than 2 years may only reflect the mineralization of the labile component of biochar-C and recommend care in extrapolating C MRT with such data to avoid underestimation. Long duration of experimentation allows a long enough time to discriminate labile and recalcitrant C pools, which facilitates the use of a two-pool model for extrapolation, thereby taking care of the differences in mineralization rates of different OC pools.

#### *6.2.2 Chronosequence measurements*

Chronosequence measurements are taken from a sequence of soil samples at varying time intervals starting from the time biochar is applied [60]. Based on the obtained data, the long-term stability of biochar is estimated using a model. A disadvantage of this technique, however, is that results are affected by transport processes such as erosion and leaching. As such, the technique is less commonly used.

#### **6.3 Gamma biochar stability measurement methods**

As defined by IBI [60], gamma methods use measurements of molecular properties and chemical composition related to the long-term stability of an OC material. The equipment needed to perform these tests are very expensive, but require a short time to complete. Gamma methods are very reliable and are often used to calibrate alpha and to lesser extent beta methods, which can be used for routine analysis. Examples of gamma stability tests commonly used are different kinds of nuclear magnetic resonance (NMR) spectroscopy, analytical pyrolysis, and a method based on the amount of polycarboxylic acids.

*Biochar Application in Soil Management Systems DOI: http://dx.doi.org/10.5772/intechopen.106337*

#### **Author details**

Theophilus Olufemi Isimikalu Faculty of Agriculture, Department of Agronomy, University of Ilorin, Ilorin Kwara State, Nigeria

\*Address all correspondence to: olufemith@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### **Chapter 7**

Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an Experimental Case the Physicochemical Property Changes of Field Aging Biochar and Its Effects on the Immobilization Mechanism for Heavy Metal

*Run-Hua Zhang, Lin-Fang Shi, Zhi-Guo Li, Guo-Lin Zhou, Yan-Lan Xie, Xing-Xue Huang, An-Hua Ye and Chu-Fa Lin*

#### **Abstract**

Heavy metal inducing contamination soil has become a serious concern. Contaminated soil can cause physiochemical and biochemical changes into soil and the plants. Thus, the plant growth and the yield were affected. In additionally, that ultimately leads to the problem of food security and human health. In recent years, many kinds of ways were used for the remediation of heavy metal contaminated soil, such as isolation, phytoremediation, immobilization, extraction, and soil washing. As a new carbon-rich material, biochar has been applied to the remediation of heavy metal pollution in soil. As biochar is rich with porous structure, high cation exchange capacity, pH value, and surface function, it has become an adsorbent for soil heavy metal remediation. While, with time, the capacity of biochar to immobilize the heavy metals may be modified as the sorption sites may get occupied with native soil organic matter or competing contaminant, etc. And that the physicochemical properties of biochar changed significantly during field aging. Thus, to clarify the mechanism of field-aged biochar for the remediation of heavy metal contaminated soil, we analysis, through an experimental case, the physicochemical property changes of field-aged biochar and its effects on the immobilization mechanism for heavy metal.

**Keywords:** field-aged biochar, heavy metal, contaminated soil, remediation, adsorption mechanism

#### **1. Introduction**

Heavy metal pollution such as cadmium (Cd), lead (Pb), arsenic (As), and chromium (Cr) is characterized by high toxicity and biological enrichment in soil. Heavy metal contamination in soil poses a potential threat to human health and food security [1–3]. In addition, different types and concentrations of heavy metals often exist at the same time, making the polluted soil environment more complex. Therefore, it is urgent to seek remediation technology to remove heavy metals from contaminated soil. Fortunately, biochar, as an economical and efficient adsorption material, has opened up a new way for the immobilization of heavy metals [4, 5].

Biochar is an organic and pyrogenic material produced by pyrolysis of animal or plant-based feedstocks under oxygen limited conditions [6, 7]. In the pyrolysis process, the fatty carbon chain (c) in the raw material finally forms aromatic C, which is considered as fixed C and can exist in the soil for hundreds or thousands of years. The intermediate products between the fixed C surfaces are called active components. When biochar was applied to the soil, they are easily decomposed or weathered and oxidized by soil microorganisms, thus reducing the content of biochar in the soil.

At the present time, increasingly studies show that the properties of biochar will change significantly due to the influence of various environmental factors. This was identified as biochar field aging [8–10]. As far as we know, biochar mainly fixes heavy metals in soil or water through precipitation, surface complexation, cation exchange, electrostatic attraction, and cation-π interaction [11]. However, the field aging of biochar will cause the interaction between biochar and organic matter, minerals, and dissolved organic matter in soil [12]. These resulted in the change of specific surface area (SSA), cation exchange capacity (CEC), element composition, acidity, and Ocontaining functional group of biochar. These will further affect the ability to absorb heavy metals of biochar and its field application performance. The research showed that most biochar increased the content of O-containing functional groups after biochar artificial aging and enhanced the adsorption capacity of heavy metal [13]. However, Lin reported that water washing aging biochar and acidification treatment had a negative impact on the biochar's aluminum toxicity reduction and the improvement of acidic soil [14]. These contradictory results can be explained by many factors. To sum up, there is no consensus on whether the adsorption capacity of biochar for heavy metals changes with biochar field aging and how it changes with field aging.

Although the related research on the aging of biochar is booming, most of the research of the biochar aging is carried out by the simulated aging method under the controllable laboratory conditions [15]. It is not under the field conditions. Actually, there are great differences between artificial aging and field aging of biochar. That is, the changes of physical and chemical properties of biochar under field aging are different from under the artificial aging. So far, there are few studies on the characterization of physical and chemical properties of aged biochar (ABC) extracted from soil [16, 17]. Not to mention the influence of physical and chemical properties of fieldaged biochar on heavy metal adsorption. Therefore, we studied the characteristics of field-aged biochar for the remediation of heavy metal contaminated soil by analysis through an experimental case the physicochemical property changes of field aging biochar and its effects on the immobilization mechanism for heavy metal.

*Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*

#### **2. Biochar**

#### **2.1 Preparation of biochar**

Biochar is carbon-rich byproduct of pyrolyzing material (feedstocks) at high temperatures and low oxygen levels. It has porous structure, larger surface area, ample surface functional groups, and good CEC [17]. These unique physiochemical properties and distinct role of biochar give it to improve the soil biological and physiochemical properties, carbon sequestration, and remediation of heavy metals in soil. Biochar's physical and chemical residences rely on the categories of feedstocks and pyrolysis situations.

The feedstocks of biochar can classify into virgin sources, residues, and municipal solid wastes. The virgin sources include forest sources and oilseed/cereal crops. The residues are timber residues, agricultural residues, and wastes of livestock residues [18]. The wood, wood pellets, tea trash, coffee hulls, biodegradable sewage sludge, wheat straw, rice straw, macro and microalgae, maize fodder are unquestionable fantastic potentials as pyrolysis feedstocks so far.

Up to now, the pyrolysis situations of biochar are conventional pyrolysis, microwave-assisted pyrolysis, impregnation pyrolysis, co-precipitation, hydrothermal carbonization, etc. The conventional pyrolysis is the standard heating system in which heat is transferred from an external supply to the biomass through conductivity, radiation, and convection. It is inefficient and slow and dependent on the biomass thermal conductivity as well as the system's convection present day [17]. Microwaveassisted pyrolysis entails strength conversion rather than mere heating. At some point of this approach, electromagnetic energy is regenerated into thermal energy by using dielectric heating. And the temperature of the feedstock at its center is higher than the temperature of the components. This system accelerates chemical reactions and shortens their duration, saving energy surface and time [18]. This approach is among the foremost promising strategies of fast and improving chemical reactions. The magnetic biochar was obtained from the impregnation-pyrolysis system. It has been studied and used extensively at present. Firstly, the biomass was impregnated in a solution that has transition metal salt and without the solvent. Then the dried residue is pyrolyzed in inert or anoxic atmosphere to get the magnetic biochar [19]. The synthesis of magnetic biochar by co-precipitation was more sophisticated than the impregnation-pyrolysis methodology. However, it is more manageable, allowing the magnetic medium to be stably adhered to the biochar matrix. In co-precipitation system, the biochar was firstly dispersed into a solution containing transition metals with the pH range of 9–11 for a while at a given temperature. And then, the supernatant of the solution was removed. Next, the residue was washed and dried at room temperature, and the magnetic biochar was obtained [20]. Different from the pyrolysis, the hydrothermal carbonization system is reacted at lower temperature with many kinds of biomass in a metal particle solution. These reactions are relatively milder reaction situations than the abovementioned methods [21].

#### **2.2 Biochar properties for remediation of contamination soil**

Biochar displays a tough morphological surface with honeycomb under the microscope. The inherent micropores engender biochar a comparatively excessive intrapore volume and low envelope density. Biochar consists frequently of amorphous, aromatic carbon and possesses over abundant oxygen containing surface functional groups (C=O, –COOH, and –OH) and a disorderly stacked graphene sheet shape. Biochar largely has negatively charged surfaces that will increase the surface assimilation capability of cation species [22]. Thus, it plays a crucial position in improving nutrient retention in soil. The biochar capabilities and applications largely depend on their structural and physicochemical properties. So, it is vital to represent the structural and physiochemical properties of biochar before its use. Different feedstocks and pyrolysis method situations contribute to different structural and physical traits of biochar, including structural complexity, extent, porosity, particle size distribution, density, and mechanical strength [23]. During pyrolysis, biomass feedstock undergoes a variety of physical, chemical, and molecular changes. Pyrolysis circumstance and feedstock type considerably affect the structural and physicochemical traits of the ensuing biochar product. Such as the aromaticity of biochar usually increases, whereas the surface practicality decreases due to the fact that pyrolysis temperature is elevated. It is often an outcome of the innovative losses of aliphatic C–H, olefinic C=C, carbonyl, carboxyl, and hydroxyl groups at a higher temperature [24].

Biochar has been explored for mitigating soil heavy metal contamination. Many reported analysis indicates that biochar is capable of efficaciously immobilizing heavy metallic elements in soil and sorbing heavy metallic cations from water. Thus, biochar serves as a promising amendment for decreasing the eco-toxicity of heavy metal contaminated soils [24]. Biochar's high sorption capability together with high surface area applicable to immobilize contaminants. It is assumed that the contaminants will not be freed into the matrix until the biochar is degraded [25]. Biochar can also immobilize heavy metals through reduction. The oxygen functional group on the surface of biochar reduces the hexavalent chromium (Cr (VI)) to the trivalent (Cr (III)) via influencing its redox response. Cr (III) is usually nontoxic and tightly attached to soil particles, whereas Cr (VI) is very poisonous and mobiles [26].

#### **3. Heavy metals contaminated soil**

#### **3.1 Sources of heavy metal contamination to soil and heavy metal toxic effects**

The metals with relatively higher specific density compared to water (>5 g/cm<sup>3</sup> ) are considered as heavy metals [27]. Naturally, heavy metals are found over the earth's crust in trace amount, so also considered as trace metals [28]. In past decades, the amounts of heavy metals are found to be increasing dramatically besides their natural occurrence. In the meantime, public health concerns due to the toxicity of these metals increased worldwide. Heavy metals such as lead (Pb), mercury (Hg), arsenic (As), lithium (Li), titanium (Ti), antimony (Sb), cadmium (Cd), chromium (Cr) are the most toxic metals with highly detrimental effects on human and animal organs and plant system [29]. Exposure to these metals beyond their permissible limit has life-threatening effects on biological world. Hg, Pb, As, Cd, and Cr are classified as top-priority toxic metal pollutants of significant concern.

Normally, the soil parent material itself contains most of the heavy metals in trace amount, which is not bioavailable. Rather anthropogenically added heavy metals have high bioavailability. There are different identified sources adding heavy metal contamination to soil, including agricultural practices, industrial and domestic effluents, natural and atmospheric sources. Sufficient Cd, Zn, Cr, and Ni will be generated due to wastewater, industrial wastes, and deposited sludge released from industrial

*Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*

applications such as milling, electroplating, etching, tanning, textile and dye, metal casting and smelting, wood preservation and processing, photography, pharmaceutical printed circuit board (PCB), glass processing, manufacturing, etc. [30].

Most of the mismanaged anthropogenic activities are responsible for rapid contamination of soil with various toxic metals. Phosphate and nitrate fertilizers also contain variable amount of Cr, Cd, Ni, Pb, Hg, and Zn in which Cd is of main concern due to its accumulation in plant leaves. Pesticides, fungicides extensively used in agriculture, horticulture, and animal husbandries are the mixture of different compounds containing metals such as Cu, Hg, Fe, Pb, Zn. All these practices contribute to elevate the background concentration of heavy metals in soil. The agriculture practices, such as cattle manure, pig manure, and livestock manure, can add a large amount of Cu, Mn, Cd, Cr, Pb, and Zn to the soil. These ascribed to the compounds of livestock were containing various metals as the animal feed in the pig and poultry industry. And the feces containing metals of these animals were reused for land application. In the long run, these heavy metals will cause a large accumulation in the soil [31].

#### **3.2 Heavy metal removal mechanism of biochar in soil**

The heavy metal contamination soils and its management are a challenging issue. Because it is hard to mineralize them into other forms and their persistence. The remediation mechanism of biochar is different for different heavy metal pollutants. And for the same heavy metal ion, the adsorption mechanism is different when the biochar is different [32]. The removal mechanism of biochar acting on the bioavailable fraction of soil heavy metals is as follows: complexation, physical sorption, electrostatic attraction, ion exchange precipitation, etc. That can be able to reduce also their leachability [33]. Biochar rich in oxygen containing acidic functional groups (phenolic, carbonyl, lactonic, carboxylic, phenolic, and hydroxyl) plays significantly important role in binding (complexation) of heavy metals and metalloids onto the biochar surface as well as inner pores. Physical adsorption involves the removal of heavy metals by diffusion of metal ions into the pores of sorbent. Since biochar is the carbon material with a well-distributed pore networks including micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) [34]. Biochar surface is negatively charged. Heavy metal immobilization via electrostatic attractions takes place between metal ions and biochar's surface charge. This is the electrostatic attraction. Ion exchange from soil matrix to the surface of biochar is another method of metal fixation by biochar. The size of functional groups and metal species on the surface of biochar are the most important factors affecting the residual efficiency of heavy metals in the ion exchange process [35]. Precipitation is considered as the most common accountable mechanism for heavy metal immobilization by biochar. During the sorption process formation of solid(s), either in solution or on a surface is known as precipitation.

#### **4. Experimental case study: Materials and methods**

#### **4.1 Field aging of biochar and separation of aged biochar particles**

The work of the field aging of biochar was carried out in Wuhan City, Hubei Province (30°28<sup>0</sup> N, 114°25<sup>0</sup> E). The biochar used in this work was got from Zhengzhou Lishe Environmental Protection Co., Ltd. in China. The biochar was made from the 3–5 mm corn straw pyrolysis at 500°C. Using the abovementioned biochar as raw material, the biochar field aging test was started in 2015. The biochar was added to the soil at a ratio of 1% (w/w) as biochar treatment, and the soil without biochar addition was as control (CK). Seven years later, we got the field-aged biochar by manually separating the biochar particles with a diameter greater than 3 mm from the soil by tweezers. We planted vegetables in the soil with or without biochar as usual during the past 7 years. The field-aged biochar is carefully washed with deionized water to remove soil particles attached to its surface and then dried at 35° C for 8 hours to eliminate moisture. The field-aged biochar (ABC) was obtained. Comparatively, the biochar purchased from Lishe Environmental Protection Company is relatively named fresh biochar (FBC). The appearance of soil, FBC, and ABC is shown in **Figure 1**.

#### **4.2 Biochar characterization**

#### *4.2.1 pH and EC of biochar*

We examined the EC and pH values of the biochar samples by deionized water at the ratio of 1: 5 (W/V). Conductivity meter (DDS-307A, Rex, China) and PH meter (phs-3c, Rex, China) were equipped respectively.

#### *4.2.2 Pore diameter and specific surface area (SSA) of biochar*

A scanning electron microscopy (SEM, Tianmei, SU8010, China) was used for observing the surface morphology of biochar. Before imaging, biochar samples were sprayed with gold for 2 minutes to improve conductivity and imaging quality of the biochar. The pore diameter and the SSA were analyzed by a pore size analyzer and an automated surface area (Mike ASAP2020, USA). About 0.5 g of each biochar sample was degassed at 125° C for 3 hours, and then SSA was determined using Brunauer– Emmett–Teller (BET) equation according to N2 adsorption/desorption data.

#### **Figure 1**.

*Appearances of FBC (a), ABC (d), original soil (b), the aged biochar-soil mixture with soil (c), and the particle size of FBC (e) and ABC (f). Note: FBC: Fresh biochar; ABC: Field-aged biochar.*

*Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*

#### *4.2.3 The crystal structures and functional groups of biochar*

X-ray powder diffractometer (XRD, Brooke, D8 ADVANCE) equipped with Cu Kα radiation (λ = 1.5406 Å) was used for determining the crystal structures of biochar samples. The XRD pattern was acquired at 0.02° step size, 5°/min scanning speed, and in the 2θ range of 5 � 90°. Fourier transform infrared spectroscopy (FTIR) (Thermo Fisher, Nicolet in10) and KBr were used for characterizing the functional groups on the surface of biochar. The spectral regions from 4000 to 400 cm�<sup>1</sup> were recorded at a resolution of 2 cm�<sup>1</sup> .

#### *4.2.4 The elements contents of C, N, O, H, and S of biochar*

The Elemental Analyzer (EA, Vario el cube) was used for analyzing the contents of C, N, O, H, and S elements in FBC and ABC samples with argon as a carrier gas. X-ray photoelectron spectroscopy (XPS, thermo escalab250xl, USA) was used to study the combined states of major elements in biochar. The elemental binding energy was corrected to C1s (284.8 eV) obtained in the experiment.

#### **4.3 Batch adsorption experiment**

#### *4.3.1 Adsorption kinetics*

In this experiment, 20 mg/L Cd2+ and Pb2+ solutions were used for subsequent adsorption experiments. Using 0.01 mol/l NaNO3 solution as background electrolyte, dissolve CdCl2 and PbCl2 respectively to prepare 1 g/L Cd2+ and Pb2+ stock solutions, respectively. Then we adjust the initial pH of the stock solution to 5.0 � 0.2 with 0.1 mol/ L NaOH or HCl solution. The adsorption kinetics experiment was carried out at room temperature. About 0.15 g of ABC or FBC respectively was added to 150 ml of 20 mg/L Cd2+ and Pb2+ solution, shaken well at 180 RPM/min. The collection time intervals of all samples were 0.083, 0.25, 0.5, 1, 2, 4, 6, 8, 12, and 24 h, respectively. There were three replicates per treatment. All samples were filtered through a 0.45 μm membrane in the adsorption experiments. Inductively coupled plasma atomic emission spectrometry (ICP-AES) (Icpe-9000, Shimadzu Corporation, Japan) was used to determine the concentrations of Pb2+ and Cd2+ and the changes of substituted Na+ , Ca2+, K+ , and Mg2+.

The adsorption capacities of biochar for Cd2+ and Pb2+ were calculated according to the formula (1):

$$Q\_{\epsilon} = \frac{(C\_0 - C\_{\epsilon})V}{m} \tag{1}$$

*Ce*(mg/L): the remaining concentration of Pb2+and Cd2+ in the solution; *C0* (mg/L): the initial concentration of Pb2+and Cd2+ in the solution; *V* (mL): the volume of heavy metal solution; *m* (g): the dosage of biochar.

Two different models were used to fit the adsorption kinetic data. The formula is as follows:

$$Q\_t = Q\_\mathcal{e} \left(\mathbf{1} - e^{-k\_1 t}\right) \tag{2}$$

$$Q\_t = \frac{Q\_\varkappa^2 k\_2 t}{1 + Q\_\varkappa k\_2 t} \tag{3}$$

*Qt*: the adsorption capacity of Cd2+ or Pb2+ on biochar at time t; *Qe* (mg/g): the adsorption capacity of Cd2+ or Pb2+ on biochar at time equilibrium; *t* (h) is the adsorption time; *k1* (h�<sup>1</sup> ): the rate constant of the pseudo-first-order kinetic equation; *k2* (g/mg/h) the rate constant of pseudo-second-order kinetic equation.

#### *4.3.2 Biochar isotherms adsorption*

The adsorption isotherms were conducted as follows: add 0.02 g of biochar to 20 ml of Cd2+ and Pb2+ Solution for adsorption isotherm, and the initial concentration is 5–120 mg/L. Equilibrium adsorption was performed at room temperature for 24 h. The rest of the operation is the same as the adsorption kinetics.

The adsorption isotherms was disclosed using Langmuir and Freundlich models. The equations are listed as follows:

$$Q\_{\epsilon} = \frac{Q\_m b C\_{\epsilon}}{1 + b C\_{\epsilon}} \tag{4}$$

$$R\_L = \mathbf{1}/(\mathbf{1} + b\mathbf{C}\_0) \tag{5}$$

$$\mathbf{Q}\_{\boldsymbol{\epsilon}} = \mathbf{K}\_{\boldsymbol{f}} \mathbf{C}\_{\boldsymbol{\epsilon}}^{\mathbf{1}\_{\boldsymbol{\eta}}} \tag{6}$$

*Qe* (mg/g): the amounts of Cd2+ or Pb2+ adsorbed on biochar; *Qm* (mg/g): the maximum saturated adsorption capacity of biochar; *C0* (mg/L): the initial concentration of Cd2+ or Pb2+ ; *Ce* (mg/L): the equilibrium concentration of Cd2+ or Pb2+ ; *b* (L/mg) and *Kf* ((mg/g) (mg/L)�<sup>n</sup> ): the corresponding constants of Langmuir and Freundlich; *n*: the Freundlich constant related to the surface site heterogeneity; *RL*: the dimensionless constant separation factor.

#### *4.3.3 Biochar saturated adsorption*

Biochar saturation adsorption experiments were executed on the basis of adsorption kinetics. About 0.5 g biochar and 100 mL 100 mg/L Cd2+ or Pb2+ solution were mixed and shaken for 24 h at180 rpm/min at 25°C. The mixture liquid was filtered by 0.45 μm acetate membrane. ICP-AES was used for determining the concentrations of Cd2+ and Pb2+ in the mixed solution before and after adsorption by biochar.

#### *4.3.4 Toxicity characteristic leaching procedure (TCLP)*

The TCLP test was with the solution. The extraction solution was obtained by dissolving glacial acetic acid with 5.7 mL into the deionized water to a total volume of 100 mL. And the solution's initial pH was adjusted at 2.9 � 0.05. Next, the 0.25 g of biochar was added into the extraction solution with 10 ml and shaken the solution at 180 rpm/min for 18 h at25 °C. Then, the Cd2+ and Pb2+ contents in the extraction solution were analyzed using ICP-AES after being filtered through a 0.45 μm acetate membrane.

$$T \text{CLP-Cd} /\_{Pb} = \frac{q\_1}{q\_2} \times 100\% \tag{7}$$

*q1* (mg/g): the contents of Cd2+ and Pb2+ in the extraction solution of TCLP; *q2* (mg/g): the biochar's total saturated sorption of Cd2+ and Pb2+.

*Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*

#### *4.3.5 Statistical analysis*

The statistical analysis in the paper is using the SPSS23.0 software (SPSS Inc., USA). Data are presented as mean standard deviation. One-way analysis of variance (ANOVA) with a least significant difference (LSD) at a 0.05 significance level was used for the standard analysis. XPS and XRD data were analyzed with Advantage and Jade6.5 software, respectively. Origin Pro 2018 (OriginLab, USA) and Microsoft Excel 2016 (Microsoft Corporation, USA) were used for drawing the figures and tables.

#### **5. Experimental case study: results and discussion**

#### **5.1 The capabilities of field-aged biochar**

#### *5.1.1 The microstructure of field-aged biochar*

Compared with FBC, the surface of ABC became smoother after 7 years of application in the soil as show in **Figure 2(c)** and **(d)**. And there were no small particles attached on its surface. However, the surface of FBC was rough and has some attached aggregates shown in **Figure 2(a)** and **(b)**. These SEM results illustrated that the small particles attached on FBC surface may be labile fractions or soluble substances (e.g., CaCO3 as shown in **Figure 3**) from the pyrolysis processes of corn straw [36].

#### *5.1.2 The surface morphology of field-aged biochar*

The SSA data demonstrated that, compared with the FBC, the ABC's pore volume, the SSA, and size were 0.02 ml/g, 8.32 m<sup>2</sup> /g, and 9.62 nm. While the FBCs were 0.0017 ml/g, 2.98 m<sup>2</sup> /g, and 2.32 nm, respectively (**Table 1**). These demonstrated that the SSA, pore volume, and pore size of ABC increased by 179%, 1076%, and 314% compared with FBC.

**Figure 2**. *Scanning electron micrographs of FBC (a, b) and ABC (c, d).*

**Figure 3**. *XRD patterns of FBC and ABC before and after adsorption of heavy metals.*

#### **5.2 The chemical properties of field-aged biochar**

#### *5.2.1 Potential of hydrogen (pH) of field-aged biochar*

The properties of biochar are presented in **Table 1**. The pH of ABC was 5.83 as acidic while FBC was 8.87 displayed alkaline. This demonstrated that biochar in field aging process remarkably decreased the pH.

#### *5.2.2 The elemental analysis of field-aged biochar*

**Table 2** shows the elemental contents of ABC and FBC. The C content in ABC was 46.89%, deceased considerably compared with the content in FBC, which is 81.01. The C content in ABC was 21.26% while was 12.47% in FBC. The atomic ratios, including H:C O:C and (N+O+S):C, increased notably with aging, suggesting that ABC was highly oxidized and exhibited lower aromaticity than FBC.

#### *5.2.3 The crystal structures of field-aged biochar*

**Figure 3** shows the crystal structures of FBC and ABC before and after the reaction with heavy metal, which were investigated by XRD and analyzed using Jade6.5 PDF cards. In ABC, there is a strong peak appearing at 26.6° where was assigned to the characteristic diffraction peak of SiO2 either before or after the absorption of heavy metal [37]. However, there is a peak located at 29.5° where is the crystalline structures of CaCO3 in FBC in the XRD pattern before the absorption of heavy metal. And interestingly, the diffraction peaks appearing at 2*θ* = 30.33°, 20.97°, and 24.79° after the heavy metal ions are adsorbed by FBC can be ascribed to CdCO3, Pb (CO3)2(OH)2, and PbCO3 [20]. Totally, the intensity of the characteristic peaks of CaCO3 and SiO2 in FBC and ABC was obviously weakened or disappeared after the absorption with heavy *Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*


#### **Table 1**.

*The physicochemical properties of FBC and ABC.*


#### **Table 2.**

*The data of XPS analysis of C1s for samples.*

metal. We can conclude that precipitation may be one of the main mechanisms for Cd2+ and Pb2+ removal by FBC.

#### *5.2.4 The functional groups of field-aged biochar*

**Figure 4** shows the FTIR spectra of ABC and FBC before and after the adsorption. There were some differences in the types and intensity of surface functional groups on ABC and FBC. The main functional groups on ABC were including C=O [38], COO [39], Si–O–Al, C-H, Al–O–Si [40], Si-O-Si [37]. While the functional groups on FBC were C=C, COOH/CHO, phenolic –OH bending, CO2–3, C-O, and C-C, the aromatic ring C-H, the aromatic C-H [40]. The -OH and C-H were in all samples. There were also changes of FBC/ABC between before and after the adsorption of heavy metals. After FBC adsorbs metal ions, the peak waves of C = C, C = O and aromatic hydrocarbons were shifted. This may be the result of chelation of heavy metal ions with

#### **Figure 4**.

*FTIR spectroscopy of FBC and ABC before and after adsorption. Note: Peak at 3418–3433 cm<sup>1</sup> : -OH; peak at 2950 cm<sup>1</sup> and 2860 cm<sup>1</sup> : Saturated C-H; peak at 1620 cm<sup>1</sup> : C=O; peak at 1585 cm<sup>1</sup> : Aromatic C=C; peak at 1430 cm<sup>1</sup> : -COOH/CHO; phenolic –OH bending; CO2–3; peak at 1384 cm<sup>1</sup> : COO; peak at 1158 cm<sup>1</sup> : C-O and C-C; peak at 1158 cm<sup>1</sup> : Si–O–Si or Si–O–Al asymmetric; peak at 874 cm<sup>1</sup> :Aromatic ring C-H bonds; peak at 804 cm<sup>1</sup> : Aromatic C-H; peak at 797 cm<sup>1</sup> : Aromatic C-H stretching; peak at 538 cm<sup>1</sup> : The symmetric of Al–O–Si; peak at 538 cm<sup>1</sup> : The antisymmetric of Si-O-Si.*

aromatic structure of biochar (cation-π interaction). In addition, we also found that the atomic ratio of (N+O+S): C and H: C in ABC increased significantly, which can measure the polarity and aromaticity of biochar. These indicated that the aromaticity of ABC decreased while the polarity increased. This would reduce the stability of heavy metal ions, which passivated by biochar and the heavy metal would be released to the soil inducing secondary pollution. These results were consistent with the increasments of TCLP leaching rate of Cd2+ and Pb2+ in ABC.

#### *5.2.5 The combination state of main elements in field aging biochar*

**Figure 5** shows that the C, O, N, S, Si, and Al were observed in ABC (**Figure 5(c)**), while only C and O elements were in FBC (**Figure 5(a)**) before the heavy metal absorption. And O element is more in ABC than in FBC interestingly. That the O:C in ABC was 0.45 while was 0.15 in FBC. This ascribed to the O-containing functional groups of biochar markedly increased after aging in field. Data as showed in **Table 2**, the C1s spectrum were divided into four peaks: 285.4 0.1 EV (C─O), 284.8 0.1 EV (C─C/C═C), 289.1 0.2 EV (O─C═O or carbonate), and 286.4 0.3 EV (C═O). The relative proportion of C─C/C═C, C-O, C═O, and O─C═O/carbonate in ABC were 42.92%, 27.34%, 22.61%, and 7.13% respectively, while they were 57.33%, 19.38%, 18.24%, and 5.05% in FBC. That is, the ABC's relative contents of C─O, C─O, and <sup>O</sup>─C═O functional groups were dramatically higher than that of FBC. During the field aging process, due to the oxidation or weathering of persistent free radicals in the soil, oxygen-containing functional groups, including phenolic groups and carboxyl, were

*Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*

**Figure 5**. *XPS spectra for typical survey scan of FBC (a, b) and ABC (c, d) before and after adsorption.*

formed on the surface of biochar [41]. This will contribute to an increase in the relative contents of C═O, O─C═O, and C─O. **Figure 6** shows that the peaks of Cd3d and Pb4f were observed in FBC and ABC before and after the adsorption of heavy metal. It was concluded that Cd2+ and Pb2+ were successfully adsorbed on the surface of biochar. It has been proved that the peaks of 412.5 EV and 405.6 EV belong to Cd 3d3/2 and Cd 3d5/2, respectively. This indicated that Cd exists in the form of Cd─<sup>O</sup> through complexation with hydroxyl (─OH) or diprotic oxygen (─O─) on the surface of biochar [42, 43]. The peaks of Pb4f appeared at around 144.4 eV and 139.4 eV, which can be attributed to Pb─O─C and Pb─O [44].

#### *5.2.6 The discussion of chemical properties of biochar*

The pH of biochar was reduced by three units after being aged in the agricultural field for 7 years. This change can be attributed to the alkaline substances in FBC leached out [5]. During aging, the alkaline substances were dissolving. In addition, the decrease in basicity of biochar may be due to O-containing functional groups' formation on the biochar's surface. These can be well explained by elemental analysis and XPS. The analysis of elements showed that the O:C of FBC is 0.15 while the O:C of ABC was increased to 0.45. That is a representative of the oxidation level of biochar. XPS results indicated that the relative content of C═O, C─O, and O─C═O functional groups of FBC were dramatically lower than that of ABC. During the field aging process, due to the oxidation by persistent free radicals in soil or weathering effects, including phenolic groups and carboxyl, oxygen-containing functional groups were

**Figure 6**. *XPS spectra of C1s, Cd3d, and Pb4f for FBC and ABC before and after adsorption.*

formed on the surface of biochar. These lead to the increase of the relative content of <sup>O</sup>─C═O, C═O, and C─O. However, the formation of acidic functional groups on the surface of biochar, such as carboxyl and phenol groups, can cause the biochar's lower pH value [12].

It was further confirmed that more O-containing functional groups were formed on aged biochar surface during aging in field [5]. Biochar has large SSA and abundant pore structure. In our study, the average pore size and SSA of ABC increased by 4331.39% and 279.19% compared with FBC, respectively. Even more interesting is that the total pore volume in ABC increased more than 9.9-fold compared with FBC. The freeze–thaw cycles and rainfall events of biochar in soil maybe related with these changes. The expansion and elimination of water molecules also occur in biochar during freeze–thaw cycles [12]. As a result, the SSA of the biochar increases. As well, aging in field may also influence the functional groups and elemental composition of biochar surface. In this study, the relative contents of S, O, and N in ABC increased significantly compared with FBC. While the relative content of C decreased in ABC compared with FBC. These results were in accordance with the results of the prevenients [8]. It was found that the O content of biochar's surface increased while the C content decreased after 5 years in the field soil, and the o content increased. These indicated the dissolution of unstable C during the aging process of biochar [12, 45]. Furthermore, the results of XPS indicate that the amounts of Al and Si in ABC increased, implying that soil minerals could have been attached on biochar surface during field aging. This result was also reported in previous studies.

*Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*

#### **5.3 Biochar sorption experiments**

#### *5.3.1 The kinetic adsorption of field-aged biochar*

**Figure 7** shows the adsorption kinetics of Cd2+ and Pb2+ on FBC and ABC in the single-metal (**Figure 7**(**a** and **b**)) and binary-metal (**Figure 7**(**c**, **d**)) systems. Correspondingly, **Table 3** gives the parameters fitted by pseudo first-order model and

**Figure 7**. *Kinetic adsorption of Cd2+ and Pb2+ by PBC and ABC in single- (a, b) and binary-metal (c, d) systems.*


#### **Table 3.**

*Fitting parameters of pseudo-first-order and pseudo-second-order kinetics Cd2+ and Pb2+ by FBC and ABC singleand binary-metal systems.*

pseudo first-order model. From the perspective of R<sup>2</sup> in **Table 3**, the pseudo secondorder kinetic model (R<sup>2</sup> = 0.9674–0.9917) was better fit for the Cd2+ and Pb2+ adsorption kinetics data for FBC and ABC in the single-metal and binary-metal systems, compared with the pseudo first-order model (R2 = 0.8623–0.9898). In addition, the calculated *Qe* values based on the pseudo second-order model were approximately the experimental *Qe* values. Overall, the adsorption of two metal ions on ABC or FBC increased dramatically within 2.5–3.0 hours and then approached to a flat with the augment of reaction time shown in **Figure 7**(**a**–**d**). And the adsorption quantity on Cd2+ and Pb2+ of ABC was stronger than FBC either in the single-metal or binarymetal system. It was interesting that the capacity of quilibrium adsorption in singlemetal system was higher than that of the binary-metal system. While the capacity of its total adsorption was weaker. That is, ABC and FBC reach the adsorption equilibrium at 8 h in the binary-metal system, while at 12 h in the bimetallic system. These were indicating that there was a competitive relationship between Cd2+ and Pb2+.

#### *5.3.2 The isothermal adsorption of field-aged biochar*

**Figure 8** shows the isothermal adsorption of Cd2+ and Pb2+ on FBC and ABC in the single-(**Figure 8(a, b)**) and binary-metal (**Figure 8(c, d**)) systems. Correspondingly, **Table 4** gives the fitting parameters of the Langmuir and Freunlich isothermals for Cd2+ and Pb2+ by FBC and ABC in single- and binary-metal systems. As shown in **Figure 8**, Langmiur model due to its higher correlation coefficient (R2 = 0.96013– 0.9910) was more reasonable than Freunlich model (R2 = 0.7924–0.9679) in this isothermal adsorption experimental data analyses. **Figure 8** shows that at low initial concentrations, the adsorption capacity of eight ABCs or FBCs increased with the

**Figure 8.** *Isothermal adsorption of Cd2+ and Pb2+ by FBC and ABC in single- (a, b) and binary-metal (c, d) systems.* *Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*


#### **Table 4.**

*Fitting parameters of Langmuir and Freundlich isotherms for Cd2+ and Pb2+ by FBC and ABC in single- and binary-metal systems.*

increasing of initial concentration of Cd2+ and Pb2+ and then gradually slowed down when the biochar reached saturation. It has been known that in Langmuir isotherm the separation factor RL commonly used to evaluate the affinity between the adsorbent and the adsorbed material [46]. They are as follows: when RL >1, adsorption is unfavorable; when RL = 1, adsorption is linear; when 0 < RL <1, adsorption is favorable; when RL = 0, adsorption is nonlinear; and when RL < 0, adsorption is irreversible [14, 47]. In this experiment, the initial concentrations of Cd2+ and Pb2+ were ranged from 5 to 120 mg/L (**Table 4**). The RL values of FBC-Pb, FBC-Cd, ABC-Pb, and ABC-Cd were between 0 and 1. These results indicated that both ABC and ABC were favorable for Cd2+ and Pb2+ adsorption. In addition, **Table 4** shows that all 1/n values were in the range of 0–1 in this study, indicating that adsorption is also favorable [48]. The above analysis illustrated that the adsorption of Cd2+ and Pb2+ on FBC and ABC was monolayer adsorption.

#### *5.3.3 The metal leachability and bioavailability of field-aged biochar*

In order to understand the metal leachability and bioavailability of ABC, we take the leaching characteristics of Cd2+ and Pb2+ in FBC and ABC by TCLP method. The results showed that the concentration of Cd2+ and Pb2+ in TCLP leachate was 39.21% and 28.62% in ABC while was 24.08% and 21.24%, respectively in FBC. This implied that the adsorption mechanisms of ABC for Cd2+ and Pb2+ were different from FBC. At same time, the increase of TCLP leachability of Cd2+ and Pb2+ suggesting the stability of ABC to immobilized heavy metals was significantly reduced.

#### *5.3.4 The discussion of biochar sorption*

The results of isothermal and kinetic adsorption experiments showed that the pseudo-secondary kinetic and Langmuir model were more fitted with the adsorption of metal ions by FBC and ABC. FBC and ABC immobilize Cd2+ and Pb2+ in binary metal system as a chemical reaction as confirmed by the above results [20, 49, 50]. ABC adsorbs more heavy metal ions than FBC. These ascribed to the aged biochar having more oxygen-containing functional groups and a larger SSA. These indicated

that ABC surface has more chemisorption active sites. Additionally, the biochar can well serve as a habitat for microorganisms, such as bacteria and fungi, due to its abundant porous structure.

In addition, the active component carbon and mineral nutrients in biochar can be used as its energy source by microorganisms in the soil environment [8, 36]. Therefore, it is likely that microorganisms will attach to the ABC surface after it is applied to the soil. Furthermore, the adsorption of heavy metals may be promoted by the beneficial microorganisms immobilized on biochar. It has been reported that the combined application of biochar and bacteria can improve the adsorption of heavy metals [33]. There are some possible mechanisms for the interaction between bacteria, biochar, and heavy metal ions. First, the respiration of bacterial cells attached to the surface of biochar form metal carbonate precipitates. Second, new adsorption sites were formed by bacteria colonized of biochar [13]. Third, bacterial cells are as transport carriers between heavy metals and biochar. That is, the heavy metal ions in the soil solution are first transferred to the cells, and then the cells adsorbed on the biochar and actively pumped out of the bacterial cells [51].

#### **6. Conclusions**

In conclusion, under field conditions, the physicochemical properties of biochar have changed in soil after 7 years of field aging. The pore volume and SSA of biochar increased with field aging, owing to the dissolution of unstable carbon or carbides in biochar. FTIR and XPS results proved that there were abundant O-containing functional groups on the surface of aged biochar. The results of adsorption kinetics and adsorption isotherm showed that the adsorption of heavy metal ions on ABC and FBC surface was controlled by chemical adsorption. FBC immobilizes Cd2+ and Pb2+ mainly through cation exchange, co-precipitation, and cation-π interaction. Whereas the main mechanism of ABC removing Cd2+ and Pb2+ may be the cation exchange and surface complexation. Compared with that of FBC, the adsorption performance of ABC for Cd2+ and Pb2+ is improved due to the increases of O-containing functional groups and SSA in ABC. Nevertheless, the stability of ABC to immobilized heavy metals was significantly reduced.

#### **Acknowledgements**

This study was financially supported by the Projects of National Natural Science Foundation of China (31201618), China Agriculture Research System (CARS-23-G27). *Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*

#### **Author details**

Run-Hua Zhang<sup>1</sup> \*, Lin-Fang Shi<sup>1</sup> , Zhi-Guo Li<sup>2</sup> , Guo-Lin Zhou<sup>1</sup> , Yan-Lan Xie<sup>1</sup> , Xing-Xue Huang<sup>1</sup> , An-Hua Ye1 and Chu-Fa Lin<sup>1</sup>

1 Wuhan Academy of Agricultural Sciences, Wuhan, China

2 Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden of Sciences, Wuhan, China

\*Address all correspondence to: rhzhanag0508@126.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[34] Lehmann J, Joseph S. Biochar for environmental management: An introduction. In: Lehmann J, Joseph S, editors. Biochar for environment management. 1st ed. Earthscan; 2009. p.1-9

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*Aged Biochar for the Remediation of Heavy Metal Contaminated Soil: Analysis through an… DOI: http://dx.doi.org/10.5772/intechopen.107523*

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#### **Chapter 8**

## Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants from Wastewater: Current Status and Perspectives

*Uplabdhi Tyagi and Neeru Anand*

#### **Abstract**

Human activities and rapid modernization have affected the ecological and economical aspects worldwide resulting in alarming situations such as global warming and the accumulation of waste disposal and toxic contaminants in water. Contaminants in water are toxic and carcinogenic, posing a serious threat to the environment. Water is a precious and limited resource and hence it is highly imperative to utilize effective remediation strategies for the removal of pollutants. Several competitive remediation techniques have been proposed due to their distinctive features including ease of operation, inexpensiveness and universal nature. The present chapter highlights the potential of ecofriendly biomass-derived biochars as adsorbents for the effective removal of toxic contaminants. This includes biochars derived from industrial solid wastes, agricultural wastes, clays minerals and municipal wastes. Biomass-derived biochars are found to be highly efficient, alternative and carbon-neutral precursors and provide a new approach to the modular adsorption process. The present chapter also includes conversion of waste materials into efficient bio-adsorbents followed by their applications for the purification of wastewater. Besides, attempts are made to discuss the techno-economic and future perspectives of eco-friendly and low-cost biochars for the treatment of wastewater.

**Keywords:** adsorption, waste management, green synthesis, biomass engineering

#### **1. Introduction**

Freshwater is a basic demand for human activities including industrial, agricultural and domestic activities. These activities produce a huge amount of contaminated water resulting from the discharge of undesirable toxic and carcinogenic contaminants (inorganic/organic/biological agents/radioactive wastes) into water bodies that impose a serious concern on the environment and living species. According to World Health Organization (WHO) and the literature available [1, 2], the majority of water

on earth is salty, requiring treatment before it can be used. The rest of the freshwater is in glaciers and underground reservoirs. Industrial activities (automobile manufacturing, textile, dyeing, paint, paper and pulp, tannery and leather industry) and Agricultural activities (excess use of fertilizers and pesticides, antibiotics, processed wastes of crop plantation) and unwanted environmental changes (damages to sewer system due to high rainfall, soil runoff, use of pesticides and fertilizers) are the major cause of water pollution [3–5]. Hence, the preservation of freshwater, as well as the quality improvement of contaminated water (decontamination of pollutants from water), is a growing challenge.

Literature reports several feasible and popular conventional separation techniques for the treatment of polluted water such as chemical precipitation, adsorption, ion-exchange, flotation, coagulation and flocculation, ultrafiltration, nanofiltration, reverse osmosis, electrochemical process, evaporation and photo-catalysis [6, 7]. Each technique is effective in its own way and offers several advantages for one process but at the same time imposes several restrictions on other processes. However, amongst these popular conventional separation techniques, the chemical and electrochemical treatment processes are ineffective even at very low pollutant concentrations, due to excessive amount of chemical usage, sensitivity towards variable wastewater input and producing a large amount of sludge that needs further treatment before releasing it to the environment [8]. Other processes such as ultrafiltration, nanofiltration, reverse osmosis and ion exchange are the most expensive to treat a large amount of wastewater adding demerits to explore at the industrial scale.

Safe drinking water demand at a reasonably low price with an effective and sustainable treatment approach is a prime focus of industrialists and academicians. Currently, adsorption is used for wastewater treatment and is gaining wide attention due to its effectiveness and feasibility. In this regard, biomass-derived biochars are gaining attention due to their high potential, sustainability, carbon neutrality, low cost, mobile capability and wide availability in nature. Synthesis of biochars from inexpensive matters (living and non-living biomass) leads to significant cost reduction in waste disposal [9, 10]. These biochars can be obtained from various sources including industries and agricultural activities, plant wastes, fruit wastes, naturally occurring inorganic materials and living and dead biomass [11]. Literature reports a wide variety of biomass-derived biochars for effective wastewater treatment such as date pits, *vermiculite* plants, coconut shell and husk, bamboo waste, rice husk, ground nutshell, shells of almond, wheat bran and *Heveabrasiliensis* seed coat [12, 13]. These waste materials not only balance the environmental problems but an unutilized and a potential resource is also managed during the process. Also, utilization of these biochars resolves several major challenges associated with up-scaling technology including pollutant selectivity, regeneration, sludge formation and pollutant recovery and also exhibits excellent adsorption ability. Many factors affect the adsorption capacity of these biochars including physical and chemical properties of pollutants (i.e. molecular weight, oxidation state and ionic radius), characteristics of biochar and the process parameters (i.e., quantity of bio-sorbent, pH, temperature and sorbate concentration). Besides influencing the dissociation of pollutant sites and solution chemistry, pH plays a crucial role in the speciation and biosorption affinity of pollutants. Other factors include the composition of biomass (cellulose, hemicellulose, lignin and extractives), pore structure, surface charge and heteroatom content in the biochar. The adsorption capacity of biochar is highly dependent on the chemical compositions and carbohydrate contents of biomass which may vary from source to

#### *Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.105534*

source and species to species. The biomass exists in diverse forms and has distinctive physical and chemical compositions, carbohydrate and lignin fractions as summarized in **Table 1**. In comparison to other wastes, agricultural and forestry wastes have high percentage composition of carbohydrates and lignin [14, 15]. Utilization



#### **Table 1.**

*Percentage Composition in Biomass (Cellulose, Hemicellulose, Lignin).*

of such biochars not only enhances the removal efficiency of various pollutants but simultaneously helps in the reduction of atmospheric carbon dioxide via the processing of waste biomass for a wide range of applications such as the synthesis of biofuels (i.e. biobutanol, bioethanol, and biomethanol), energy storage and soil refinement [16]. Apart from biochars, hydrochars have gained significant importance. Hydrochar is a char which is made by hydrothermal carbonization (a process where biomass is heated to a temperature range of 200–300°C in the presence of water), and is comprised of two phases: liquid and solid. Hydrochars offer advantages like low oxygen and ash content, zero hazardous chemical waste generation, high production yield (approximately 30-60 wt%), mild temperature processing (180-250°C), large surface areas and porosity. These materials offer several applications in many areas including soil amelioration, energy storage and water purification.

#### **2. Remediation techniques employed for the removal of contaminants from wastewater**

Several techniques are commercially available, to remove various contaminants including (inorganic and organic chemicals in dissolved and non-dissolved forms, biologically active agents, radioactive substances, polychlorinated biphenyls and pesticides) from wastewaters and some are summarized in **Table 2**. The following **Table 2** lists the most common commercially available techniques used in different sectors for pollutant removal due to their distinctive characteristics such as low-cost operation, flexibility and design simplicity.

#### **2.1 Ion exchange**

The ion exchange reaction is a reversible chemical reaction that involves the removal of dissolved ions from a solution and their replacement with other ions of the same or similar electrical charge. This process uses an insoluble matrix (or support structure) which is in the form of small microbeads (0.25–1.43 mm radius), usually white or yellowish, and are fabricated from an organic polymer substrate. This process has been widely employed for the separation of ionic dyes and heavy metal ions from aqueous streams. The widely used materials for this process are ion-exchange


*Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.105534*

> **Table 2.**

*Performance summary of different adsorption techniques using different wastewater.*

resins that can be natural or synthetic having the ability to exchange their cations with the solutes present in the aqueous streams. Several parameters affecting the ionexchange process are temperature, solution pH, initial metal concentration, contact time and ionic charges. Zeolites (silicate minerals) are most abundant in nature and have been extensively used to separate heavy metal ions from aqueous streams under different conditions [16, 18]. Although natural Zeolites show good performance in a few cases scale-up of the process at an industrial level is still restricted. In contrast, synthetic resins show high efficiency in comparison with natural resins. Literature reports that macroporous anion exchangers (MP62, weak basic and S6328a, strong basic) are more effective with higher affinity and adsorptive capacity to separate pollutants from wastewaters originating from textile industries [19, 20].

#### **2.2 Advanced oxidation processes (AOP)**

Advanced Oxidation Process (AOP) is a treatment technology designed to remove organic matter from wastewater by oxidation through a reaction with hydroxyl radicals. As opposed to direct oxidation, AOPs usually consume less energy. In AOPs, a sufficient amount of hydroxyl radicals are produced that impact water purification. Hybrid advanced oxidation processes such as photocatalytic fenton, photo-fenton, H2O2/O3/photocatalysis and photo-electrocatalysis have drawn the attention of industrialists and academicians due to their efficiency and cost-effectiveness [18, 19]. Currently, several nano-particle supported AOPs have been discovered for the remediation of several contaminants from wastewater such as methyl orange, methylene blue, 2,4-dichlorophenol and pentachlorophenol. Recently, research has also been carried out to explore the activity of photo-Fenton and/or heterogeneous Fenton catalysts for the simultaneous removal of multiple contaminants from waste streams. TiO2 photocatalyst mixed with fly ash has also been employed for simultaneous separation of Cd+2 ion and methyl orange dye from an aqueous stream (removal efficiencies of Cd+2: 88% and methyl orange: 70%) [21]. Similarly, heterogeneous catalyst FeIIFe2 IIIO4 nanoparticles supported on activated carbon have been utilized in a photo-Fenton process for remediation of pollutants (aniline and benzotriazole) and maximum removal efficiency for aniline was found to be 70.4% and benzotriazole to be 99.5% [22]. Although, the studies based on nano-particle supported AOPs proved to be promising at the pilot-scale, this process has no valid evidence to prove its costeffectiveness and its eco-friendly operation due to the toxicity of nanoparticles. Also, there is no reliable information available about the commercialization of AOPs for the simultaneous treatment of multi-component pollutant systems.

#### **2.3 Flotation**

The flotation technique has been extensively used to remove inorganic heavy metal ions from aqueous streams. Flotation is a separation process that works on the introduction of gas bubbles as the transport medium. Suspended particulate matter, being hydrophobic or adhering to gas bubbles and move towards the water solution surface—i.e., contrary to the direction of gravity. In this technique, heavy metal ions are made hydrophobic by the use of some hydrophobic agents such as surfactants (surface-active chemicals) and separated with the assistance of air bubbles. The surface-active agents consist of a hydrophilic head (water-loving part, polar) and hydrophobic carbon chains (non-polar, water-hating part). The air bubbles loaded by solutes float over the water surface and are separated as a metal-rich froth [17].

*Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.105534*

This process is highly effective for the removal of sulfide minerals. Despite several advantages (i.e. almost all minerals can be removed, surface properties are highly governed and controlled by flotation agents used), this process has some disadvantages such as high cost and complex. According to recent research, open tank settling clarifiers are currently used as primary, secondary and tertiary clarifiers. This is primarily due to their reluctance to embrace new technologies in the development of dissolved air flotation (DAF), especially in paper mills. The specific clarification is limited to 0.5 GPM per square foot. Chemical treatment improves specific load and transparency while the residence time of settling is still 60-200 minutes.

#### **2.4 Adsorption**

Adsorption is a well-established separation process used widely for the removal of inorganic and organic compounds from wastewater. This process is proved to be superior as compared to other remediation techniques due to its ease of operation. It is simple and flexible in design, capable to treat dye wastewater effectively even at higher concentrations and also insensitive to the toxicity of contaminants [3, 5, 23]. The adsorption technique is dependent upon the affinity of contaminants towards the adsorbing materials. It is influenced by many other factors such as specific surface area of adsorbent, interactions between pollutant and sorbent, particle size distributions, solution pH, system temperature and contact time. The proper selection criteria of any adsorbent for separation are based on several characteristics such as adsorption capacity of adsorbent, selectivity, regeneration power, mechanical strength and low cost. Several adsorbents have been extensively utilized and show high sorption capacity for simultaneous removal of organic and inorganic solutes from wastewater as shown in **Table 3**. For instance, Fly ash has been successfully utilized for the separation of heavy metals and dyes from a multi-component aqueous solution; Ca(PO3)2-modified carbon can be used for the separation of heavy metal ions and dye (acid blue 25); Nano-particles (TiO2) for removal of organic dye, copper and silver heavy metals; Graphene oxide nano-composite can be used for separation of cadmium and ionic dyes; Magnetic metal–organic frameworks composite i.e. (Cu-MOFs/ Fe3O4) have been used for separation of malachite green dye and lead ions; Zr-based magnetic Composites i.e. Zr-MFCs and Amino-decorated for separation of lead and methylene blue [17, 24]. The use of this technology for the treatment of textile wastewaters is still limited due to excessive maintenance cost, high regeneration cost, issues regarding proper disposal of used adsorbents and the requirement of pretreatment to


#### **Table 3.**

*Summary of adsorption capacity of various biomass-derived biochar with different operating conditions.*

reduce suspended solids into feed for acceptable operational range. Thus, the adsorption technique shows promising outcomes at a commercial scale and resolves several challenges associated with waste disposal and regeneration.

#### *2.4.1 Utilization of biochar as an adsorbent*

All the above processes show their advantage and disadvantage concerning process efficiency, high costs (capital or operational), adsorbents, process conditions and removal percentage of pollutants. In this regard, biochars are receiving increasing attention and are highly recommended as a bio-adsorbent since they can both mitigate climate change by capturing carbon dioxide from the atmosphere into soil and increase the removal of organic pollutants. Biochar is defined as a carbon-rich material produced during the pyrolysis process that is a thermochemical decomposition of biomass with a temperature of about ≤700°C in the absence or limited supply of oxygen. As it is having a high-carbon content (approximately 60–90%), the application of biochar for the removal of a wide variety of contaminants from wastewater is considered a significant and long-term approach to sink atmospheric CO2 in terrestrial ecosystems. Several kinds of biomass can be used as sources of biochar, such as wood chips, animal manure, and crop residues. Biochars have the ability to enhance the recycling of agricultural and forestry wastes. Biochar adsorbents are relatively cost-effective, environment-friendly and will be a beneficial tool for environmental remediation. Thus biochar research is gaining attention.

#### *2.4.1.1 Characteristics of Biochar*

The properties of biochar are determined by the pyrolysis temperature, the residence time, the feedstock considered, and the technology used for conversion. These factors influence the effectiveness of contamination removal. It was found that the amount of carbonized matter, the surface area, the pores, and the hydrophobicity of biochar increased with increasing temperature, consequently increasing the affinity of organic pollutants for adsorption. The presence of a high amount of carbonized matter in biochar favours the adsorption of contaminants, especially for the compounds having oxygen and hydrogen functional groups. According to research, activated carbon derived from wheat residue at 500-700°C was well carbonized and had a high surface area (>300 m2 /g), whereas charcoal made at 300-400°C was partially carbonized and had a lower surface area (<200 m2 /g) [25]. Hence, the former material exhibits high sorption capability for the removal of organic pollutants. Biochar can be made of diverse materials exhibiting different properties. The change of properties of biochar can be correlated to their function. Additionally, improving the adsorption capability of biochar through different treatments, such as chemical activation and surface modifications are found to be effective in improving its properties. This may be due to the enhanced porous structure and sorption properties that occurs after activation process [25]. Apart from activation of biochar, magnetization is also a useful method to improve biochar property. **Tables 4** and **5** summarizes the different preparation methods of biochar under different operating conditions.

#### *2.4.1.2 Biochar adsorption mechanism*

Adsorption is a surface phenomenon with a common mechanism for the removal of organic and inorganic pollutants. When a solution containing an adsorbent solute comes into contact with a solid with a very porous surface structure, the intermolecular


*Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.105534*


*Biochar - Productive Technologies, Properties and Applications*

**Table 4.**

 *Several preparation methods of biochar under different operating conditions.*


*Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.105534*


#### **Table 5.**

*Overview of synthesis of biochar-based sorbents.*

attractive force between the liquid and the solid causes some of the solute molecules to concentrate from the solution or deposit on the solid surface. The mechanisms for the removal of organic pollutants with biochar involves surface sorption, cation/ion exchange, electrostatic interactions, precipitation and complexation [28]. All these mechanism as an individual or together plays important role and show great effect on adsorption capacity.

Surface sorption: In this process, metal ions diffuse into the pores of the sorbent to form chemical bonds. The pore volume and the surface area of the sorbent (biochar) depend upon the carbonization temperature.

*Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.105534*

**Figure 1.** *Properties and mechanism of biochar functioning.*

Electrostatic interaction: It is a mechanism that uses electrostatic interaction between the charged biochar particles and the metal ions to prevent metal ion mobilization.

Cation/ion exchange: The major principle of this mechanism is the exchange between protons and ionized cations on the surface of the biochar. As a result, its ability to remove heavy metals depends on the size of the contaminated surface and the surface functional groups of the biochar.

Precipitation: It is one of the main mechanisms that can be used to remove inorganic pollutants from biochar. As a result, mineral precipitates are formed either within the solution or on the surface of the sorbing material. In particular, this occurs for biochar produced from pyrolysis of cellulose and hemicelluloses with a temperature exceeding 300°C and with an alkaline property.

Complexation: Metal complexation involves the formation of multi-atom structures through the interaction of specific metal ligands. Due to the oxygen-containing functional groups present in low-temperature biochar such as phenolic, lactonic, and carboxyl, it can bind with heavy metals. The oxygen content of the biochar can lead to an increase in surface oxidation and metal complexation.

Biochar's remediation effect is achieved by these mechanisms as shown in **Figure 1** and the nature of bonding working together, rather than acting separately. The nature of the bonding depends on the type of species interaction while the adsorption process is usually classified as physisorption (characteristic of weak Van Der Waals forces) or chemisorption (characteristic of covalent bonding) [29].

#### **3. Development of economic and sustainable biomass derived biochar**

#### **3.1 Preparation methods of biochar**

Several techniques including pyrolysis, gasification, hydro carbonization have been used for the synthesis of biochar affecting the adsorption capacity and are discussed in **Table 4**. Pyrolysis of biomass is found to be the most widely used

technique and can be carried out in the absence of oxygen at high temperature. Pyrolysis process can be classified as slow, fast and flash depending on the temperature and residence time. A slower heating rate and a lower pyrolysis temperature can result in the high yield of solid products [27]. It was found that slow pyrolysis results in the formation of ~35% solid yield indicating the effectiveness of the process among other three-pyrolysis techniques. Hydrothermal carbonization (HTC) is another important technique used for the synthesis of biochar. Biochar obtained from HTC exhibit superior adsorption properties with zero production of toxic substances. The main limitations of this method are requirement of high pressure, reactor cost and high temperature that limits the practical applications. Recent literature shows that the treatment of sewage sludge is found to be more effective and feasible using HTC as compared to other thermochemical processes due to low energy consumption and high thermal and mechanical stability of biochar. In addition to slow pyrolysis and HTC, other methods such as rapid pyrolysis, flash pyrolysis and gasification are also efficient and cost effective. However, such methods have low product yield and are typically used to produce bio-oil or gaseous materials.

There is a strong relationship between the preparation method and the physicochemical properties of biochar as shown in **Figure 2**. Biochar can be produced from wide range of biomass such as municipal, agricultural, aquatic or forestry having different physical, chemical and structural properties. There are several factors affecting the physicochemical properties of biochar including type of the raw material, source of biomass, pyrolysis type (slow, rapid or flash), duration of pyrolysis, size of the substrate, temperature and heating rate [26]. These operating parameters results in the number of surface functional groups including hydroxyl, carbonyl, methyl and carboxyl. In addition, several factors affect the structure of biochar including oxygen-containing aromatic functional groups, high carbon content, surface area and high porosity. These factors significantly favour the adsorption of pollutants onto the surface of biochar.

**Figure 2.**

*Biochar preparation methods and its applications.*

*Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.105534*

#### **3.2 Biochar's properties influencing its activity**

As discussed above, properties of biochar are influenced by pyrolysis temperature, residence time, feedstock, and the thermal conversion technology. The variations in these parameters results in the variation in the removal efficiency of pollutants as shown in **Figure 3**. The selection of biochar for a specific purpose depends on several factors such as mechanical strength, adsorption efficiency, cost, regeneration, ease of synthesis, selectivity for different pollutants, reusability and rate of adsorption and desorption. Due to high porosity, sorption ability of biochar is highly dependent on the surface area. The high surface area enhances the ability to adsorb the pollutants on the surface of biochar. This can be done either by physical modification (such as purging of steam and gas) or by chemical activation using various chemical reagents (concentrated or diluted). In addition to porosity, several other factors including pH, temperature, adsorbent dose, and agitation speed affects the adsorption process [20]. pH is the most crucial parameter that affects the dissociation of functional groups and the charge on the active sites, thereby affecting the adsorption capacity. Another significant parameter is the biochar dosage. Significant increase in the adsorption of pollutants has been found with increase in the biochar dosage due to the availability of sufficient active sites on the surface of biochar. While further increase in the biochar dose than the optimal dosage declines the adsorption of pollutants due to the saturation or blockage of active sites. Generally, adsorption processes are endothermic in nature thus on increasing the temperature, increase in the adsorption of pollutants was observed. It has been observed that high temperature leads to the degradation of molecules that results in the decline of adsorption capacity [18]. Thus, maintaining an optimum temperature is highly essential. Agitation speed is another critical parameter that influences the adsorption capacity and reaction mechanism. With increase in the agitation speed, gradual increase in the adsorption capacity has been observed.

**Figure 3.** *Factors affecting the properties of biochar.*

This may be due to the increase in the turbulence and reduction in the thickness of the boundary layer around the biochar that improves the interaction between adsorbate and adsorbent. According to the literature, the boundary layer and intraparticle diffusion are the controlling steps for the adsorption mechanism, and the optimum speed for adsorption process is usually in the range of 120 rpm to 200 rpm.

#### **4. Future perspectives**

It is evident from the above studies that the biochars are potential and economical candidates for water purification. This study covers the advancement in the field of biochars followed by their utilization in various fields. However, detailed research is still required in terms of physical and chemical modifications to enhance porosity and surface area of biochar. Further, polymorphs of biomass for the production of biochar and their effect on multicomponent systems still needs exploration. A more underlying mechanistic approach is required to understand the role and performance of individual components i.e. cellulose, hemicellulose and lignin as these polymers provide heterogeneity to the biomass matrix. Differences in the magnitude of adsorption capacities using different biochars having the same origin and composition is an indicative of unexplained correlation between morphological patterns and molecular structure of biochars. Besides this, a critical investigation is required to determine the effectiveness of surface area, porosity and functional groups of biochars. Many studies cover the technical performance of biochars while the economic feasibility and environmental impact is neglected. Studies need to be carried out in detail to suggest an effective binding mechanism of several pollutants with biochars. Also, no study has been reported on the removal of anions, radionuclides and pesticides using biochars. Further, limited data is available for the competitive adsorption of contaminants especially on phenols and dyes. In addition, some biochars are incapable to perform under neutral conditions (pH 7.0) and at low concentrations (μg/mL), therefore it is essential to develop biochars which are effective at normal temperature and short residence time. Despite the limited price information and widespread utilization, scale-up technology of biochars is strongly recommended due to their engineering applicability, easy availability and techno-economic feasibility.

#### **5. Drawbacks of biochar**

Although there is a growing consensus on the benefits of biochar in various areas and at the same time different point of view exists. Several concerns have been raised on the sustainability and carbon neutrality in the utilization of biochar. Some of the challenges which limit the usage of biochar for scale-up production include (i) incompetence while supressing the emission of greenhouse gases (ii) effectiveness of biochar for all type of organic pollutants (iii) toxicity of biochar. For instance, production of biochar from different raw material may contain chlorinated organic compounds such as polyvinyl chloride or pentachlorophenol and may result in the formation of polychlorinated biphenyl-p-dioxins, PAH and furans. However, if there is a sudden increment in the level of such compounds in the biochar then it imposes threat to the environment and human health. Therefore, it is essential to suitably select the feedstock and synthesis conditions including temperature, residence time and technology that could control the concentrations of potentially toxic compounds

in the desired biochar. Safe usage of biochar materials ensuring human health and environment benefit along with comprehensive life cycle analysis and environmental risk assessment is recommended.

#### **6. Conclusion**

This chapter attempts to cover wide range of low-cost biochars for the effective removal of toxic contaminants from wastewater. These materials offer several advantages including technical feasibility and engineering applicability and serves as a boon for the environmental scientists and government authorities. The suitable selection of biochars not only minimizes the cost inefficiency but also improves profitability and adds promising benefits for the scale-up technologies in future. In addition, some biomass derived materials with and without prior pretreatment can be used as biochars in non-industrialized sectors. The purpose is to implement sustainable development policies at local and national levels. With few exceptions, it appears from the literature that biochars having good carbon content are usually versatile adsorbents that can be successfully used to remove contaminants from wastewater. Besides the technological progression, some limitations that still need to be overcome are (i) low surface area of biochar (ii) critical balance between pH and operating temperature during adsorption (iii) relationship between composition and constituents of the biochars is essential. Last but not the least, exploration of the possibility of recovering or reusing adsorbed substances needs attention.

#### **Acknowledgements**

The authors acknowledge Guru Gobind Singh Indraprastha University, New Delhi, India.

#### **Declaration of competing interests**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this chapter.

#### **Author details**

Uplabdhi Tyagi and Neeru Anand\* University School of Chemical Technology, Guru Gobind Singh Indraprastha University, India

\*Address all correspondence to: neeruanand@ipu.ac.in

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Sustainable and Eco-Friendly Biomass Derived Biochars for the Removal of Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.105534*

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### Section 3

## Biochar Uses in Energy Sector and Chemical Productions

#### **Chapter 9**

## Prospects of Biochar as a Renewable Resource for Electricity

*Ariharaputhiran Anitha and Nagarajan Ramila Devi*

#### **Abstract**

To face the change in energy paradigm, we need to devise technology that utilizes renewable resources and eventually realizes sustainability. Fuel cells generate electricity in a greener way, the efficiency and its cost-effectiveness depend mainly on the electrode material. Biochar serves as the promising electrode material, fuel, and separator membrane for fuel cells by being cheap, renewable, and possessing excellent electrochemical performance. The chapter is expected to provide a database of knowledge on how biochar with diversified physical and chemical features and functionalities can be effectively utilized for the possible application as electrode material for energy systems. The chapter appreciates the immense wealth of choice of biochar available with us for an important application in the area of energy as electrode material, fuel, and separator membrane for fuel cells.

**Keywords:** biochar, biomass carbon, fuel cells, electrode, separator

#### **1. Introduction**

The enormous usage of fossil fuels leads to harmful environmental damage, viz., global warming, depletion of energy resource, and lack of sustainable growth. To overcome this situation, now the world is in need of pollution-free green energy source. Moreover, the energy source should be available anytime anywhere in order to be a perennial source and to have a sustainable development. In regard of this, waste has to be used in a vast amount as an energy source instead of traditional fossil fuels which liberates huge amount of carbon dioxide, i.e., waste to wealth conversion. The energy derived from biomass termed biomass energy is the fourth largest energy source next to three fossil fuels, viz., coal, petroleum and natural gas. About 5% of the United States' primary energy need is fulfilled by biomass in 2021. As biomass is a carbon-neutral resource, the energy produced out of it is considered clean green energy.

It is noteworthy to know about the biomass and biochar. Biomass is the matter from biological organisms and biochar is the product obtained by the thermal/chemical processing of biomass. The sources of biomass include forest residue, agricultural crops and residues, domestic waste, municipal waste, marine waste, and industrial

waste [1]. Biomass though worthless, but it is a great source for valuable biochar. The biochar finds its multifarious applications such as:


The widespread utility of these electrochemical energy devices is hampered due to the high cost of the electrode materials. There arises revolution in the field of energy due to the utility of biochar as electrode material in electrochemical energy devices as it replaces the costlier electrode materials, thereby paves the way to the production of electrical energy at low cost. In addition to that, it gives value-added utilization of biomass in the field of energy.

A brief description of fuel cells is worthwhile here. Fuel cell converts the chemical energy of fuel and oxidizing agent into electricity by electrochemical redox reactions. It serves as an endless power source for space vehicles and submarines. The main components of fuel cells are the cathode, anode, and electrolyte which facilitate the passage of ions. Fuel cells are of many types, such as proton exchange membrane fuel cell (PEMFC), solid oxide fuel cell (SOFC), microbial fuel cell (MFC), and direct carbon fuel cell (DCFC).

In this chapter, detailed survey of utility of biochar as electrode material and separator in MFC and as fuel in DCFC is given in systematic way. This enables us to understand the value-added utilization of biomass in the field of energy and to explore many other biomass for its utility in the near future.

#### **2. Microbial fuel cell (MFC)**

Microbial fuel cell (MFC), a bioelectrochemical device, employs organic waste to produce electrical energy [10, 11]. In MFC, proton exchange membrane separates the anode and cathode. Electrodes used in MFC are crucial in determining its efficiency [12]. Electrogenic bacteria on the surface of the anode oxidizes the organic matter to generate electrons and protons [13, 14]. The electrons generated flow reaches the cathode to combine with an electron acceptor via an external circuit [15, 16]. Due to superior electrochemical oxidation capacity, great abundance, and clean reaction product, oxygen is the most widely used electron acceptor [17, 18]. However, the poor cathode oxygen reduction reaction and high oxygen mass transfer resistance significantly lower the performance of MFC [19]. The working mechanism of MFC is given as follows:

Acetate and glucose in organic compounds are oxidized in MFCs, which generate electrons, flow down to an external circuit, and produce electricity, whereas organic compounds are anaerobically oxidized and result in the evolution of protons, electrons, and CO2. In MFC, water is generated in cathode by the reduction of protons and electrons with the usage of oxygen supplied from outside. The protons and electrons thus liberated reach the cathode through an electrical circuit in presence of electrolyte medium. The formation of water in MFC is represented by the equation given as follows [20]:

$$2\text{CH}\_3\text{COO}^- + 4\text{ H}\_2\text{O} \rightarrow 2\text{HCO}\_3^- + 9\text{ H}^+ + 8\text{ e}^-. \tag{1}$$

$$\text{2O}\_2 + \text{8H} + + \text{8e}^- \rightarrow \text{4H}.\tag{2}$$

In MFC, oxidation occurs at the anode and reduction at the cathode, thereby creates the potential difference between the electrodes, leading to the generation of bioelectricity as shown in **Figure 1**.

Biochar can act as electrodes (anode/cathode), electrocatalysts, and proton exchange membranes b [21]. Biochar acting as electrodes should possess high porosity, rich carbon content, excellent electrical conductivity, large surface area, and costeffective. Moreover, it should be nonbiodegradable, biocompatible, and pave way to waste to wealth conversion. **Table 1** lists the biochar derived from various biomass, its method of preparation, processing temperature, its utility as electrode material for MFC, and the power density derived from it.

#### **2.1 Biochar as separator**

Proton exchange membrane (PEM) in MFC is superior in its performance when it possesses large proton conductivity, minimal oxygen, and substrate crossover, decreased biofouling rate, and low cost. As the source of biochar is abundant and easily available, the biochar possesses strong cation exchange properties, a high concentration of surface-active sites, and excellent porous nature supports its use in PEMs. In PEM fuel cell, the biochar acts as the unique separator which usually replaces the Nafion polymer membrane. Moreover, the biochar acts as a porous membrane having expanding sustainability than the other membrane and costeffective eco-friendly catalyst material. The biochar-built PEM fuel cell was applicable for the Industrial and lab scale preparation.

**Figure 1.** *Working of MFC.*


#### **Table 1.**

*Sources of biochar and its utility in MFC.*

#### **2.2 Biochar based catalyst**

The basic mechanism of oxygen reduction reaction (ORR) is the absorption of proton from the electrolyte by the oxygen molecule at the cathode. Followed by this, the transfer of electrons takes place from anode to metallic wire. Reaction requires more energy for the production of fuels. For the good performance of MFC, the anode and cathode play the catalyst role. For enhancing the sustainability, stability, and activity, the cathode fabrication is very important. The necessity of good cathodic material is to reduce the activation potential of ORR reaction and the cost-effective process. Initially, the fuel reactions were carried out using a platinum catalyst which serves as the catalyst for the reduction of oxygen and reduction. The economic preparation of the material was not affordable for the large-scale preparation as well as not suitable for the domestic purpose application. For the replacement of notable platinum catalyst, the non-transition metal, 2D material, carbon material, and porous


#### **Table 2.**

*Sources of biochar, its preparation, functions in MFC, and the power obtained.*

material are used for the fuel reaction. In recent research, the usage of biochar material from natural sources acts as the cathodic material for the ORR reduction and showed the good performance than the other catalyst materials. The mechanistic reactions are explained in various ways.

List of biochar derived from various biomass and its utility as separator, PEM, and catalyst for ORR are given in **Table 2**.

#### **3. Direct carbon fuel cell**

Global energy demand depends mainly on conventional sources such as coal, petroleum, and natural gas since earlier days. The excess use of coal as an energy source in the past is due to its low cost, abundance, and extensive distribution throughout the world [40]. As these sources are nonrenewable, its continuous usage leads to scarcity. Moreover, continuous usage of conventional sources leads to environmental damage, thereby exploration of clean energy source is the need of the hour. This put forward the steps to initiate energy generation from renewable sources like biomass. The direct carbon fuel cell (DCFC) employs carbon as anode operates on a high-temperature range of 700–900°C. It is superior over other fuel cells by attaining 80% efficiency (for power generation) [41, 42]. The carbon used as fuel in DCFC may be coal, biomass, and organic waste which are abundantly found in nature. DCFC transforms chemical energy trapped in the solid carbon fuel into electrical energy. DCFC accounts for green energy generation as it does not require any gasification processes and other conventional electric generators [43, 44]. The elemental carbon act as fuel contains high-energy density and is oxidized electrochemically at the electrodes [43, 44].

Based on the working electrolyte, there are three main categories of DCFCs, namely, molten hydroxide, molten carbonate, and solid oxide DCFCs. Molten hydroxide and molten carbonate DCFC utilizes NaOH/KOH and carbonates,

respectively, as their electrolyte. The electrolyte is filled in a metal vessel, which acts as the cathode. The carbon materials function both as fuel and the anode and are dipped into the electrolyte. Solid oxide DCFC resembles the other two DCFCs except using an oxygen ion (O2�) conducting ceramic electrolyte. The most commonly used electrolyte is Y2O3 stabilized zirconia due to high ionic conductivity, better stability, chemical and thermal compatibility, mechanical robustness, easy fabrication, and low cost [45–47]. Solid oxide DCFC though favorable for its simple design, but it has the limitation of low output [48]. This low output is explicitly due to the limited reaction zone at the carbon fuel and the electrolyte interface.

A DCFC consumes solid carbon and oxygen to produce electrical energy through electrochemical anodic and cathodic reactions [49, 50]. The overall reaction involved in DCFC is the simple combination of carbon and oxygen to form carbon dioxide. Electrooxidation of carbon occurs at the anode, whereas electroreduction of oxygen occurs at the cathode [40, 49, 50]. Working of different types of DCFC is shown in **Figure 2**.

**Reaction at the anode (oxidation of carbon).**

$$\text{C} + \left(\text{electrolyte}\right) \text{4OH}^- \rightarrow \text{CO}\_2 + 2\text{H}\_2\text{O} + 4\text{e}^-. \tag{3}$$

$$\text{C} + \text{(electrolyte)} \; 2\text{CO} \\ \text{3}^{2-} \rightarrow \text{3CO}\_2 + 4\text{e}^-. \tag{4}$$

**Overall anodic reaction.**

$$\text{C} + 2\text{O}^{2-} \rightarrow \text{CO}\_2 + 4\text{e}^-.\tag{5}$$

**Reaction at the cathode(reduction of oxygen).**

$$\text{O}\_2 + 2\text{H}\_2\text{O} + 4\text{e}^- \rightarrow 4\text{OH}^-.\tag{6}$$

$$\text{2CO}\_2 + \text{O}\_2 + 4\text{e}^- \rightarrow 2\text{CO}\_3\text{}^{2-}.\tag{7}$$

$$\text{O}\_2 + 4\text{e}^- \rightarrow 2\text{O}^{2-}.\tag{8}$$

If DCFC is supposed to operate at a high temperature (above 700°C), carbon electrooxidation is overlooked by the reverse Boudouard reaction (electrooxidation of CO). So direct electrooxidation of solid carbon is overlooked by direct electrooxidation of CO, termed as CO shuttling mechanism [51] which leads to low carbon fuel utilization [52].

$$\text{C} + \text{CO}\_2 \leftrightarrow \text{2CO}.\tag{9}$$

Being reverse Boudouard reaction is endothermic, decreasing the working temperature of DCFC will enhance the CO reduction in the anode exhaust, thereby increase carbon fuel usage [49, 53]. DCFC attracts the scholarly attraction owing to its low-maintenance cost, simple cell structure, and rich availability of carbon feedstocks, i.e., biomass. List of biochar derived from biomass which finds its utility in various types of DCFC is given in **Table 3**.

*Prospects of Biochar as a Renewable Resource for Electricity DOI: http://dx.doi.org/10.5772/intechopen.108161*

**Figure 2.**

*Working of different types of DCFC.*



#### **Table 3.**

*List of Biochar served as fuel in DCFC.*

#### **4. Challenges and future perspective**

To understand the mechanism behind the electrochemical reaction kinetics and carbon oxidation at the anode/electrolyte interface is still a challenge for us. In addition to that, metallic components in the biochar inhibit the electrochemical performance of the fuel cell. Determining the amount of ash accumulation is also a key factor and should be researched into as well to determine the lifetime of DCFC. Technological expertise in the cell design along with the clear understanding of kinetics will solve the issues in near future.

#### **5. Conclusion**

Biochar an inexhaustive renewable resource solves many environmental issues arised in recent decades, viz., pollution, remediation in soil, and water. Research studies in recent years advocate the multifarious utility of biochar as fuel in DCFC and as electrodes, separator, and catalyst for ORR in MFC. Moreover, biochar-based MFCs remove hazardous chemicals from wastewater, DCFC utilizes carbon from zero-cost sources as fuel along with the generation of electricity.

### **Author details**

Ariharaputhiran Anitha\* and Nagarajan Ramila Devi Department of Chemistry, V.V. Vanniaperumal College for Women, Virudhunagar, India

\*Address all correspondence to: anithaa@vvvcollege.org

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Prospects of Biochar as a Renewable Resource for Electricity DOI: http://dx.doi.org/10.5772/intechopen.108161*

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#### **Chapter 10**

## Biochar Synergistic New Ammonia Capture of CO2 and High-Value Utilization of Intermediate Products

*Yu Zhang, Yalong Zhang, Dongdong Feng, Jiabo Wu, Jianmin Gao, Qian Du and Yudong Huang*

#### **Abstract**

In the face of global warming and the urgent need for CO2 reduction, carbon capture, utilization, and storage, technology plays an important role. Based on the traditional liquid-phase and solid-phase CO2 capture technologies, the liquid-phase ammonia and biochar CO2 capture technologies are reviewed with emphasis. A multiphase carbon capture technology that uses biochar to enhance the mass transfercrystallization process of the new ammonia CO2 capture technology is proposed. High CO2 capture efficiency, limited ammonia escape, and low system energy consumption can be achieved through the orderly construction of three-dimensional graded pore channels and the directional functionalization of biochar. The intermediate products of CO2 captured by the ammonia process and the special agricultural waste rice husk components were considered. The use of rice husk-based biochar for CO2 capture by synergistic new ammonia method and the process regulation of intermediate products to prepare nano-silica to achieve high-value utilization of interstitial products of carbon capture. This technology may be important to promote the development of CO2 capture technology and CO2 reduction.

**Keywords:** CO2 capture, biochar, new ammonia, rice husk, nano-silica

#### **1. Introduction**

#### **1.1 Current status of CO2 emissions and CCUS technology**

Carbon is cycled between different sources (atmosphere, ocean, terrestrial biota, and marine biota) in the form of carbon dioxide, carbonates, and organic compounds. Human activities have disrupted the balance of this cycle, and a large amount of CO2 emissions has led to an increasingly serious greenhouse effect. Global climate change has caused widespread concern in the international community. According to the report of the International Energy Agency (IEA) [1], to achieve the target of global average temperature increase within 2°C above the pre-industrial level by 2100 and

to try to limit it to 1.5°C, direct CO2 emissions from industrial production need to be reduced by about 30%, and CO2 emissions per unit of GDP need to be reduced by about 60% by 2050 compared with the current level. However, as things stand today, global CO2 emissions from energy combustion and industrial processes will rebound in 2021 to the highest annual level ever recorded (**Figure 1(a)**). Emissions increased by 6% compared with 2020 (**Figure 1(b)**). The largest increase in CO2 emissions by sector in 2021 is from electricity and heat production, accounting for 46% of global emissions (**Figure 1(d)**). Coal accounts for more than 40% of the increase in total global CO2 emissions, a record high (**Figure 1(c)**). As the most important coal-consuming industry, coal-fired power plants are the most important source of CO2 emissions. Hence, the research on CO2 reduction technology for coal-fired power plants has profound significance.

Carbon Capture, Utilization, and Storage (CCUS) technology are considered the most economical and feasible way to reduce greenhouse gas emissions and mitigate global warming on a large scale in a short period. CCUS technology captures CO2 from large point sources such as power plants or directly from the atmosphere. The captured CO2 will be compressed and transported for various applications or injected into deep geological layers for permanent storage. As early as 2005, the Intergovernmental Panel on Climate Change (IPCC) identified CCUS as a key technology in mitigating the greenhouse effect [2]. Today, strengthened climate goals and new investment incentives have created unprecedented momentum for CCUS, and many countries have taken steps to develop CCUS technologies [3–7]. Projections indicate [8] that the least-cost pathway to "≤2°C" is to capture and sequester about 4 billion tons of CO2 per year by 2040 and that the current CO2 capture capacity is still

**Figure 1.**

*(a) CO2 emissions from energy combustion and industrial processes, 1900–2021, (b) annual change in CO2 emissions from energy combustion and industrial processes, 1900–2021, (c) change in CO2 emissions from fossil fuels, 2019–2021, relative to 2019 levels, (d) annual change in CO2 emissions by sector, 2020–2021 [1].*

far from the required amount, making CO2 capture technology critical in the overall carbon reduction and CCUS system.

#### **1.2 CO2 capture**

#### *1.2.1 CO2 capture technology*

There are four main CO2 capture technology routes: pre-combustion capture, oxygen-enriched combustion, post-combustion capture, and chemical loop combustion. In pre-combustion capture technology, fossil fuels are converted to a syngas of carbon dioxide and hydrogen before combustion using gasification or reforming technology so that the "carbon" in the fuel does not participate in the combustion process [9]. Oxyfuel combustion uses oxygen instead of air for combustion and can be used without considering the separation of nitrogen and carbon dioxide, a technically feasible process [10]. Post-combustion capture technologies remove CO2 from the flue gas after combustion has occurred. In recent years, chemical loop combustion (CLC) has also been developed. It uses metal oxides to transport the oxygen required for combustion to prevent direct contact between fuel and air, with its inherent CO2 capture capability [11]. Of the above capture technologies, post-combustion CO2 capture is the most mature and most thorough and is the preferred option for retrofitting existing power plants.

#### *1.2.2 Post-combustion CO2 capture*

Post-combustion CO2 capture technologies mainly include adsorption, absorption, membrane separation, and low-temperature distillation. Low-temperature distillation is a method of separation using the difference in boiling point or volatility of each component gas in the gas mixture. This method has high CO2 separation efficiency and purity and can directly produce liquid CO2 for storage and transportation [12]. The absorption method includes chemical absorption and physical absorption. Physical absorption involves using a physical solvent to dissolve a component gas. The solubility increases with increasing pressure and decreasing temperature; therefore, the optimal conditions for the CO2 absorption process are high pressure and low temperature [13]. The chemical absorption method uses an alkaline absorber to contact and react with CO2 in the flue gas to remove CO2. The salts generated by the reaction will decompose and release CO2 under certain conditions, thus removing and enriching CO2 from the flue gas [14]. The principle of membrane separation is that different components pass through the membrane with different selectivity. The membrane allows only specific gases to pass through, thus achieving separation and enrichment. The performance of the membrane system is influenced by the flue gas conditions [15], the enriched CO2 concentration is low, and the separation conditions are demanding. Adsorption can be divided into physical adsorption and chemisorption, with physical adsorption having a weak binding force, a relatively small heat of adsorption, and easy desorption. On the other hand, chemisorption is caused by chemical bonding between the adsorbent and the adsorbent, the adsorption is often irreversible, and the heat of adsorption is usually larger [16]. Adsorption differs from the absorption process in that the adsorption efficiency is mainly influenced by the specific surface area, selectivity, and regeneration characteristics. **Table 1** compares the above four post-combustion CO2 capture technologies, and all of these methods inevitably have various problems. Therefore, the development of new ammonia decarbonization technology will become the main


#### **Table 1.**

*Comparison of different CO2 capture technologies.*

theme of CO2 capture technology. However, its ammonia escape problem also needs to be further strengthened. The study of solid-phase adsorption combined with ammonia liquid-phase absorption to achieve two-phase synergistic CO2 capture will have farreaching significance in the future.

#### **2. Ammonia-based liquid-phase CO2 capture technology**

#### **2.1 Liquid-phase chemical absorption of CO2**

The commonly used absorbents for chemical absorption targeting CO2 capture are monoethanolamine (MEA), ammonia, and potassium carbonate. The CO2 capture efficiency of MEA is very high, but it has high regeneration energy, a high corrosion rate, and is susceptible to oxidative degradation. The high energy consumption of CO2 capture using aqueous amines is also one of the main drawbacks that limit its wide application. Non-aqueous absorbents have an absorption capacity comparable to aqueous MEAs and higher desorption efficiency, leading to a larger cycle capacity and nearly half the energy consumption (**Figure 2(a)**). Bougie et al. [27] investigated new non-aqueous MEA absorbers that greatly reduced energy consumption and improved CO2 absorption kinetics. The regeneration of MEAs is also a major challenge, with approximately 80% of the total energy consumption in the CO2 capture process occurring in the solvent regeneration process [28]. Many studies have shown that carbonate solutions can be used for CO2 uptake, and K2CO3 solutions have higher capture capacity than other carbonate solutions and are more commonly used in industry [29]. Although carbonate solutions have been extensively studied, the kinetics and thermodynamics of their absorption solutions still need to be investigated, and K2CO3 solvents may be subject to corrosion due to flue gas contaminants and solvent degradation.

#### **2.2 Absorption of CO2 by ammonia**

#### *2.2.1 Ammonia CO2 capture technology*

**Figure 2(c)** shows that reliable absorbents for low concentration CO2 capture without pressurization are amine-based and ammonia-based CO2 capture technologies. Ammonia-based CO2 capture is considered a viable carbon capture technology *Biochar Synergistic New Ammonia Capture of CO2 and High-Value Utilization of Intermediate… DOI: http://dx.doi.org/10.5772/intechopen.105405*

#### **Figure 2.**

*(a) Comparison of aqueous and non-aqueous MEA absorbents [23], (b) Schematic diagram of NH3-CO2-H2O three-phase system [24], (c) Applicable range of different CO2 capture technologies (based on operating pressure and CO2 concentration) [25], (d) Reaction mechanism of CO2 absorption by ammonia method [26].*

due to the high corrosiveness of MEA and regeneration problems over conventional amine-based CO2 capture technologies in terms of technical and economic advantages. The CO2-NH3-H2O system (**Figure 2(b)**) thermochemical properties have been reasonably well explained in recent studies. Although ammonia is the simplest amine, its interaction with CO2 is quite complex, involving gas-liquid-solid three-phase reactions, making the application of CO2-NH3-H2O systems in CO2 capture poses some challenges. Thomsen and Rasmussen [30] developed a thermodynamic model with a temperature. The model can be used not only for gas-liquid systems but also for gas-liquid-solid equilibria, including forming NH4HCO3, (NH4)2CO3-H2O, and NH2COONH4. Que and Chen [31] developed an electrolyte NRTL activity coefficient model that can well represent the thermodynamic properties of the NH3-CO2-H2O system when the CO2 loading reaches a consistent level. The availability of these models allows to reliably calculate the thermochemical properties of the CO2-NH3- H2O system under various conditions and to assess the energy performance of the capture process [32]. The uptake of CO2 by ammonia is a relatively slow process; therefore, it is important to understand the reaction mechanisms/kinetics involved in the uptake chemistry. The most important reaction in the presence of free ammonia is the reaction of NH3 with CO2, and the reaction scheme is shown in **Figure 2(d)**. The equilibrium constant of carbamate of MEA is much higher than that of ammonia, and the yield of ammonia-derived carbamate is lower than that of the equivalent monoethanolic ammonium carbamate, indicating that ammonia possesses a higher CO2 capture capacity.

#### *2.2.2 Ammonia escape*

Ammonia CO2 capture technology has many advantages, but it also has drawbacks in current applications: (1) low CO2 absorption rate; (2) serious ammonia escape; and (3) high energy consumption for desorption and regeneration. The high volatility of ammonia is the main drawback of ammonia CO2 capture technology. The concentration of NH3 escaping from the emission gas of this technology is usually above 10,000 ppm [33], which is much higher than the emission standard of 50 ppm. The high NH3 escape rate also decreases the concentration of NH3 in the solution, which reduces the CO2 absorption capacity [34]. Therefore, it is imperative to develop effective methods to suppress ammonia leakage or recover the leaked ammonia. The use of acid washing, membrane technology, and additives are common strategies to control ammonia escape.

#### *2.2.3 Ammonia-ethanol mixture absorber*

To better solve the above problems, many scholars have proposed the modification of CO2 absorption by ammonia solution using additives, which can inhibit not only NH3 escape but also improve CO2 absorption performance. Many scholars have studied the CO2 capture performance of ammonia with additives [35–41], among which Gao and Zhang et al. [39, 41] have shown significant advantages in various aspects of using ethanol as an additive. Ammonia and additives can, to some extent, promote each other to improve the CO2 uptake rate of ammonia [42]. However, a slight contradiction emerged between the hybrid absorber improving the absorption rate and inhibiting ammonia escape [43]. The additive mainly binds the free ammonia in the ammonia solution by hydrogen bonding and thus inhibits ammonia escape. However, the additive cannot achieve effective ammonia release when this hybrid absorber absorbs CO2, which will reduce the liquid-phase partial pressure of free ammonia and adversely affect the absorption process. The advantages of an "ammonia-ethanol adsorbent mixture" for CO2 absorption and capture are significant [41]. However, many aspects still need to be improved. It is urgent to develop a new ammonia carbon capture technology based on this idea to maintain its advantages and avoid its shortcomings.

#### **3. Biochar-new ammonia synergistic carbon capture**

#### **3.1 Solid-phase adsorption CO2 capture**

The adsorption of CO2 by porous carbon materials is an exothermic process, with the heat of adsorption of physical adsorption processes ranging from −25 to −40 kJ/ mol [44], and the amount of adsorption is directly related to the porous structure of the adsorbent and the active functional groups on the surface. The molecular kinetic diameter of CO2 is 0.33 nm, so micropores (<1 nm) are the main sites for CO2 adsorption (**Figure 3(a)**). Still, only micropores cannot achieve high adsorption capacity, and a suitable pore structure is required [45]. Macropores and mesopores act as channels for diffusive CO2 transport and can facilitate CO2 adsorption in micropores. CO2 being polar and acidic molecules, basic and polar functional groups (e.g., pyridine, pyrrole nitrogen) also plays an important role in adsorption [45]. Therefore, when selecting CO2 adsorbent, the economy and reliability should be satisfied. The adsorbent's pore structure and surface functional groups should be considered to ensure that the distribution of the two reaches a certain balance. Too much pursuit of one side will lead to the deterioration of the other side, resulting in a worse adsorption effect. (**Figure 3(b–e)**) [47].

Non-carbon-based solid adsorbents, mainly MOF and zeolite, are well studied and widely used. Almost all metals and a large amount of organic matter can make MOF, which is widely used in adsorption due to its extremely high porosity and specific

*Biochar Synergistic New Ammonia Capture of CO2 and High-Value Utilization of Intermediate… DOI: http://dx.doi.org/10.5772/intechopen.105405*

**Figure 3.**

*(a) Molecular dynamics and quantum chemical simulation of CO2 adsorption by porous carbon materials [45], (b) various structures in biochar micropores that effectively enhance CO2 adsorption [46], (c) effect of adsorbent porosity and chemical properties on CO2 adsorption performance [47], (d) correlation between CO2 adsorption, micropore, and mesopore volumes at 25°C and 5 bar [48], (e) correlation between different pore ratios and relative CO2 adsorption [45].*

surface area. When the partial pressure of CO2 is low (<0.2 bar), the adsorption capacity of MOF is poor [49], and impurity gases replace the skeletal ligands during the CO2 capture process, leading to degradation of MOF and a decrease in the capture capacity. Zeolites have a regular pore size of 0.5–1.2 nm [50] and have been widely investigated for CO2 capture due to the strong electrostatic interaction between CO2 and alkali metal cations in the zeolite skeleton [51]. Siriwardane et al. [52] showed that natural zeolites with high sodium content exhibited high CO2 adsorption capacity. However, the electrostatic interaction between CO2 and alkali metal cations in the zeolite skeleton is reduced by water [53], and therefore only in dry gas streams is CO2 separation effective. Among the carbon-based materials, activated carbon is one of the most commonly used adsorbents in industry. It is less costly than other adsorbents [54], but its adsorption capacity is only comparable to that of zeolites at high CO2 pressure [55], and the heat of adsorption is lower than that of zeolites. By introducing impurity atoms or acid-base sites, activated carbon can appropriately improve adsorption selectivity and adsorption capacity. As a new carbon-based material, carbon nanotubes have also received attention in gas adsorption [56, 57].

#### **3.2 Biochar adsorbent**

The raw materials of biochar are widely sourced, and the cost is lower than other adsorbents. The biochar prepared from different raw materials is different due to their intrinsic elemental composition ratio and structure. Biochar prepared from raw materials with high strength and carbon content, such as wood chips, coconut shells, date kernels, and rice husks, has a more desirable CO2 adsorption capacity [58]. During preparation, the carbonization temperature affects the structure, surface functional groups, and elemental composition of the final material and 500–800°C is considered

**Figure 4.**

*(a) Dynamic molecular structure of biochar derived from plant biomass [60], (b) Infrared spectra of biochar after heat treatment at different temperatures and comparison of XPS spectra of raw biochar and biochar after heat treatment at 300°C [61], (c) SEM images of different stages of biochar preparation [62], (d) Mechanism of biochar pore classification and group functionalization [2].*

the optimal temperature range for carbonization [59]. Thermal degradation of biomass at high temperatures in limited or complete anoxia is central to biomass conversion into porous carbon. Most biomass consists of lignin, cellulose, and hemicellulose, prepared under different pyrolysis and activation conditions to obtain different pore structures, group ratios, and surface chemistry (**Figure 4(a** and **b)**).

#### **3.3 Biochar modification**

The adsorption of CO2 by biochar is highly dependent on the pore structure and surface physicochemical properties, and the optimal pore size is about twice the kinetic diameter of CO2 molecules. However, the CO2 adsorption capacity of directly carbonized biochar is low. The authors' previous studies [45] have been conducted to enhance the CO2 adsorption capacity of biochar by sequential construction of pore channels and surface functionalization modification through activation (physical and chemical activation). Molecular dynamics simulations were also performed by clearly modeling the hierarchical pore channels to explain the experimental phenomena from a microscopic perspective. The mechanism is shown in **Figure 4(d)**. On the other hand, the carbonization temperature plays a key role in controlling activated porous biochar's functional groups and specific surface area. Therefore, the reasonable selection of the amount of activator and carbonization temperature becomes a necessary part of preparing activated porous biochar materials.

#### *3.3.1 Biochar pore hierarchy construction*

According to the International Union of Pure and Applied Chemistry (IUPAC) standards, biochar is classified as macroporous, mesoporous, and microporous.

*Biochar Synergistic New Ammonia Capture of CO2 and High-Value Utilization of Intermediate… DOI: http://dx.doi.org/10.5772/intechopen.105405*

**Figure 5.** *Relationship between specific surface area and pore volume of biochar and CO2 adsorption capacity [64–74].*

Usually, the pore size of macroporous exceeds 50 nm, mesoporous is 2–50 nm, and microporous is less than 2 nm. For the CO2 adsorption process, macropores and mesopores help diffusive transport of CO2 molecules, while micropores provide adsorption sites as direct storage sites for CO2. Therefore, a reasonable construction of graded pores can effectively enhance the CO2 capture performance of biochar. Lingyu et al. [46] prepared biochar from seven types of straw and wood biomass to study their CO2 adsorption performance and found that wood biochar has better pore structure than straw biochar with 2.73–4.40 times larger specific surface area, and biochar with super pore structure has higher CO2 adsorption capacity. Capacity was higher, and good pore structure played a crucial role in the CO2 adsorption. Avanthi et al. [63] prepared biochar using pine sawdust and steam activated it at the same temperature for 45 min after completion of pyrolysis. Due to the high surface area and microporosity, pine sawdust biochar showed significantly higher CO2 adsorption capacity than paper mill sludge biochar, which may be due to the Steam activation increased the microporosity, surface area, and oxygen-containing basic functional groups. In this paper, we summarized the literature that studied the CO2 adsorption capacity of biochar with different pore structures in recent years, and the relationship between their specific surface area, pore-volume, and biochar CO2 capture capacity is shown in **Figure 5**.

#### *3.3.2 Biochar functionalization construction*

The adsorption of CO2 on the biochar surface is influenced by the chemical properties of the biochar surface. Many studies have shown that the introduction of basic nitrogen functional groups can increase the alkaline sites on biochar and enhance the adsorption of acidic CO2 [75]. Nitrogen-containing functional groups are the main contributors to the alkalinity of the biochar surface, and activation in different nitrogen-containing reagents was performed to introduce nitrogen-containing functional groups to the biochar surface. The commonly used activation reagents are

KOH, NaOH, CO2, and K2CO3. Activation of biochar with KOH or NaOH can dissolve compounds such as ash, lignin, and cellulose, thus increasing the O content and surface alkalinity of biochar. Some new activation reagents such as NaNH2, CH2COOK, and H2SO4 have been gradually investigated. He et al. [76] prepared activated carbon by KOH activation using rice husk as raw material and modified biochar with chitosan as a nitrogen source. They found that the modified AC exhibited better CO2 adsorption performance in comparison. Yang et al. [77] prepared N doped porous carbon, and the CO2 adsorption capacity could reach 6.33 mmol/g at 273.15 K and 100 kPa, which was significantly higher than most of the carbon-based adsorbents reported in the literature due to the introduction of nitrogen-containing functional groups that increased the CO2 adsorption sites. In addition, unlike the acid-base interactions between CO2 and biochar surfaces, it has been shown that the presence of oxygen-containing acidic functional groups such as hydroxyl and carboxyl groups also promotes hydrogen bonding between CO2 molecules and carbon surfaces, thus increasing CO2 adsorption on carbon-containing surfaces [78]. Ma et al. [79] synthesized a series of carbon materials with different functional group contents. The experimental results showed that introducing oxygen functional groups into the carbon framework can again improve CO2 capture efficiency in N-doped porous carbon. According to the theoretical calculations (**Figure 6**), the carbon framework with high oxygen content further enhanced the hydrogen bonding and electrostatic interaction for CO2 adsorption. Wu et al. [80] prepared biochar from corn kernels by KOH activation, and the samples possessed a very high number of oxygen functional groups (45.5%) and exhibited a large CO2 adsorption capacity. The presence of alkali and alkaline earth metal (AAEM) elements such as Na, K, Ca, and Mg can also promote the formation of basic sites, which have a strong affinity for CO2 with acidic properties [81]. Therefore, the presence of biochar's AAEM elements may enhance the CO2 adsorption capacity of biochar, and the introduction of alkaline metal sites in the biochar skeleton may also enhance the CO2 adsorption of biochar in the order of Mg > Al > Fe > Ni > Ca > Raw biochar > Na [75].

### **3.4 "Biochar-new ammonia" CO2 capture system**

The excellent CO2 adsorption performance and low regeneration energy consumption of biochar are closely related to its well-developed specific surface area,

#### **Figure 6.**

*(a) modification of rice husk-based biochar and CO2 adsorption capacity [76], (b) preparation of N-doped porous carbon from chitosan and NaNH2 for CO2 adsorption [77], (c) hydrogen bonding between CO2 and functionalized biochar surface [79], (d) adsorption energy of different functional groups of biochar for CO2 [45].*

#### *Biochar Synergistic New Ammonia Capture of CO2 and High-Value Utilization of Intermediate… DOI: http://dx.doi.org/10.5772/intechopen.105405*

three-dimensional through-gradient pore structure, and unique oxygen/nitrogen surface chemistry. Dagaonkar et al. [82] estimated the effective diffusion coefficient of CO2 within biochar to be 9.645 × 10−7 m2 s−1. Suppose biochar particles are used as a modified material to enhance the mass transfer properties of the liquid phase. In that case, their stronger CO2 diffusion properties can be fully utilized to improve the overall reaction rate of the carbon capture system. Biochar has also been used to adsorb ammonia nitrogen, and more than half of the mass of ammonia nitrogen adsorbed was completed within 2 h [83]. In view of this, biochar can be applied in ammonia water CO2 absorption systems to achieve effective inhibition of ammonia escape through the sequestration of free ammonia by its active surface groups.

By combining the respective development potentials of biochar adsorbent and ammonia-ethanol absorber, biochar adsorbent was cross-linked with ammoniaethanol absorber to realize the functionalized cross-linking of the biochar-enhanced new ammonia carbon capture mass transfer-crystallization process (**Figure 7(a)**).

**Figure 7.**

*(a) Functionalized cross-linking of biochar-enhanced novel ammonia-based carbon capture mass transfercrystallization processes, (b) Enhanced mass transfer mechanism of biochar in ammonia-ethanol mixed absorbent [2].*

This system transforms the carbon capture process from the traditional ammonia carbon capture gas-liquid two-phase reaction to a gas-liquid-solid three-phase process. The system can achieve CO2 adsorption and enrichment in micropores, CO2 diffusion and transport in mesopores/macropores, and dynamic sequestration of free ammonia by regulating the hierarchical structure of biochar nanopores and the orderly grouping of active functional groups on the surface. The cross-scale multiphase system processes, such as functionalization, pore gradation, biochar/NH4HCO3 dissolution and crystallization, and adsorption/absorption coupling, are cross-linked by crystal regeneration instead of liquid-rich regeneration. The synergistic effect of "graded adsorption—efficient absorption—dissolution crystallization—crystal regeneration" in the system is accomplished. The multiple goals of ammonia carbon capture, such as increasing absorption rate, suppressing ammonia escape, and reducing system energy consumption, are achieved. This process can improve a series of shortcomings of ammonia CO2 capture and overcome the shortcomings of biochar adsorbents. The synergy of the CO2 capture process in the solid-liquid system of "biocharammonia-ethanol" is achieved by "taking the advantages of each and avoiding the shortcomings."

#### *3.4.1 Biochar efficiency transfer*

The absorption of CO2 by ammonia is a typical non-homogeneous reaction process in which CO2 in the gas phase is first dissolved in the absorption solution and then reacts with NH3 in the liquid phase. Therefore, the absorption rate is controlled by the "chemical reaction in the liquid phase" and the "mass transfer characteristics between gas and liquid." The generation and hydrolysis of carbamate in the reaction process is the most important factor affecting the chemical reaction rate, roughly divided by the carbonation degree of ammonia absorption CO2 solution ≈0.5, as shown in **Figure 7(a)**. The liquid membrane mainly controls the mass transfer resistance of ammonia absorption CO2 reaction process, when the hydrolysis of ammonium carbamate mainly controls the carbonation degree >0.5, ammonia absorption CO2, so that the liquid phase carbon capture rate is significantly reduced, and this process has been the bottleneck to improve the absorption rate in the later stage of the reaction in the traditional process. This process has been the bottleneck to improving the absorption rate in the later stage of the reaction in the traditional process. The key to reducing the liquid film mass transfer resistance in the process of CO2 adsorption by ammonia and improving the low CO2 absorption rate is to get rid of the influence of carbonation degree on the regeneration energy consumption and to control the CO2 absorption reaction by ammonia only in the rapid generation phase of ammonium carbamate with carbonation degree <0.5. The mechanism of mass transfer characteristics of the new ammonia carbon capture process with biochar efficiency enhancement is shown in **Figure 7(b)**. Using the highly efficient adsorption performance of biochar hierarchical pore channels, the initial rapid CO2 sequestration is completed, and the biochar is used as a carrier to bring CO2 into the ammonia absorption system. Subsequently, the transfer of CO2 from solid particles' adsorption space to the ammonia liquid phase's absorption space is further realized. The release of CO2 from biochar and the absorption of CO2 by ammonia is completed, which greatly increases the contact time between CO2 and ammonia liquid phase, thus realizing the reduction of liquid film resistance and prolonging the residence time of CO2 in the solid-liquid phase system to increase the material transfer and chemical absorption rate in the ammonia liquid phase system. The liquid-liquid phase ammonia system can be used to increase the rate of material transfer and chemical absorption.

*Biochar Synergistic New Ammonia Capture of CO2 and High-Value Utilization of Intermediate… DOI: http://dx.doi.org/10.5772/intechopen.105405*

#### *3.4.2 Limiting ammonia escape*

The ammonia escape process is shown in **Figure 8**. Among many parameters affecting ammonia escape, the temperature is one of the most sensitive [84]. From the ammonia escape point of view, the absorption temperature should be as low as possible, requiring a large amount of energy to maintain cold ammonia. For the solid-liquid two-phase CO2 capture system, the pore surface functional groups in the solution permeable region of biochar/macropore can undergo cation exchange with NH4+ in solution [85], which promotes the reverse migration of the hydrolyzing process of ammonia monohydrate. At the same time, the free ammonia in the liquid phase was held by the van der Waals force and chemical hybrid force [86] so that the production of free ammonia in the liquid phase could be effectively controlled. Therefore, the hierarchical functionalized construction of biochar pore structure ensures the hierarchical adsorption of CO2/NH3 by biochar particle pore, improves the material transfer and chemical absorption rate in the ammonia liquid phase system, and makes the dynamic balance of NH3 adsorption and fixation in unsaturated solution impregnation space present in biochar pore. To a great extent, the effective concentration of free ammonia that can participate in the reaction in the liquid phase system is ensured, the dynamic partial pressure of free ammonia in the liquid phase is maintained, and the ammonia escape is limited.

#### *3.4.3 Dissolution crystallization instead of rich liquid regeneration*

The traditional ammonia-rich liquid thermal regeneration process is the largest energy-consuming part of the whole ammonia carbon capture process. The regeneration energy consumption is mainly composed of three parts: the sensible heat of rich liquid warming, the latent heat of vaporization, and the heat absorption of

**Figure 8.** *Ammonia escape mechanism.*

regeneration reaction, of which 50–70% of the energy is consumed in the warming and vaporization of rich liquid solvent [87]. The solubility of the product of the reaction process of CO2 absorption by ammonia is known: ammonium carbamate is soluble in water and ethanol; ammonium bicarbonate is soluble in water-insoluble in ethanol. The main mechanism of solvation crystallization is to use the different chemical structures of the main solvent molecules and solvating agent molecules to make a difference in the microscopic forces between the ions of the substances to be separated and to change the macroscopic properties of the mixed solvent by changing the microscopic forces of the particles in the solution, thus greatly reducing the solubility of the solute, and using the solubility difference as the driving force to make the solute continuously precipitate out of the liquid phase in the form of crystals, so that the solvent and the solute are separated. The solvent and solute are separated. In the "biochar-ammonia-ethanol" carbon capture system, the crystallization process is strengthened by the solvation and precipitation method, and the regeneration of crystals replaces the regeneration of carbon-rich liquid, which greatly reduces the energy consumption of regeneration. The biochar functionalized Meso-/macropore pores ensure the NH3/NH4+ concentration in the liquid phase. The pores' active surface structure provides nucleation sites for the crystallization process, which accelerates the formation and growth of carbonated liquid solvation crystals in the liquid phase system. The dynamic balance between the crystallization process's residence time and the biochar's saturation time for efficient adsorption can provide a stable CO2 adsorption-absorption-crystallization series process.

#### **3.5 High-value utilization of carbon capture products**

The main crystallization product of the novel ammonia CO2 capture technology described above is ammonium bicarbonate, which is widely used in agriculture, food, pharmacy, and ecological management, but its utilization process's complexity and economics have prevented its use widespread development. Therefore, further optimization of the new ammonia CO2 capture technology and high-value utilization of the intermediate product ammonium bicarbonate have also become key issues. Rice husk is widely available, and its internal structure has a lignocellulose-SiO2 crossover network, and the SiO2 in it can be dissolved to construct pore channels of a specific size. With this unique structural advantage, rice husk is the best raw material for preparing graded porous carbon [88]. The particle size of SiO2 in rice husk is mainly concentrated in the range of 8–22 nm, with a small fraction of SiO2 in the range of 1–7 nm [89], indicating that SiO2 in rice husk can be used as a natural template to induce mesopore generation in situ after solubilization. The chemical activation of agricultural waste rice husk as a raw material enables the orderly construction of high-quality rice husk-based biochar with a hierarchical pore structure and high specific surface area. Combined with the new ammonia carbon capture technology, the rice husk-based biochar-ammonia-ethanol system was constructed, and the nano-silica carbon black was produced by the acid-base neutralization and redecomposition reaction between NH4HCO3, an intermediate product of the new ammonia carbon capture, and silicate, an intermediate product of the rice husk-based biochar (**Figure 9**). This route greatly solved the problems of carbon capture product consumption and agricultural waste pollution and produced high-value products of rice husk-based biochar carbon and nano-silica at the same time.

*Biochar Synergistic New Ammonia Capture of CO2 and High-Value Utilization of Intermediate… DOI: http://dx.doi.org/10.5772/intechopen.105405*

#### **Figure 9.**

*Technology roadmap for high-value utilization of process products from rice husk-based biochar-new ammoniabased carbon capture technology.*

#### **4. Summary and outlook**

CO2 capture is a crucial part of CCUS technology, and absorption and adsorption have been widely studied as the main means of CO2 capture. The mainstream CO2 liquid-phase chemical absorption method is difficult to avoid high regeneration energy consumption, degradation problems, and high corrosiveness. In contrast, the "ammonia-ethanol" system effectively avoids these problems but still has serious ammonia escape problems and crystallization control difficulties, and the technology needs to be improved. Biochar has excellent CO2 adsorption performance due to its specific surface area, three-dimensional through-gradient pore structure, and unique oxygen/nitrogen surface chemistry. However, it still has many problems, such as poor CO2 selectivity, limited adsorption capacity, high cost, and short service life. Combining the above-mentioned new ammonia carbon capture technology, the carbon capture process is transformed from the traditional ammonia carbon capture gas-liquid two-phase reaction to a gas-liquid-solid three-phase process, which maximizes the efficiency of CO2 capture by graded adsorption of biochar and efficient absorption of ammonia-ethanol solution:

1.Enhancement of mass transfer between solid and liquid phases to improve the carbon capture rate;


In order to realize the high-value utilization of intermediate products, we propose a system of rice husk-based biochar—new ammonia method—process product resource synthesis—process regulation, which provides new ideas and directions for the CO2 capture industry.

### **Author details**

Yu Zhang, Yalong Zhang, Dongdong Feng\*, Jiabo Wu, Jianmin Gao, Qian Du and Yudong Huang School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, China

\*Address all correspondence to: 08031175@163.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Biochar Synergistic New Ammonia Capture of CO2 and High-Value Utilization of Intermediate… DOI: http://dx.doi.org/10.5772/intechopen.105405*

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#### **Chapter 11**

## Microwaved Flux Matter- Char Sand Production of Waste Coal Char/Biochar/Gypsium Ash and Fly Ash Mixtures for Mortar- Fire Retardent Composite

*Yıldırım İsmail Tosun*

#### **Abstract**

Fire inhibiting materials as cement filler are used in mortar constructions especially using gypsium board, similar isolator mortars. The mortar covered char and ash sand mixtures insulate heat and reduce fire blazing activity. Ytong, or porous briquettes and clay is the world's most popular insulating construction material retarding blaze due to its porous durability, processability, and cost. However, producing concrete or mortar with high isolation with HD styrene panels is insulating the structure, protecting the cement board against flammable fire risk. Slag-type masonry requires high heavier fire inhibiting matter in construction. Styrene type isolation provides fire inhibiting at lightweight masonry or mortar generation with the use of waste gypsium fines and waste coal slimes and high ash char "char sands" and ash fines. The growing environmental concerns motivated researchers to search for char waste slag-type inhibiting materials using gypsium fines and biomass waste char fines leading to alternative routes of fire-retardant mortar construction. In this way, several alternative materials of isolation mortar have prompted.

**Keywords:** microwave, fire retardent, composite mortar, waste fire retardant, plaster, analysis of gradations, porous structure, light weight retardent, heat absorbance, composite plaster durability

#### **1. Introduction**

Molten plastic extruded belts or strips may easily be produced through the nozzle hole of pressed waste plastic fluids by microwave radiation till 300°C for recycling waste materials as granule compost [1, 2]. The use of waste concrete debris and broken glass or plastic slags cause an important cost decrease in masonry stone production [3, 4]. Even the use of waste materials as aggregate and sand size make them beneficial in concrete mixture evaluation in most light weight constructions [5, 6]. The melted plastics and bitumen asphalt may be replaced by cement in masonry brick and roof tile production as binder compound while providing impermeable and high resistive durability to thaw and freezing in cold climates. Plastic extrusion may need suitable fluidization quality and antifouling powders use such as clay at a certain amount. Presently, around 70% of the construction is produced through the conventional slag-type masonry as inhibiting masonry constructions [3, 4].

In the region, the municipal bottom ash wastes of asphaltite combustion in boilers as wastes, containing the high porous content. 70–80% content of bottom ash is over 25 mm size suitable as lightweight aggregate discarded and collected. The villagers for heating house collect agricultural oak tree and bush waste, municipal waste and agricultural, manure waste products such as forest waste at 21% of total waste [5, 6]. The biomass waste collected in the region is combusted and bottom ash mixed with asphaltite bottom ash at the density of 0.7 kg/l is about 450 thousand tons for wet production [7–9]. The wood char fine in Siirt and Hakkari is evaluated for fire inhibitor. The waste plastics are collected as sludge waste and shredded wet and converted plastic pellet noodles. The plastic noddle products and belts both should be evaluated by pyrolysis oil content below 350°C and the other slag plastics is becoming hard porous slag such as, fine matters gradation of aggregate subjected to mixing and melted asphalt briquetting of the sludge waste and subsequently briquetted products for concrete compost aggregate below 25-50 mm [10–12].

In this study; the effect on the physical parameters of briquetting, shear model patterns making preliminary tests to determine the briquetting and processing conditions, indention and sawing shear rate were investigated for rock and waste plastic or asphalt compost aggregate concrete in comparison with cemented aggregate [13]. This assay has been determined to be advantageous in the plastic and asphalt bound aggregate briquette production from sludge content solution with the waste plastic and their mixture rate with porous local stone [14–16].

#### **1.1 Carbon source-biomass potential of Turkey**

In the cement and retardant material consumption, use of waste materials as a carbon source from agricultural biomass waste and forest biomass waste depending on crop production in the market and waste straw used for various purposes, such as other waste cotton stalks, corn stalks, sunflower stalks, nut leafs are evaluated in production of retardend carbon source at finer sizes as filler material. The total amount of different wastes are given in the **Table 1** [14]. The total waste field crops in Turkey and waste quantities are given in **Table 2** [17, 18].

Biomass wastes are evaluated in char carbon production as active carbon or fireretardant carbon even in the low-quality high ash containing matter as waste source. The biochar carbon resources may be produced from country oil resources, or crop oil, oily wastes as composted sources as given in **Table 2** in Turkey [17].

The asphaltite coal type is widely deposited in the Şırnak province with high amount of shale content. The shale ash content is illustrated in **Figure 1**. The combustion is retardent act over 45% ash content leaving about 20% unburned carbon in the ash [19, 20]. The ash content change of Şırnak asphaltite coal and char used as fire-retardant in this study in terms of density is illustrated in **Figure 1**.

#### **1.2 Gradation**

Aggregate size distribution is changing by ASTM standards of soil classification over the foundation stability research in detailed [14] is given with Sieve analysis

*Microwaved Flux Matter- Char Sand Production of Waste Coal Char/Biochar/Gypsium Ash… DOI: http://dx.doi.org/10.5772/intechopen.101559*


#### **Table 1.**

*The total amount of municipal waste divided into actual values in Turkey and eastern Anatolian region in 2019 [17].*


#### **Table 2.**

*The total annual production of biomass waste in Şırnak and eastern Anatolian region [17].*

**Figure 2.** *The soil -aggregate classification in ASTM standard [14].*

results as shown in **Figure 2**. The permeability of soil is also determined regarding the chart illustrated in Standard as **Figure 3**. Physical properties of the clay material [21, 22] Parameter Value Color Dark Brown Specific Weight 2.69 Sand Content (%) 17.33 Silt Content (%) 6.22 Clay Content (%) 76.44 Liquid Limit (%) 43.9 Plastic Limit (%) 21.8 Ground Class (USCS) CL 10%, 30%, 50% liquid limit values of the waste Şırnak asphaltite slime and clay material mixed into the clay sample were calculated. The liquid limit value of the clay sample containing 10% waste slime clay corresponding to the sinking of 20 mm was determined as 28%. The textural and strength properties of the Şırnak shale clay showed that the water absorption of the texture is high and the chlorite mineral is suitable for volume changes. Due to this structure of the clay, the plasticity and strength of the material changes and the swelling and shrinkage activities of the clay can lead to different behaviors and cause structural problems. For this reason, it is of great importance to perform volumetric shrinkage tests of asphaltite slime or ash slime.

#### **1.3 Fire retardent slags**

The recycling needs of waste plastics in housing in cities forced to energy use and construction use of polymer wastes and many other filler areas such as fly ash composted ornaments and masonry areas are increasing. The large-scale reconstruction projects offer the use of demolished buildings concrete, transportation of those debris materials and crushing and compacted with water and cement cause a high amount of cement and water even increase cost elements. The dams, factories and the construction sector, which aims to protect the stability of concretre structures, gradually need much cheap aggregate production. To meet the cost reductions of all masonry and mortar construction, waste materials are evaluated similarly to masonry bricks regarding strength and durability [23, 24]. Although, the aggregate materials obtained from the quarries can be widely used in the construction

*Microwaved Flux Matter- Char Sand Production of Waste Coal Char/Biochar/Gypsium Ash… DOI: http://dx.doi.org/10.5772/intechopen.101559*

**Figure 3.** *The soil permeability regarding void in soil classification in ASTM standard [13].*

**Figure 4.**

*(a) extruding ball die, (b) plastic waste and asphaltite slag and slime asphaltite char mixing briquetting [22].*

industry. Lightweight materials are waiting in high house constructions as surplus stock. Crusher residue fine-grained materials are scattered around by 10–15% at depending on the crusher type. As a result, the waste plastic nodules or belts may compost as slag waste sand, fine material that remain in the dust collectors. Therefore, the general standard provisions stated in the construction materials fire resistivity for plastic contents not over 30% volume regarding bitumen asphalt or other masonry mixing fines [25–28]. In the mortar tests, the mixed waste briquettes are aimed to prevent fire reaction that occurs as a result in the blazing fire contact, degradation of stability of residents. The importance of fire inhibiting or control practices in the evaluation, the regulation was emphasized on that way. Disposal of plastic waste heaps is in the form of shredded waste and can be used by extruding.

Many plastic waste recycling articles dealing with many issues such as such are included in the literature [29–32]. As an industrial raw material, lightweight volcanic cinder instead of broken glass is used as the main raw material and additive material in lightweight brick sectors depending on the masonry use. The aggregates obtained by crushing, sieving and sizing according to the sector in which porous stone will be used are evaluated in appropriate gradation sizes according to the geotechnical strength purpose in briquetted brick use. However, the fine-grained material remaining under the sieve during the sizing process is awaiting stock surplus in the local dumps' areas. Şırnak asphaltite bottom ash slag with high porosity is in search of new areas of use to utilize the organic soils they obtain as waste other than dumping activities. In this respect, the light weight mixture with the recycled waste plastic product that occurs in retort furnace is searched detailed for lightweight briquette production without causing environmental pollution. In addition, shale fine in soil environments, which are quite commonly layered in certain regions of Şırnak is used for gradation mixing encountered for high strength. In this study, it was aimed to examine the behavior of these two different types of materials by mixing them in variable ratios because of the optimum gradation amount of these materials in the region and the specific characteristics of briquetted materials without cement are evaluated. It is stated that pumice is a suitable additive for the stabilization of high plasticity clay. It is emphasized the usability of plastic waste materials in improving the engineering properties of briquette with shale powder and porous limestone added to briquette blocks cemented in certain proportions. It was also determined that in the asphalt-based mixtures prepared by using fly ash and limestone fine in the improvement of the fine-grained ground sample, the limestone aggregate decreased the shrear strength by 35% and the volcanic cinder increased by 22% [33–36]. Indentation and shear properties of the briquetted materials by plastic waste melted and asphalt melted to be used in the experiment were carried out in the Şırnak University.

#### **1.4 Fire retardant chemical materials**

Fire retardent salts such as the construction materials used in the environments we live, the building materials are non-combustible and are produced from salt hydrates, chlorides as chemical materials. Since slag chars or melted/foam salts ignite and shine more quickly than natural materials, a possible fire spreads quickly. The heat from the flame source destroys the oxygen in the environment very quickly and starts to pose life risks in 90 seconds. It is not possible to prevent a fire in a closed environment after 3 minutes without external intervention. In a fire, blazing hazard and toxic gases hazard, chemical gas hazards, explosion and sustainable fire hazard, structure collapse occur. Again, in a possible fire, the temperature rises to 550°C in the first 5 minutes and to 720°C after half an hour. The temperature can reach 950°C degrees after 90 minutes and 1100°C degrees after 3 hours. In some large fires, it is claimed that a temperature of 1500–1700°C occurs [35]. From dripping bricks in buildings fire, which is an exothermic chemical reaction, continuously generates heat and enlarges and spreads the adjacent materials in a chain way by reaching their ignition temperature. In order to eliminate the devastating effects of risk factors in an indoor fire prompt the use of fire retardent construction materials that slow down and stop the progression and spread of fire; Various substances were Flame Retarder. The commonly used salt materials are

*Microwaved Flux Matter- Char Sand Production of Waste Coal Char/Biochar/Gypsium Ash… DOI: http://dx.doi.org/10.5772/intechopen.101559*


Regarded the fires encountered the retardant material need have revealed the importance. The necessity of fire-proof materials in the world, especially the wood industry, cable (plastic) industry, and the textile industry have started to produce fire-resistant materials [36–38].

Fire retardant coating and mortars are needed for insulater's coverings and construction boards and plates. The gypsium is providing a good retardant protection however the strength and heat insulation change the strength stability of boards as given in **Table 3** [38].

#### **1.5 Şırnak fly ash, waste ash slag and Şırnak asphaltite char with ash materials**

The geological petrographic, geochemical and physical properties of the fine-grained slag and cinder material are found in the local quarry in Tatvan and Şırnak region. The volcanic cinder such as pumice stone, iso foam stone, ash slag stone, are two types of porous texture and contain at least 70–80% porous formation as a result of basic volcanic gaseous activities. The Tatvan basic volcanic cinder is similar to acidic pumice, which is the most widely found and used in the world, has a white dirty appearance and a grayish-white color. The silica ratio is higher in acidic pumices, and it can be widely used in the construction industry [12–15]. A volcanic cinder is a browny reddish porous, glassy volcanic rock that is formed as a result of volcanic gaseous eruptions highly sponge and resistant to chemical reactions at high abrasion strength. It contains pores from macro to micro scale due to the sudden release of the gases in the body during its formation and its sudden cooling. Volcanic cinders have high permeability and high heat and sound insulation. Its hardness is 5–6 according to the Mohs scale. In Eastern Anatolia, severe volcanic events have occurred in very wide areas since the Middle Miocene. Tectonic activity is covering wide areas near Van Lake as volcanic craters lake,


#### **Table 3.**

*Fire retardent chemical materials classification.*

craters heel, disseminated tuff covers and tuff debris lava remnants carried by waterfloods. It has been active starting from the Mid Miocene period until the end of the Quaternary [36]. Tatvan unit consists of volcanic cinders with 78–83% porous cinders as block flows, debris of flow tuffs, and andesitic, basaltic and rhyolitic lavas [37]. Pumices are light browny macroscopically and dark gray colored in certain places. It has a vesicular texture formed by the cavities left by the gases that expand as a result of sudden pressure decrease under atmospheric conditions. The gray acidic cinder contains coarse plagioclase minerals showing feldspat, biotite minerals and chlorite minerals as accessories are observed in the rock [38–44].

#### **2. Method**

The method of compaction for retardant wet material at 15% optimum fluid weight rate as water muddy content pushed to nozzles of the extruder for board plaque production as illustrated in **Figure 4a**. The aggregate mixing the retort mixer is used in laboratory-scale a meter and 30 cm diameter retort used in 10 minutes for mortar homogenized wetting at optimum retardant compositions as showed in flowsheet procedure followed in **Figure 4b**.

In the fire-retardant mixture, preparation used volcanic cinder prepared as slag based on the main element iron, manganese oxide and trace elements given in **Table 4**, the analysis results of the waste Şırnak Asphaltite bottom ash slag material of ultrabasic magma (**Table 5**).

#### **2.1 Porous char slag asphalt sand production**


Material is located in Şırnak Province, Southeastern Anatolia Region, are located in the south part of Tatvan and chlorite shale formations limestone formations

#### **Table 4.**

*Composition of the waste cinder and Şırnak bottom ash slag material.*

*Microwaved Flux Matter- Char Sand Production of Waste Coal Char/Biochar/Gypsium Ash… DOI: http://dx.doi.org/10.5772/intechopen.101559*


#### **Table 5.**

*Physical properties of waste volcanic cinder.*

contained quartz, feldspar, calcite, dolomite and limonite, hematite minerals and asphaltite slag is red color due to the hematite mineral in its composition at 17%. It can be found in light yellow colors depending on the ratio of limonite in the gray shale ground [21–23]. The porous limestones shale texture, marl shows a heterogeneous texture (**Figure 2**).

#### **2.2 Particle size distribution- gradation**

M mass of aggregate is, the void is affected by compaction of briquetting and binder distribution. Especially melted asphalt and ash distribution are controlled by volume % of compaction. The bulk sand eating by microwave will also be controlled by the amount of little as 1% binder ash bound as a volume.

where, γ*g* = \_density of aggregate, g/cm3; *V(r)* and *dN(r)* are the volume and particle amount of aggregate in the size region of integration of cumulative pile from *r, to r* + *dr*), respectively. *V*e volumetric equation is calculated as,

$$d\mathbf{M}r = \eta \mathbf{g} \text{ V}(\mathbf{r}) d\mathbf{N}(\mathbf{r}) \tag{1}$$

$$V(\mathbf{r}) = k \, r^3 \tag{2}$$

where, *k* is the shape factor. Substitute Eqs. (1) and (2) using.

#### *2.2.1 Aggregate particle size distribution*

Particle size distribution is defined by aggregate crushing matter, the type of milling affects the size distribution and the fineness matter ranged below 20 microns determined as given the Eq. (3) below; and RRS logarithmic size distribution is defined as given I Eq. (4) below [24].

$$\mathbf{u}(\mathbf{x}, \mathbf{d}\_{\mathbf{f}}, \mathbf{c}) = \left(\chi/\mathbf{d}\_{\mathbf{f}}\right) \left(\mathbf{1} + \mathbf{k}/\mathbf{d}\_{\mathbf{f}} \cdot (\mathbf{x} - \chi)\right)^{-1/\mathbf{k}} \tag{3}$$

$$R\mathbf{s}(n) = f(n)W(n)\sum\_{m=1}^{n} \mathbf{x}'/(\mathbf{x} - r)^{m} \tag{4}$$

**Figure 5.** *The Şırnak Fly ash and Şırnak asphaltite slime particle size distribution in gradation in ASTM standard.*

The weight of fineness below 100 microns is determined by hydrolic settling analysis. The rate of material used in the experimentation is illustrated in **Figure 5**. The d60 values of the particle distribution of Şırnak Fly ash, Şırnak char slime and waste slime are below 100-micron fine size.

#### **2.3 Fire tests**

Flame gas brulor is blazed on the thick 10 mm board and the resistance to fracture and bubbling on a 5 minutes time flash burning at a distance far from 10 mm. The depth of disturbed face of board in the fire resistivity test is determined as an opened hole or as weight rate of burning natter weight. The time of burning of fire contact according to ASTM D-635 was investigated over the extent of depth measured by extensometer of mortar boards reported if the specimen does not burn on the board of 10 mm thickness. An average burning depth rate was also determined.

*Microwaved Flux Matter- Char Sand Production of Waste Coal Char/Biochar/Gypsium Ash… DOI: http://dx.doi.org/10.5772/intechopen.101559*

#### **3. Results and discussion**

#### **3.1 Gradation of retardent mixture-** *asphalt ash/char amount and briquette porosity*

#### *3.1.1 Particle İndex*

The coarse particle distribution avoids the heat conduction so that fineness of particle size distribution in the construction gradation provides optimum fire retardent heat activity on the surface without breaking the mortar face.

$$I\mathfrak{a} = \mathfrak{1}, \mathfrak{2}\mathfrak{F}V\_{10} - \mathfrak{0}, \mathfrak{2}\mathfrak{F}V\_{\mathfrak{5}0} - \mathfrak{3}\mathfrak{2} \tag{5}$$

where Ia is particle index, V10 is voided in aggregate compacted at 10 drops per layer, V50 is voided in aggregate compacted at 50 drops per layer. Especially fly ash content in the retardent mixture was decreasing compaction ability. The amount of reaching 30% fly ash addition reduced the permeability of texture compacted in the mortar briquettes at 27% volume rate decrease.

The sand matters are thought as rounded and smooth particles as an ideal form. This may have a low particle index of around 6 or 7, while silty sands composed of angular, rough particles may have a high particle index of between 15 and 20 or more.

#### *3.1.2 Fineness modulus*

The fire retardent mortar sands may contain optimum gradation with very fine clays or fly ash on standard content description as happening in ASTM C 125 with a gradation curve as illustrated in **Figure 2**. In this study, the fly ash fineness is determined by the RRS diagram and n distribution coefficient as in Eq. (6) below and illustrated in **Figure 6** for the samples studied as fire retardent.

$$\mathbf{F}(\mathbf{d}; \mathbf{t}; i) = \sum\_{i=0}^{n} \mathbf{u}(\mathbf{x}, \mathbf{t}) + \phi(\mathbf{x}; \mathbf{t}; i).e^{-\mathrm{int}} \tag{6}$$

#### **3.2 Compressive strength test**

It is based on the determination of the compressive strength from the indenting of the briquette sample in seconds as drilling bit penetration on rock sample [21]. Then, the indenting depth is determined using the extensometer dipping measure by the pattern is obtained for rock samples used in Şırnak. The fire retardent additives show stable porosity and strength suitable for mortar mixture while cement is locking the fixed coverage over wood in the fire flame tests (**Figures 6** and **7**).

#### *3.2.1 Mortarcompost - porous texture strength*

The massive mortar mixture of rock sands show different porous structures and strengths. The compaction indentation depth for porous rock stones and fire retardent materials are depended on particle size and fines amount as given below Equations;

$$Elasticity(\mathbf{0}) = -qf \sum\_{m=1}^{M} Cm(\mathbf{1} + Xr)^{m} \tag{7}$$

#### **Figure 6.**

*The fineness of Şırnak fly ash and Şırnak asphaltite slime and char slime regarding gradation, RRS distribution factor of 0.45.*

**Figure 7.**

*The Şırnak Fly ash and Şırnak asphaltite slime compost compaction regarding gradation factor below 100 microns solid.*

$$E\operatorname{deformation}(\mathbf{0}) = f \sum\_{m=1}^{M} \operatorname{Cm}(\mathbf{1} + Xr)^{m} \tag{8}$$

After this process, the sinking amount of the cone was determined from the electronic measuring stick on the device. Some samples taken from the submerged *Microwaved Flux Matter- Char Sand Production of Waste Coal Char/Biochar/Gypsium Ash… DOI: http://dx.doi.org/10.5772/intechopen.101559*

part of the cone were dried in the oven and the water content corresponding to the determined sinking was found.

Samples with volume 10%, 20%, 30% and 40% waste phospahate salt and char/ salt flux composts blazed on the depth-averaged from three different points for fire retardent manner. The sample used is thick at 5.425 mm for 10 mm wood. The advantage of this experiment is that it minimizes the errors of the candle fire flame over 50 mm experiment according to the standard gradation.

Considering inferences, extreme deformations can be observed under fire load on wood-covered fire retardent mortar that is saturated with a dried binder depending on firing time. Due to these negative weight effects, various chemical burning weight changes on the wood are required for the unflammable ground environment to reduce fire weight decrease, reduce cracking and prevent the negative consequences of bubbling melting in the ground structure. The plastic slag and char regulate the air mixing and permeability on the wood substrates where it is criticized in **Figure 8**, while Şırnak asphaltite char addition reduces air diffusion and reduces heat conduction to wood (**Figure 9**).

The optimum mixing fineness content of the ash sample containing 3% plastic was determined as 30.5% and the maximum dry unit volume weight was 17.52 kN /m<sup>3</sup> . The dry unit weight graph of the plastic slag sample containing 50% waste plastic, the optimum mixing binder content of 25.25% and the maximum dry unit weight of 16.13 kN /m<sup>3</sup> for Şırnak asphaltite char.

Accordingly, the results revealed that the asphalt mixing values decrease with the increase of the ratio of plastic slag chars because plastic char slag is a binding material. As a result of the indentation experiments, the optimum Optimum mixing fineness content as below 5-micron content and dry unit weight reduction in fire tests as given in **Figure 10**.

**Figure 8.** *The char/ ash and phosphate salt slime with retardation to board depth.*

**Figure 9.** *The Şırnak fire-retardant mortar sand types regarding strength vs. porosity change.*

*Microwaved Flux Matter- Char Sand Production of Waste Coal Char/Biochar/Gypsium Ash… DOI: http://dx.doi.org/10.5772/intechopen.101559*

#### **4. Conclusions**

All materials undergo deformation when the load is applied. It is predicted that soils are also compact without shear deformation together with the decrease in volume under stress. However, this decreases in the volume of the soil mass, the compression of the plastergrains, the type of voids, the structure and its continuity reveal different types of behavior depending on the way and duration of removal of mixing light weight plaster and air in the cavities. In this context, the study emphasizes the importance of positively improving engineering properties such as compaction and fire inhibiting mortar mixing, char, plastic slag binder content by using different types of materials together.

Salt content over 20% with char and fly ash fire retardent sands the depth of deterioration decreased 200%.

As the amount of Şırnak asphaltite char and plastic slag in the briquette sample increases fineness weight rate with fire weight rate reduction rate, the optimum mixing binder content increases and the maximum dry unit volume weight decreases. This behavior is an expected situation by adding certain proportions of aggregate high- sand rate mixtures because of the plastic unit weight value of the used sand to low ash. The unit weight value of briquette with cinder decreased the bulk density of the mixture. The utility of briquette as lightweight concrete takes attention to the low cost of the material. In this text, the gradation of the much finer potential of fly ash is reducing density with char being critical in fire retardation for oxygen uptake as construction materials.

#### **Author details**

Yıldırım İsmail Tosun Engineering Faculty, Mining Engineering Department, Şirnak University, Şirnak, Turkey

\*Address all correspondence to: yildirimismailtosun@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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**Chapter 12**

## Biofuel and Biorefinery Technologies

*Abdulkareem Ghassan Alsultan, Nurul Asikin-Mijan, Laith Kareem Obeas, Aminul Isalam, Nasar Mansir, Maadh Fawzi Nassar, Siti Zulaika Razali, Robiah Yunus and Yun Hin Taufiq-Yap*

#### **Abstract**

The global demand for energy is expected to rise up to 59% by the year 2035. This is due to the increasing technology developments and contemporary industrialization. Continues trends of these simultaneously will affects the crude fossil oil reserves progressively. Therefore, biofuels that are predominantly produced from the biomass based feedstocks such as plant, algae material and animal waste. Liquid or gaseous biofuels are the most simple to ship, deliver, and burn since they are easier to transport, deliver, and burn cleanly. The key contributor to the elevated green house gaseous concentration is carbon dioxide (CO2). Two-thirds of global anthropogenic CO2 emissions are due to fossil fuel combustion, with the remaining third attributed to land-use changes. Interestingly, recent literature has announced that the utilization of liquid biofuels capable of reducing the CO and CO2 emissions. Other positive impacts of the liquid biofuels are; (1) reduce the external energy dependence, (2) promote the regional engineering, (3) increase the Research & Development activities, (4) reduce the environmental effects of electricity generation and transformation, (5) improve the quality of services for rural residents and (6) provide job opportunities.

**Keywords:** catalysis, bioenergy, biofuel, hydrogen energy, green fuel

#### **1. Introduction to biofuel**

The global demand for energy is expected to rise up to 59% by the year 2035 [1–7]. This is due to the increasing technology developments and contemporary industrialization. Continues trends of these simultaneously will affects the crude fossil oil reserves progressively. Therefore, biofuels that are predominantly produced from the biomass based feedstocks such as plant, algae material and animal waste [2, 3]. Liquid or gaseous biofuels are the most simple to ship, deliver, and burn since they are easier to transport, deliver, and burn cleanly [4]. The key contributor to the elevated green house gaseous concentration is carbon dioxide (CO2). Two-thirds of global anthropogenic CO2 emissions are due to fossil fuel combustion, with the remaining

third attributed to land-use changes. Interestingly, recent literature has announced that the utilization of liquid biofuels capable of reducing the CO and CO2 emissions [5, 6]. Other positive impacts of the liquid biofuels are; (1) reduce the external energy dependence, (2) promote the regional engineering, (3) increase the Research & Development activities, (4) reduce the environmental effects of electricity generation and transformation, (5) improve the quality of services for rural residents and (6) provide job opportunities [7].

#### **1.1 Type of biofuels**

The oxygen content is the most important difference between biofuels and petroleum based fuels [8]. The biofuels produced from different renewable resources are typically non-toxic, accessible and abundant. Biogas, syngas, biobuthanol, bioethanol, biodiesel, bio-ether, and green fuel are various forms of biofuels. Gaseous biofuels are commonly used for heat and energy production purposes.

Biogas, is a gas fuel that burns much like fossil fuels, and for this reason, it gradually gains its position. Biogas consists mainly of methane gas, although it is produced from the degradation of anaerobic biomass. Many agricultural businesses use biogas, and the fuel is currently being packaged for domestic use in gas cylinders. The fuel is extracted from a combination of flora and fauna since each provides a specific ingredient (animals and plants). Plants have significant carbon and hydrogen in them, while they have nitrogen in them for animals. The components above are necessary and required for the production of biofuels.

Liquid biofuels, on the other hand, are widely used in the automotive industry. Biogas, obtained by anaerobic fermentation from organic materials, has a 40–70% CH4 composition, 30–60% CO2 and other gases such as H2S, H2, N2 and CO. Biobutanol is capable of replacing both petrol and diesel. Using a bacterium to ferment biomass appears to be the most promising approach for processing biobutanol at the moment. The acetone-butanol-ethanol process is the name of the method. Acetone (propanone), butanol and ethanol are produced here. Clostridium species such as *Clostridium beijerinckii, Clostridium acetobutylicum*, *Clostridium saccharoperbutylacetonicum* and *Clostridium saccharobutylicum,* are used for the fermentation process of acetone-butanol-ethanol to produce biobased butanol [9]. Algae, sugar beet, sugar cane, maize, sorghum and cassava are the feedstocks used successfully so far. To make pure butanol, the materials are fractionally distilled. Bioethanol is a transparent liquid that is biodegradable, non-toxic, and environmentally safe. It's chemically known as ethyl alcohol, and it's made from the plant's fermentable sugars (such as glucose, sucrose, and other sugars) through microorganisms. It is possible to mix bioethanol with gasoline as well. Fermentation of bioethanol is a biological process in which microorganisms convert sugars to produce bioethanol and CO2. Yeasts are the most widely used microorganism in the fermentation process and Saccharomyces cerevisiae is the preferred option for bioethanol fermentation among yeasts [10]. For example, fermentation bacteria used are *Clostridium acetobutylicum, Lactobacillus fermentum* etc. The basic schematic fermentation process is as follow (**Figure 1a**).

Green fuels (green-diesel and green-gasoline) are an oxygen-free hydrocarbon comprised of short chain and long chain carbon fractions within a range of C8–C12 and C13–C20, respectively. The green fuels also free from sulfur and aromatic compounds. They contain *n*-alkanes and *n*-alkenes, which similar to those found in the petroleum-based gasoline and diesel. As the green fuel is entirely compatible as the petroleum-derived fuels, their fuel properties are vastly similar to each other.

*Biofuel and Biorefinery Technologies DOI: http://dx.doi.org/10.5772/intechopen.104984*

**Figure 1.**

*(a) Fermentation process, (b) Transesterifcation reaction of triacylglycerol using methanol and (c) Transesterification of triacylglycerol using ethanol.*

However, green fuel is completely different in chemical structure as compared to the well-established commercialized biodiesel [11–13]. To produce green diesel, biological oil feedstocks such as algae, vegetable or plant oils, and animal fats are thermally catalytically hydrocracked. Hydrocracking is a refinery process that uses elevated temperatures and pressure to break down these larger molecules (natural oils) into a shorter mixture of hydrocarbon chains in the presence of strong chemical heterogeneous catalysts. Green diesel also known as renewable diesel [14]. One of the most well-established liquid biofuels is biodiesel or fatty acid methyl ester (FAME), not only because of its lower environmental effect, but also because it provides the benefits of being renewable, biodegradable and non-toxic. By transesterification reaction with alcohol, biodiesel will be fabricated from either vegetable oils or animal fats using required catalyst [15]. It can be used as a diesel engine fuel in its pure form (B100) or in mixtures (B10, B15), but it is typically used as a diesel additive to reduce the levels of diesel-powered particulates, CO and hydrocarbons. The following is a diagram of the transesterification process (**Figure 1b**). Biodiesel fuel used in car and lorry engines typically comprised of higher boiling FAME fraction and its alkanes contain 14–22 carbon atoms [16]. Biodiesel also can be a mixture between their FAME and fatty acid ethyl ester (FAEE) and FAEE is prepared as following reaction (**Figure 1c**).

**Table 1** depicts the most recent global developments on modern transportation fuels in 2019, published by NS Energy [17]. There are three liquid biofuels – bioethanol, biogasoline and biodiesel account for the vast majority of global biofuel production and use today. The United States is on the first world rank of biofuel


#### **Table 1.**

*Top countries for biofuel production across the globe in 2019 [17].*

manufacturer followed by Brazil. In Europe, Germany is the largest producer. Argentina produced almost similar production amount with the Germany. As been expected, China is the leading country for biofuel production in Asia. The majority of biofuels were produced from soybean, sugarcane, rapeseed, corn and waste cooking oil (WCO). In the case of China, bioethanol and ethanol-blended gasoline are primary products. In the United States and Brazil, soybean oil is widely used. Many European countries, primarily Germany, use rapeseed oil for biofuel production. Note that Germany also utilized the WCO as a biofuel feedstock. It should be noted that the WCO derived biofuels initiative is also used in many nations, including Australia, China, Italy, Portugal, the United Kingdom, the United States, Austria and Spain. Brazil also produced WCO-based biodiesel, but it only accounted for 0.5% of total biodiesel production. In Brazil, only 2.5% of the WCO produced in Brazil is estimated to be reused for biofuel production, while the rest is improperly discarded [18]. Sugarcane is widely used in Brazil and Argentina. Brazil is the global leader in producing bioethanol from sugarcane.

Overall, all liquid biofuels are made mainly from agricultural commodities, such as grain and sugar (bioethanol) and vegetable oil, based on the above results (biodiesel, bio-gasoline). It can be observed that each nation concentrated on distinct feedstock. Lowest oil price, high oil content, required fatty acid composition (saturated or unsaturated acid), low cultivation maintenance and expense, controllable growth and harvesting season, consistent seed maturity rates, and potential demand for agricultural by-products are all desirable characteristics when choosing the best biofuel feedstock [19].

#### **1.2 Biofuel uses**

There are applications of biofuel other than an alternative to diesel fuel. Most claim that the material is used for transportation only. But hydrogen, cleaning oil, cooking oil, and more can be provided by biofuel. As an alternative to substitute energy needs from vehicle fuel to core home heating, biofuels can work.

Here are the top ten biofuel applications.

#### *1.2.1 Transportation services*

In the United States, about 30% of the energy consumed is used for moving cars. Transport accounts for 24% of electricity and more than 60% of the absorbed oil worldwide. This means that more than a third of the oil is used for vehicle operations.

The key issue with alternatives is that it is not feasible for transport to use solar, wind, and other renewable energies. Experts think that successful breakthroughs in developments in practical technology are still decades away.

In short, biofuel can be converted into steam of hydrogen that is intended to be used in the fuel cell adjacent to it. More important automotive brands have already invested in biodiesel vehicle stations.

#### *1.2.2 Generating electricity*

Fuel cells provide a power-generating application that is used for electricity and providing fuel for transport. In backup systems where pollutants matter the most, biofuels could be used to generate electricity. This involves facilities located in suburban areas, such as schools, hospitals, and other styles. In reality, the greatest biofuel market in the United Kingdom will turn over 350,000 homes from landfill gas into power generation.

#### *1.2.3 Provide heat*

Over the last few years, bioheat has developed. The heat coming from hydraulic fracking would contribute to natural gas development as the primary use of natural gas that comes from fossil fuels. Although there is no need for natural gas to come from fossil materials, it can also derive from newly grown materials.

There is a large amount of biofuel that is used for heating. Since wood is the most practical heating process, houses that use wood-burning stoves instead of gas or electricity are used. A biodiesel blend would reduce the production of both nitrogen and sulfur dioxide.

#### *1.2.4 Electronics charging*

According to scientists from Saint Luis University, a fuel cell was built with cooking oil and sugar to produce electricity; customers would be able to use these cells instead of generating electricity. Instead of batteries, customers will be able to use fuel cells to charge everything from laptops to mobile phones. Cells have the ability to become a ready source of power when they are still in the process of growth.

#### *1.2.5 Spills and grease from clean oil*

Biofuel is considered to be environmentally friendly and can also help clean up oil and grease spills. For areas where crude oil polluted the waters, it was checked to act as a possible cleaning agent.

It has also been found that the results improve the areas of recovery and allow it to be extracted from the water. Biofuel can also be used for metal cleaning as an industrial solvent, which is also useful because of its lack of harmful effects.

#### *1.2.6 Cooking*

Although the most common ingredient to be used for stoves and non-wick lanterns is kerosene, biodiesel works equally well.

#### *1.2.7 As a lubricant*

In order to decrease the Sulphur concentration, diesel fuel is needed as Sulphur offers the most fuel lubricity. When it comes to maintaining the engine running correctly and preventing infection's premature failure, this is critical.

#### *1.2.8 Remove paint and adhesive*

Biofuels can replace toxic materials in order to eliminate paint and adhesives. The best approach for eliminating non-critical applications is often known to be biofuel.

#### *1.2.9 Create energy when fossil fuel runs out*

As the supply of oil is beginning to run out. This has led us to ask how, without damaging the ecosystem, fuel can be extracted. Biofuel would assist the government in forming a sustainable, cost-effective method of generating energy.

#### *1.2.10 Reduce cost and need for imported oil*

In the United States, over 84% of the world's petroleum is used. The U.S. has recently begun to reduce the need since 2006, despite the rise in fuel requirements. This makes it possible for biofuels to become the strongest emission reduction factor.

Analysts claim that when oil is disrupted, substituting imported oil with biofuel would help to balance the Economy. It does not matter how much the Americans Spend on oil imports, but how to balance the overall Economy (Top 10 Uses for Biofuel, 2016).

#### **1.3 Biofuel feedstock**

Biomass feedstocks for energy production can be produced from plants directly grown for energy use or parts of plants, waste, residues, and materials extracted from humans and animals' activities. In 2005, the U.S. Department of Energy evaluated these feedstocks' results and found that it was possible to sustainably harvest and deliver more than 1 billion tons of agricultural and forestry-related biofuels to biorefineries. Feedstocks may be defined by plant or residue types, the energy products they make, or any other way. The following categories of feedstocks will be used for discussion purposes.

#### *1.3.1 Sugar and starch crops*

Many of the sugar and starch crops that are contenders for biofuel production are already being used for agricultural and food grains or sweetener sugars. Root and tuber starches are usually used across the globe as food staples. Via conventional fermentation methods, these crops and their particular products can easily be transferred to ethanol and related alcohols for transport and other uses.

#### *Biofuel and Biorefinery Technologies DOI: http://dx.doi.org/10.5772/intechopen.104984*

Competition for resources and the need for genetic, development, and manufacturing modifications to increase energy production sustainably would be the unique challenges facing most of these crops. Some examples are:


#### *1.3.2 Fibre and grass cellulosic crops*

Many of the grass and related crops that have been cultivated for decades as pasture and grazing for feeding livestock or for soil conservation can be used as an energy resource. In general, these crops are higher in fiber (cellulose, hemicellulose, lignin) and poorer in carbohydrates, proteins, and oils. A variety of methods may turn these crops into energy, including direct heat and/or power combustion, cellulosic conversion into ethanol, thermochemical processing for fuel supplements, or anaerobic methane digestion. Some examples are:


#### *1.3.3 Oil crops*

While several crops generate at least a small amount of vegetable oil, 15–50% of oil is provided by various crops. By grinding the seed and squeezing the oil out, oil can be extracted. To produce biodiesel, the oil is transesterified. Oil crops may also be transformed as alternatives to fossil fuel materials into high-value biochemicals and biomaterials, thus reducing the use of fossil fuels in turn. Some examples are:


#### *1.3.4 Crop residues, manures, and organic wastes*

Critical biomass residues remain after corn, sugar, starch, or oil plants are harvested for feed and food components. Abundant crop residues that can be transformed into renewable fuels are corn stover, corn cobs, wheat, and small grain straw. Sustainable maintenance of the agricultural production system is a crucial obstacle when removing crop residues. To increase soil organic matter quality as well as soil and water conservation objectives, crop residues are usually incorporated into the soil. In order to evaluate the influence of stover removal on the sustainability of crop production, a great deal of research is performed to study the effects on ecosystem services and the diversity of insects, vertebrates, and microbes. Some examples are:


Manures are a result of livestock's digestion of plants. Anaerobic digestion techniques have been used for years to transform these and other organic waste to methane and related gases in addition to their usual land application for nutrient content. In exchange, methane can be used explicitly for combustion heat, fueling diesel generators, or supplementing natural gas with further processing and cleaning.

In metropolitan areas, food production and industrial waste, including restaurant grease, leaves, grass cuttings, and other garden waste, are contained in large quantities and can be processed and converted to electricity through a number of methods.

#### *1.3.5 Wood products*

Trees and their associated products were used as a direct source of ignition and combustion for heating and cooking for decades. The thermal efficiency of these wood products, when dry, is about two-thirds that of coal and about 10% greater than that of deciduous plant biomass. As the fuel source for gasification and cellulosic conversion to ethanol, wood and its derivatives have also been used. Although it is generally possible to use any wood supply, different research projects have been ongoing to create so-called "energy forests" or "wood energy farms." Woody Crops [20]. Some examples are:


#### **1.4 Advantages of biofuels**

Biofuels offer a wide range of benefits.

#### *1.4.1 Renewable energy sources*

Globally, there is strong energy demand. Nonetheless, most power sources are non-renewable, lead to the greenhouse effect, or, as is the case with nuclear energy, may lead to major ecological problems. Biofuels, which are renewable fuel sources and environmentally friendly, are derived from plant and animal manure.

The majority of fossil fuels will expire and one day wind up in flames. Since most sources, such as manure, maize, switchgrass, soybeans, crop, and plant waste, are renewable and are not likely to be running out any time soon, the use of biofuels in nature is effective. These crops can also be replanted again and again, as well.

#### *1.4.2 Sovereignty*

Unlike fossil fuels, whose deposits aren't really found in all countries, any country can undertake biofuels' development without interference with all other countries' energy sources. By impacting or determining the world's fuel prices and petroleum-based goods, countries with fossil fuel reserves have always taken full advantage of their economic resources. If a nation can manufacture its own biofuel, it can easily set its own prices for goods without many regional and global constraints.

Although local crops have decreased the nation's reliance on fossil fuels, many experts agree that addressing our energy needs will take a very long time. We need more renewable energy options to reduce our dependence on fossil fuels as crude oil prices are hitting sky high**.**

#### *1.4.3 Ensure economy's sustainability*

The sustainable quality of biofuels has contributed to states worldwide adopting them and supporting a decrease in fossil fuel use. Instead of high-cost imports of fossil fuels from Middle Eastern countries, policymakers should reduce this reliance and instead fund biofuel plants that are cheaper in the long term.

Locally generated biofuels can minimize reliance on other fuels and thus increase the security of energy and economic prosperity. Fewer imports imply more exports and, therefore, greater self-dependence.

#### *1.4.4 Low expenses*

The majority of biofuels are easy to manufacture and cheaper than fossil fuels. Therefore, their use will make life easier for ordinary citizens and help boost people's living standards by reducing the increasing cost of living globally due to reliance on fossil fuels. As of now, as gasoline does, biofuels cost the same as the market. However, the net cost-benefit of using them is much more significant. They are safer fuels, which implies that they generate lower burning pollutants. They also have the ability to become cheaper in the future with the growing demands for biofuels.

#### *1.4.5 Clean fuel*

A lot of carbon is emitted by fossil fuels, which results in large levels of air pollution. This carbon also mixes with other greenhouse gases, such as methane, which contributes to unfavorable weather conditions. On the other hand, since they are clean fuels, biofuels do not release this amount of carbon into the environment.

#### *1.4.6 Efficient fuel*

Biofuel is made from renewable resources and, compared to fossil diesel, is relatively less combustible. It has considerably stronger hydrating characteristics. Compared to standard diesel, this produces less toxic carbon emissions. It is possible to manufacture biofuels from an extensive range of materials. The net cost-benefit of someone using them is considerably greater.

#### *1.4.7 Extensive durability of vehicles' engine*

In most conditions, biofuels are able to adapt to existing engine designs and perform very well. It has higher levels of cetane and more robust lubricating properties. The longevity of the engine improves when biodiesel is being used as a flammable fuel.

Engine conversion is not required. This allows the engine to run for longer, needs less maintenance, and reduces the cost of pollution control overall. Engines intended to run on biofuels generate fewer emissions than other diesel engines.

#### *1.4.8 Less smoke generation*

Automobiles and factories using fossil fuels such as petroleum and diesel commonly create a lot of atmospheric smoke. As biodiesels have oxygen atoms in their chemical structure, they burn better and contain less carbon deposits. Biodiesels emit less smoke as a byproduct and are more environmentally friendly.

#### *1.4.9 Minimize monopoly*

Fossil fuels are more likely to be favored by biofuels due to their widespread use. This has created a monopoly over the years, contributing to price increases and the ever-increasing standard of living. Since biofuels are equivalent replacements for fossil fuels, they can be used to help minimize the fossil-fuel monopoly.

Biogas could be used in the same way as fossil fuels, for instance. Consequently, people have the option of converting to Biogas when natural gas prices go up. And vehicles can opt for ethanol or butanol when fossil diesel rates increase.

#### *1.4.10 Less toxic*

As a result of combustion, all forms of fuels, fossil fuels, and biofuels form carbon compounds. In the atmosphere, fossil fuels emit toxic carbon dioxide, especially in the presence of water vapor and methane gas. On the other side, biofuels' carbon occurs in nature and is used for photosynthesis by plants, serving as an energy source for plants.

#### *1.4.11 Employment source for locals*

Most of the bio plants are geographically set up, and human capital is required in the process, such as construction engineers, farmers, project managers, fuel distributors, and logisticians. This helps to generate new work opportunities for locals.

#### *1.4.12 Lower levels of pollution*

Using fossil fuels such as coal, sulfur, and lead can be produced along with acid rain. Unlike biofuels, sulfur is not contained in biofuels. Biofuels are renewable resources that emit fewer emissions into the environment. Nevertheless, this is only one reason why biofuels are promoted.

They emit lower levels of various contaminants, such as carbon dioxide than conventional diesel. It plays a part in lowering air pollution. Furthermore, biofuels are biodegradable, reducing the risk of soil degradation during transport, storage, or use.

#### *Biofuel and Biorefinery Technologies DOI: http://dx.doi.org/10.5772/intechopen.104984*

Social and environmental studies reveal that biofuels minimize greenhouse gas emissions by up to 65%. When fossil fuels are burnt, they release huge amounts of greenhouse gases into the atmosphere, affecting the environment. The greenhouse gases absorb the sunlight, which causes the earth to be hot. Besides, burning coal and oil is a source of climate change. Various countries are opting to use biofuels as a way to reduce greenhouse gases.

#### *1.4.13 Agricultural promotion*

Increased demand for the production of biofuels will lead to further farming of the appropriate crops. Crops with high carbon and cellulose composition can be planted on a massive scale, and after harvesting the edibles, the rest of the plant components can be used for the production of biodiesel.

#### **1.5 Disadvantages of biofuels**

#### *1.5.1 High production cost*

Biofuels are very costly to manufacture in the current market, even with all the advantages associated with biofuels. The interest and capital investment put into the biofuels production are relatively low as of now, but it can balance demand.

If demand rises, then it will be a long-term process to raise the supply, which will be very costly. Such a downside also prevents the use of biofuels from becoming popular globally.

#### *1.5.2 Monoculture*

Monoculture is the method of growing the same crops year after year in a single field, rather than generating different crops in multiple fields. Although this may be lucrative for farmers, growing the same crop every year would deprive the soil of nutrients returned by cover crops and farming overused areas. The reasons for planting a single crop over large tracts of land are discussed. First of all, the environment changes when only one crop is grown, and pests can ruin the entire crop.

Besides, complete pest control can be accomplished with pesticides. Even certain pest insects would inevitably develop resistance to the chemicals we use to fight them, and they would be able to live in a single crop area.

As we intend to encourage insect resistance to our pest, the next obstacle comes with genetically modified species. The change is not likely to impact any species, and the related problem remains.

Biodiversity, which requires various varieties of plants and animals, is thus the key to healthy agricultural fields.

#### *1.5.3 Application of fertilizers*

Biofuels are derived from crops, and to grow better, these crops need fertilizers. The drawback to using fertilizers is that they can cause water contamination and have adverse effects on the surrounding environment. Nitrogen and phosphorus are found in fertilizers. It is possible to wash them away from the soil into surrounding lakes, rivers, or ponds.

#### *1.5.4 Food scarcity*

Biofuels are obtained from plants and crops which have high sugar levels in them. Many of these crops are also used as food crops. Even though plants' waste material may be used as raw resources, there will still be a need for such food crops. Other crops can take up farm space, which can cause several problems.

The use of existing biofuel land may not lead to acute food shortages, but it will undoubtedly pressure current plant growth. One big problem that people face is that the rising use of biofuels could also increase food prices.

Algae, which grows in rather inhospitable regions and has a small impact on land use, is favored by some people. The issue with algae, however, is water use.

#### *1.5.5 Pollution in the industry*

When burned, the carbon footprint of biofuels is smaller than the conventional sources of fuel. The method in which they are made makes up for that. Production depends to a large degree on lots of water and oil.

It is understood that large-scale industries intended for biofuel production produce large quantities of emissions and also cause small-scale water pollution. The total carbon pollution would not have a very significant dent in it unless more effective production means are placed.

#### *1.5.6 Extensive use of water*

In order to irrigate biofuel crops, large quantities of water are needed and can, if not handled wisely, place a strain on local and regional water supplies. Vast amounts of water that could place unnecessary pressure on local water supplies have been used in order to manufacture maize-based ethanol to satisfy consumer demands for biofuels.

#### *1.5.7 Future price hike*

The existing technology used for biofuel production is not as effective as it should be. Scientists are interested in the creation of better measurements that enable us to extract this fuel. However, testing and potential installation expense means that a significant increase will be seen in biofuels' price.

As of now, gasoline prices are equivalent and are still practicable. The use of biofuels can be as tough on the economy as the rising gas prices are doing right now.

#### *1.5.8 Land use changes*

Land must be cleared of natural vegetation if it is used to produce a biofuel feedstock, contributing to ecological harm done in three ways.

First, the harm is caused by community habitat loss, animal dwellings, microecosystems, and the general wellbeing of the region's resources will be diminished.

In extracting CO2 from the atmosphere, the native forest is almost always better than a biofuel feedstock, partially because the CO2 stays trapped and is never extracted by burning as with the fuel stock.

Secondly, the damage of the generated carbon debt is significant. This contributes to the production of greenhouse gases as it is necessary to deforest an area and prepare it for agriculture as well as to grow a crop, and puts the region at a net positive

#### *Biofuel and Biorefinery Technologies DOI: http://dx.doi.org/10.5772/intechopen.104984*

development of GHG even before the production of a specific biofuel. Estimates have shown that a carbon debt that can take up to 500 years to repay can actually be created by deforestation of native land.

Finally, almost always converting land to an agricultural status means that fertilizers can be used to get the most yields per area. Runoff and other agricultural emissions are a problem.

#### *1.5.9 Global warming*

The biofuels, which mainly burn hydrogen and carbon, create carbon dioxide that causes global warming. Biofuels generate less GHG emissions than fossil fuels, but that can only help slow down global warming and not avoid or reverse it.

Biofuels could therefore be able to help alleviate our energy requirements, but they will not solve all of our issues. In the short term, it can only act as a replacement as we invest in other technologies.

#### *1.5.10 Weather issues*

For use at low temperatures, biofuel is less satisfactory. It is more likely than fossil diesel to draw moisture, which in winter conditions causes problems. The engine that coats the engine filters also enhances microbial growth (Various Advantages and Disadvantages of Biofuels, 2020

#### **2. Biorefinery technologies**

#### **2.1 What is biorefinery?**

A biorefinery is a specialized facility that uses tools and materials for processing biomass into fuels, electricity, and useful chemicals. The biorefinery is situated next to the petrochemical industry, making various petroleum products, including gasoline and plastic. A biorefinery benefits from using multiple biomasses and intermediate items, thereby maximizing the value of biomass feedstocks. One potential example for biorefinery product is low-volume yet high-value chemical products and high volume but low-value fluid or liquid transportation fuel such as biodiesel or bioenergy. Because of its high efficiencies, energy efficient technology that generates electricity and captures the heat (CHP) technology can generate electricity for its own use and sell surplus electricity to the community. High-value products boost profitability, highvolume fuels help meet energy needs, and power generation helps lower energy costs and mitigate greenhouse gas emissions from traditional power plant installations.

Nevertheless, the production of fuel and chemicals in biorefinery is limited, which may become harder to do. Interdependent societies and trading firms demand vast quantities of fossil fuels to supply the bulk of their energy and chemical supplies. The manufacture and use of fossil fuels cause environmental degradation, resulting in toxins, greenhouse gases, and dangerous materials. The increasing demand for energy and chemicals makes the environment more dependent. The amount of waste produced is also continuously rising alongside the growth in our global population. Our country generates roughly 250 million tons of municipal solid waste per year (MSW). Overall, 35% of MSW is recycled and composted, 13% of this waste is used to generate power, and 53% of MSW is buried in landfills.

It is important to find alternative resources to make energy and chemicals due to limited fossil fuel resources and the increasing demand for energy and chemicals. In this process, biomass has been recognized as a possible future source of chemicals and energy and addresses the environmental risks of burning fossil fuels. Various biomass sources are available but can also be collected from various waste sources such as agricultural waste, municipal solid waste, and industrial contaminants such as paper factories and pulp factories. In a holistic waste management plan, biomass waste valorization also plays a prominent role in waste recycling. A biorefinery that uses renewable biomass as a feedstock for the production of chemicals is more sustainable as opposed to using fossil fuels. A biorefinery may contribute to economic expansion while also reducing air pollution in the environment [21].

#### **2.2 Biorefinery technology, product, and application**

To create an integrated biorefinery that aims to build on the biomass conversion process in such a way that the maximum added value can be derived from the sustainable biomass feedstock, it integrates a range of different technologies. New methods of bio-refining are being explored in hopes of saving the world. Biorefineries combine/integrate different technologies for the conversion of biomass into a variety of products (i.e., food, feed, chemicals, materials, petroleum, coal, heat, and/or electricity) and are defined as 'sustainable processing of biomass into a marketable commodity and energy spectrum' by IEA Bioenergy Task 42. The concept focuses on the method of how various petroleum products are refined and made into usable fuels.

As it is subject to unpredictable circumstances, such as when farmers use various farming methods, and climate changes, a biorefinery's precise technical specification can vary from case to case. Environmental and social factors decide which feedstock is available for processing. Numerous varieties of switchgrass, sugarcane, wheat, corn, wood, crop waste, sugar cane, surplus food, straw, freshwater biomass, and the biomass component of municipal and other sources of waste (MSW) can be used in a bio-refinery. Chemicals, biofuels, energy and heat, materials, food and feed, minerals and CO2, are the main product groups in a biorefinery (Biorefinery, ctc).

Biorefineries can be classified based on the number of main characteristics they have. Various feedstocks for biofuel production include perennial grasses, starch crops (e.g., wheat and maize), sugar crops (e.g., beet and cane), lignocellulose crops (e.g., controlled forest, short growing coppice, switchgrass), lignocellulose residues (e.g., stover and straw), oil crops (e.g., palm and oilseed rape), and inorganic matter (e.g., stover and straw) (e.g., industrial, commercial and post-consumer waste).

Feedstocks can be treated on a number of platforms used by biorefineries. These platforms include biogas, consisting of single carbon molecules such as methane and carbon dioxide, starch, sucrose, or cellulose carbon carbohydrates; a mixed stream of 5 and 6 hemicellulose-derived carbon carbohydrates, lignin, oils (plant-based or algal), organic grass solutions, pyrolytic liquids. By integrating biological, thermal, and chemical processes, these main platforms can be changed to produce different products. Awareness of the feedstock, platform, and product that a biorefinery uses allows the business to be represented critically. Biorefinery practice creation helps compare biorefinery systems, improves understanding of global biorefinery growth, and allows technology differences to be developed.

The biorefinery classification examples include:


#### **2.3 Biorefinery pathways**

Biomass can be used as food, heating fuel, or converted into a liquid or gaseous form that can then be used for energy resources. There are different methods to transform biomass into biofuels. A distinction is made between biochemical conversion and thermochemical conversion. Anaerobic digestion, saccharification, and hydrolysis are common conversion technologies used throughout industries. 5 distinct sub-categories of thermochemical conversion include gasification, pyrolysis, liquefaction, gasification, and combustion.

As oxygen is completely removed, the aerobic decomposition of organic carbon diminishes organic nonwoody content. It sells easier, less volatile chemicals, including methane and carbon dioxide. However, the biochemical conversion process takes a lot of time and uses just a biomass portion. Thermochemical conversion technologies are more generally regarded as being superior for their flexibility and efficacy.

Most second-generation biofuels are generated by concentrating lignocellulosic biomass into different products. Containing three main constituents: cellulose, hemicellulose, and lignin. Under conditions between 200 and 380°C, hemicellulose is the easiest to break down, and cellulose decomposes between 320 and 400°C. The most stable substituent that breaks down when heated to 400°C is cellulose.

Temperature, heating rate, and residence time are three important moving parts in chemical-reaction thermochemistry. Combustion is currently the leading source of energy (approximately 80%) in the worldwide supply. Many alternative methods for pollution control, such as gasification and pyrolysis, are still in the research and development stage due to their high cost and low performance [22].

Biomass can be converted into an extended range of chemicals in two main ways:

#### a.Thermochemical pathway

b.Biochemical pathway

#### *2.3.1 Thermochemical pathway*

Thermochemical processing is used for two major routes:

One method that produces biomass is heating biomass with regulated oxygen quantities at high temperatures and pressure (a mixture of carbon monoxide and hydrogen). The process is called gasification. The gasification of solid biomass produces many industrial compounds. The effect of gasification on various liquid fuels will be addressed in the biofuels unit.

The second technical approach involves high-temperature heating of the biomass, but it works without relying on the atmosphere. As a method, pyrolysis is wellknown. Glue must be used easily, so the reaction time must be short. If not, the top sector will be the carbon industry (char). This process is known as rapid pyrolysis, and the main product produced is organic oil [23].

The thermochemical conversion aims to minimize the entire biomass used in the chemical-making phase to steam. The Fischer–Tropsch process is a thermochemical conversion, which is why it is an example. Thermochemical biomass conversion is not a key application of chemical transformation. The main driver of this five-way conversion route is the output of thermal energy:

#### 1.The Combustion


The biomass is first transformed into syngas in the thermochemical pathway, converted by synthesis or some other method into ethanol.

#### *2.3.1.1 Combustion*

Considering that humans started with fire discovery, combustion was the first method of using living materials for energy production. Wood-burning forestry has taught people how to survive, heat and cook. Chemically, biomass combustion is an oxygen-based exothermic reaction. Here the biomass is oxidized by two major stable compounds, hydrogen and carbon dioxide. The heat generated by the reaction currently accounts for over 90% of energy consumption.

Bio-mass derived energy is mainly generated from heat and electricity. Biomass also provides renewable cooking fuel and heat in rural communities. Combustion of biomass also includes industrial heating and regional heating. Pellet stoves and woodburning fireplaces are widely used in areas with cold climates. The use of biomass for electricity is important for modern-day environmental practices. Combustion of biomass in boilers and the power-producing steam turbine are the most common activities. Biomass is used as an alternative to fossil fuel in a boiler, typically for heating. The latter approach is more effective in lowering carbon dioxide emissions from a high-emission fossil-fuel plant than the prior solution.

#### *2.3.1.2 Carbonization*

Like torrefaction, carbonization is recommended for the productivity of biomass as a safe and efficient solid fuel. The biomass is continuously heated up to 200–300°C with little or no oxygen contact in torrefaction. This process changes the biomass hydrocarbon's chemical composition to increase its carbon content while reducing its oxygen content. Torrefaction also raises the density of biomass and increases hygroscopic biomass. These qualities thus increase the commercial value of the timber for electricity manufacturing and transport. The general goal for other carbonization processes is to form carbon-rich reliable products under different conditions.

#### *2.3.1.3 Pyrolysis*

Pyrolysis occurs in an oxygen-free environment, unlike combustion, even when using partial combustion to heat the reaction. It can be used to quickly and effectively transform biomass into gases, liquids, and solids.

Parts of biomass are broken down in the pyrolysis process. Slow pyrolysis end products include reliable charcoal and gas, while rapid pyrolysis produces only bio-oil. In order to turn biomass into liquid fuels, pyrolysis is suitable. It is not an endothermic reaction, while combustion releases heat.

#### *2.3.1.4 Gasification*

Solid, liquid, and gaseous fossil fuels are converted into usable gases. One needs a medium for gasification reactions, water, or steam. An air, oxygen, or both make up the gaseous environment.

Natural gas production through fossil fuel emissions is more common than biogasification to produce biogasoline. Gasification moves the fuel from one type of fuel to another. There are numerous reasons for the change from one form of language to another.


The higher the hydrogen content in fuel, the more likely the fuel will be in its gaseous state. The relative hydrogen content in the substance is produced by adding air to the material by using gasification or pyrolysis.


Compared to the amount of oxygen in cellulosic biomass, the oxygen concentration in natural gas is decreased. Gasification decreases the overall carbon footprint and creates a more usable commodity.

Some natural gas is gasified in order to use as a source of energy and as a means of ammonia production. Nature gas reforming helps in steam output (a mixture of H2 and CO).CO, which is present in the biogas, is indirectly hydrogenated through the smog to produce methanol. However, these systems use natural gas, causing more carbon dioxide to be produced than other systems. Biomass can be used as a replacement for fossil hydrocarbons in various manufacturing processes.

Changing liquid transportation fuels generates biomass gasification, providing a strong basis for carbon dioxide and hydrogen. The process may also generate methane, useful as a source of energy.

#### *2.3.1.5 Liquefaction*

There are several methods of bringing solid biomass into liquid fuels, including pyrolysis, gasification, and hydrothermal processes. The conversion of biomass into an oily liquid can be achieved by heating the biomass at higher temperatures (300–350°C) and under high pressure [24].

#### *2.3.2 Biochemical pathway*

Biochemical conversion requires the breakdown of biomass to make the carbohydrates usable for refining into sugars, which can then be transformed using microbes and catalysts into biofuels and bioproducts. The following are possible stocks of fuel mixtures and other bioproducts:


The significant challenges of breaking down the complex structures of cellulosic biomass include key challenges for biomass's biochemical conversion. To have access to these beneficial sugars, the Bioenergy Technologies Office explores more effective and affordable means of processing the sugars.

The critical challenge is to turn sugars into biofuels more efficiently and effectively. To achieve our target, the Bureau has developed new directions and technologies.

#### *2.3.2.1 Step by step chemical conversion*

In addition to heat and other chemicals, the biochemical conversion uses biocatalysts to convert the hemicellulose and cellulose into an intermediate stream of sugar. Such sugars are an intermediate stage in the manufacture of advanced biofuels and other biochemicals or are catalyzed chemically to generate useful substances. The whole method is made up of the following necessary steps.


#### **2.4 Integration of biorefining in the processing industry**

Due to the wide variety of biorefinery systems and their component selection, there are major energy properties variations. The process will be affected by the form of feedstock, crude oil metabolism, and end product. The freedom to select and change these parameters is a complex task that takes a great deal of thinking. The biorefinery portion should be considered to optimize the biorefinery's energy characteristics to better conform the overall process integration.

The model is dynamic and implemented several degrees of freedom. The optimization is difficult to do due to the substantial uncertainty in potential energy markets, investment costs, and, especially, the cost of CO2. Optimization research on the different levels of these key parameters should also be carried out in order to find "robust" solutions, i.e., perfect ones for technical, environmental, and economic performance at multiple levels of these variables. The incorporation of a biorefinery model into a process industry is very comparable, in theory, to the foreground/background approach or, often, the Complete Site o ne for a given biorefinery design.


One experience from integrated biorefinery process integration studies is that a very non-integrated host process may be more appropriate for integration with the biorefinery than an integrated biorefinery process. Therefore, if a biorefinery in an organization is considered shortly, any planned energy-saving measures should be postponed or carefully evaluated. This will jeopardize the possibility of effective

overall integration. It also proves that the above fourth alternative that combines all the streams must always be done as a first step.

The standard theoretical integration technique is used in the article. The third alternative will then be checked by deciding whether the flows could be integrated into structures of various kinds. The distance between the gas measurements indicates the ability to save electricity. It is the complicated approach that suits the problem and the straightforward solution that is easier to manage. Functional limitations can often make the most powerful solutions impractical, but an efficient targeting method is essential.

In certain cases, the integration possibilities often depend on the availability of energy or excess heat in the refinery and part of the biorefinery. Since heat is not extracted by convection in the original form, it is only cooled by the cheapest possible means of a cooling system. Therefore, it is important that the possible amounts of excess heat from the process can be analyzed when reasonable heat exchange is applied to increase these temperatures and provide a targeted protocol.

#### *2.4.1 Biorefinery concepts in different types of process industry*

In the process industry, there is a vast variety of proposed and researched ideas for biorefineries. Only some important examples of concepts are presented below. While the integration of processes would support all forms of biorefineries, refineries processing bulk goods will be of greater importance than chemical products. The examples are, therefore, all bulk goods.

Examples of Process Integration Results Studies

#### *2.4.2 The industry for pulp and paper*


The overall outcomes of these studies are:

• In almost all situations, energy-saving steps can be found by process integration between the biorefinery definition and the process industry.


#### **3. Production of solid fuel biochar from waste biomass**

Due to the possibility of exhausting fossil energy and the increase in climate change resulting from the excessive use of fossil fuels, there is a growing need to use renewable energy sources in future in place of fossil energy. Biomass energy is becoming an increasingly popular type of renewable energy, primarily due to its worldwide availability [1]. At present, biomass combustion alone (or co-combustion with coal) to produce heat and power in current coal-fired systems is widely accepted as a low-risk process and one of the least expensive methods of reducing CO2 levels in the atmosphere [2]. However, raw biomass is not an effective energy carrier and there are significant barriers hindering the direct use of biomass due to its innate properties, including high moisture content, poor grindability and low energy density [3]. For instance, biomass has a fibrous structure that generates poor grindability, and this causes substantial increases in energy consumption and impairs the processes of fuel preparation and feeding. Moreover, in terms of high moisture content, it can reduce the maximum combustion temperature, which in turn reduces thermal efficiency and increases toxic emissions [3]. To address these issues, a pre-treatment process can be carried out to enhance the fuel quality of raw biomass before combustion.

To convert biomass feedstock into biofuels with a high-energy density, pyrolysis has been carried out. This process generates three primary product streams, namely liquid (bio-oil), solid (biochar) and gas products. The pyrolysis conditions largely determine the distributions and properties of these product streams. At present, a majority of research attention is focused on the liquid and gaseous products, and a number of different processes have been developed to generate increased yields and enhance the quality of the two target products [4–7]. Nonetheless, given the high instability and complexity of its composition, it is not possible to use bio-oil. Rather, more upgrading is necessary [8]. With regard to gaseous products, there is a low yield, whilst the separation and purification processes are largely complex. This ultimately limits the large-scale application of such products in practice [7]. In such cases, it can be beneficial to optimise the use of biochar, and this can ultimately enhance biomass utilization efficiency.

In comparison to liquid and gaseous products, very few studies have investigated the production of solid fuel biochars from waste biomass. In fact, a majority of these studies have focused on examining the improved physicochemical properties of woody biomass [9–16]. On the other hand, a few studies have examined the biochars produced from abundant agricultural wastes, although studies comparing the fuel quality of biochars obtained from woody biomass and agricultural residue are lacking. Moreover, biomass combustion has low thermal efficiency and produces extremely pollutant emissions. It also generates serious ash-related problems (i.e., fouling and slagging). Nonetheless, even though they are key issues when applying biomass as a solid fuel, very little research has examined the combustion qualities and ash issues relating to pyrolytic biochars in relevant literature [17, 18].

#### **3.1 Biochar production**

Several different biomass materials can be used to make biochar, including agriculture waste, animal waste, sewage sludge waste and algal waste. To produce biochar from biomass materials, a number of methods have been developed, such as pyrolysis, hydrothermal carbonisation and gasification.

#### *3.1.1 Pyrolysis*

Pyrolysis is a process carried out to thermally decompose biomass without oxygen. There are two stages involved in this decomposition process, namely the primary and secondary stages. Dehydration, dehydrogenation, and decarboxylation take place during the primary stage [19], after which a secondary reaction starts to take place, in which larger molecules are cracked and the solids are converted into gases and biochar. Moreover, there are two key types of pyrolysis, namely slow and fast pyrolysis and this is determined by the operating conditions. During slow pyrolysis, heat is set at a lower rate (0.1–1°C s−1) and the reaction takes place over a long period of time (hours to days) at a temperature (300–900°C). This provides a favourable environment that facilitates the secondary processes and increases the production of biochar. By contrast, the biomass in fast pyrolysis is heated at a higher temperature (300–1000°C) and with a high heating rate (10–1000°C s−1) for a short period of (0.5–2 s) [22], and this process results in the production of a solid (biochar), liquid (bio-oil) and gas (syngas) [24]. The solid carbonaceous substance biochar can be employed as either a catalyst, adsorbent or fuel. On the other hand, syngas (which is made up of CH4, CO2, H2, CO, and other low molecular gases) is typically used in gas engines. Bio-oil is made up of water, phenolic compounds, alcohol, nitrogenous compounds (pyrazine, pyridine, and amines) and aliphatic and aromatic hydrocarbons. Thus, bio-oils are often used in boilers to produce heat [19]. The key properties of biochar (i.e., porosity, surface area and functional groups) are determined by the temperatures used during the pyrolysis process. At higher temperatures, the biochar's surface area and porosity increase. This is because the aliphatic alkyl and esters groups in the organic compounds break and this facilitates the removal of pore-blocking substances [25]. When lower temperatures are used during pyrolysis, the resultant biochar is hydrophilic and has a graphene structure with fewer functional groups on the surface. On the other hand, biochar created at higher temperatures are hydrophobic and with functional groups being reshuffled and new groups (carboxyl, lactone, phenol, pyridine) being introduced that serve as electron donors and acceptors [26]. In an experiment, Akinfalabi et al. [27] carried out pyrolysis at 400 °C for two hours to produce biochar from sugarcane bagasse biomass. During the process, the latter was sulphonated with ClSO3H, which increased the surface area from 98 to 298 m2 g−1. When used as a catalyst to produce biodiesel production, a yield of 98.6% fatty acids methyl esters (FAMEs) was generated [28]. It is important to note that the yield and quality of the resultant biochar

#### *Biofuel and Biorefinery Technologies DOI: http://dx.doi.org/10.5772/intechopen.104984*

are determined by the types of biomass feedstock used. Conducting pyrolysis sing forestry plants generates a 30% biochar yield, whilst using lignin produces a slightly higher yield of 45.69%. Thus, this suggests that the biochar yield is largely determined by the lignin content [29]. Moreover, Zhang et al. [30] pyrolyzed a lotus stem at 800°C and found that it produced biochar with 55% greater surface area (1610 m2 g−1) than porous carbon made from leaves. This is because there is a greater number of metal ions in the stem.

#### *3.1.2 Gasification*

Gasification refers to the processes of converting biomass into gaseous fuel through decomposition (H2, CO, CH4, etc.). To do this, higher temperatures (500– 1400 °C) and oxygen-deficient conditions are required. To enhance the production of the gaseous product, different gasification agents (e.g., steam, CO2 and some gas mixtures) can be used. Approximately >50% of the biomass converted into gaseous fuel and biochar during this process was smaller in size, and resistant to chemical oxidation [28]. Temperature plays a critical role in the gasification process and results in the increased production of hydrogen and carbon monoxide. At higher temperatures, however, the levels of carbon dioxide, methane, and hydrocarbon are reduced [31]. In general, the surface area of biochar created during the gasification process is smaller and possesses fewer functional groups (i.e., hydroxyl, carbonyl and carboxyl groups) than that yielded during the pyrolysis process [32]. It is important to note that the equivalence ratio (ER) also impacts the yield and quality of biochar. A higher ER indicates that a high quantity of oxygen has been added to the gasifier and it can positively or negatively impact the properties of the resultant biochar. Yao et al. [33] conducted a study and found that there was a decrease in biochar yield from 0.22 to 0.14 kg g−1 and a reduction in carbon content from 88.17% to 71.6%. Additionally, the ER increased from 0.1 to 0.6 [34]. In general, if oxygen molecules are present in the compound, more ash content will be produced, whilst the yield and mechanical strength of the biochar will be reduced. A further study carried out by James et al. [35] investigated the impacts that airflow has on the properties of Pine woodchip biochar [22]. Their findings indicated that airflow of 8 to 20 L min−1 produced basic biochar (pH > 7.0) and that there were no acidic functional groups at a high airflow rate. The content of alkalis and alkaline earth metals is higher when biochars are created through gasification, although the exact content varies based on the type of biomass used [36]. There are several different gasifiers that can be used in such processes, including fixed bed, fluidized bed and circulating fluidized reactors. These will be discussed in more detail at a later stage [20].

#### *3.1.3 Hydrothermal technology*

During this process, wet biomass is thermochemical converted into hydrochar. Hydrochar can be produced in the same ways as biochar (i.e., using the methods discussed above). Moreover, the process is very much like the process of forming natural coal. Moderate temperatures (150–350°C) and conditions under 10–15 bar are established to perform the hydrothermal treatment process [25]. Interestingly, the properties of water change significantly at higher temperatures and pressure to become more of an organic solvent. In such cases, reactions involving acid-based catalysts are favourable in promoting biomass decomposition [37]. At present, the exact mechanism facilitating the hydrothermal treatment process is unknown.

However, it likely involves dehydration, hydrolysis, decarboxylation, aromatization and recondensation. Biomass breaks down to form saccharides and lignin throughout the hydrolysis process. Moreover, during the dehydration process, the hydroxyl group is eliminated, which ultimately removes water from the biomass. Meanwhile, during decarboxylation, all CO2 is removed and this facilitates subsequent aromatization. A number of different compounds are produced during these processes, including phenols, aldehydes and acids and all such compounds are subjected to recondensation with aromatic polymers, which results in the production of hydrochar [38]. The key benefit of using hydrothermal technology is it can transform wet biomass into carbonaceous solids with no extensive drying required. Additionally, this produces a high yield. Other benefits include adaptable surface functionalities, conductive behaviour, the production of natural binders and high calorific value [39]. However, the hydrochar yield is reduced at higher temperatures (~350°C) (29%), as is the yield of bio-oil (31%). However, there is a substantial increase in gas fractions (67%) [40]. The O/C and H/C rations also decline with increased temperatures. Wang et al. [41] used a combination of thermal carbonization and activation to convert sunflower stalks into hydrochar [42]. The resultant hydrochar had a large surface area (1505 m<sup>2</sup> g−1), as well as a 35.7 Wh kg−1 energy density. Several different chemical and physical methods can be used to produce hydrochar. For instance, KOH is a chemical activating agent that facilitates the production of hydrocar with a higher surface area than other chemicals (ZnCl2, HCl, NaCl, and MgCO3). This is because KOH can easily reach the outer surface layer of carbon material [21, 23, 25, 43–49].

#### **4. Conclusion**

Solar and wind energy have the potential to supplement the existing energy resources in order to meet the growing global demand for electricity. It is necessary to make renewable energy more economically and efficiently efficient in order to make it more accessible. Biochar has been investigated for use in the fabrication of electrode materials and catalysts for use in the catalysis of processes involved in the generation of biodiesel and biohydrogen. It is necessary to conduct research into the synthesis of biochar with the needed qualities in order to increase the efficiency of the biocharmediated process. Recent advancements in biochar-based material research in the field of renewable energy indicate that it has the potential to be a source of future energy in the future. The scarcity of fossil fuel, environmental pollution and the rise of energy demand have triggered many researchers to find alternatives fossil fuels feedstock. Various sources of biomass have been studied and this present study has provided the most recent promising feedstock for the alternative liquid fuel production. Criteria of excellent biomass-derived biofuel were discussed. Price, availability, the total content of oil in seed and quality are critical factors. Edible palm oil and soybean oil were proven highly promising feedstock for biofuel production; nevertheless, due to the "*food versus fuel*" issue make the use of these edible oil as biofuel feedstock not viable. Non-edible oil derived from jatropha oil and WCO presented promising feedstock for biofuel production. Based on the chemical composition and physicochemical properties of raw vegetable feedstocks, all feedstocks have failed to meet diesel standards (ASTM D6751 and EN 14214) specification, which in turn strong affirmed that raw vegetable is unacceptable to be used in the diesel engines.

### **Author details**

Abdulkareem Ghassan Alsultan1,2,3\*, Nurul Asikin-Mijan2,4, Laith Kareem Obeas5 , Aminul Isalam6 , Nasar Mansir7 , Maadh Fawzi Nassar8 , Siti Zulaika Razali9 , Robiah Yunus10 and Yun Hin Taufiq-Yap1,2,3

1 Faculty of Science and Natural Resources, Universiti Malaysia Sabah, Kota Kinabalu, Sabah, Malaysia

2 Faculty of Science, Catalysis Science and Technology Research Centre (PutraCat), Universiti Putra Malaysia, Serdang, Selangor, Malaysia

3 Universiti Malaysia Sabah, Kota Kinabalu, Sabah, Malaysia

4 Faculty of Science and Technology, Department of Chemical Sciences, Universiti Kebangsaan Malaysia (UKM), Bangi, Selangor Darul Ehsan, Malaysia

5 Technical Institute of Babylon, Al-Furat Al-Awsat Technical University (ATU), Iraq

6 Department of Petroleum and Mining Engineering (PME), Jashore University of Science and Technology, Jasho, Bangladesh

7 Faculty of Science, Department of Chemistry, Federal University Dutse, Dutse, Jigawa State, Nigeria

8 Faculty of Science, Department of Chemistry, Universiti Putra Malaysia, Serdang, Selangor, Malaysia

9 Institute of Nanoscience and Nanotechnology, Universiti Putra Malaysia − UPM, Serdang, Selangor, Malaysia

10 Institut Kajian Perladangan (IKP), Pejabat Pentadbiran, Universiti Putra Malaysia (UPM), Serdang, Malaysia

\*Address all correspondence to: kreem.alsultan@yahoo.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 13**

## Biochar Development as a Catalyst and Its Application

*Stephen Okiemute Akpasi, Ifeanyi Michael Smarte Anekwe, Jeremiah Adedeji and Sammy Lewis Kiambi*

#### **Abstract**

Biochar is a carbon-rich pyrogenic material that is made from carbon-neutral sources (i.e., biomass). It offers key strategies for carbon capture and storage (CCS) as well as being an environmentally friendly means of soil amendment. The recent recognition of biochar as a versatile media for catalytic applications has prompted preliminary research into biochar's catalytic capacity and mechanistic practices via various routes. This chapter provides a review of biochar production technologies, biochar's catalyst development, and its application in various catalytic processes as well as descriptions of the benefits and drawbacks of the various applications currently available. The characteristics of biochar-based catalysts, challenges of effective application of this catalyst system, emerging application, prospects, and future work consideration for effective utilization of biochar-based catalysts were presented.

**Keywords:** biochar, biodiesel, biomass, catalyst, pyrolysis, tar reforming, wastewater treatment

#### **1. Introduction**

With energy shortages and pollution escalating worldwide, renewable feedstocks are crucial for human long-term development. There are many natural sources of animal fats, including lignocellulosic biomass, crops, aquatic culture, biowaste generated by waste management, and domestic and urban waste recycling [1]. Utilizing thermochemical decomposition processes like gasification or pyrolysis, biofuels (bio-oil and syngas) can be produced from biomass and a carbon-based solid residue called biochar [2].

As a porous solid with high carbon content, biochar is formed during the thermal decomposition of biomass at moderate temperatures (e.g., 350–700°C) and under oxygen-limited conditions [3–7]. Despite its chemical and physical properties, biochar's thermochemical process and the intrinsic properties of biomass feedstock are two of the factors that influence its properties [4]. Due to its porosity and large surface area, biochar is classified as activated carbon (AC), yet it also contains numerous surface functional groups (carbon monoxide, hydroxyl, carbonyl, carboxylic acid, among others) that can be easily tuned and used to make various functionalized carbon materials. As well as being used for AC production and soil amendment, biochar serves as an adsorbent for pollutants in water and air [5].

Recent research has revealed that biochar is widely utilized as support for metals in catalysis, due to its feedstock availability, large surface area (for good metal phase dispersion and stability), low cost, and stability in basic and acidic media [6]. In addition to catalysis, biochar's excellent performance in supporting and catalyzing a wide range of reactions has been demonstrated: electrochemical reactions, hydrolysis, gasification/pyrolysis, catalytic reforming/cracking, esterification/transesterification, peroxide/peroxynmonosulfate oxidation, and many more.

Biochar-based catalysts have been utilized for a variety of applications, including water and soil remediation. On the other hand, current perspectives tend to concentrate on applications designed to remediate soils, revegetate, and restore them, convert energy, and remove contaminants from water and wastewater. Despite this, there is still a lack of understanding regarding the synthesis, development, and novel applications of biochar-based catalysts. This chapter provides a comprehensive overview of recent developments in the production, application, and limitations of biochar-based catalysts. Various emerging catalytic applications of biochar-based catalysts are also addressed in this chapter. Further, the benefits of using biochar as catalysts and catalyst supports, as well as the correlations between structural and physical properties of biochar, which provide insights into the development of effective and promising biochar-based catalysts will be highlighted. The challenges and future advancements of using biochar-based catalyst materials are further discussed.

#### **1.1 Properties of biochar**

Biochar is a form of organic material that is mostly rich in carbon and other elements such as nitrogen (N), oxygen (O), and hydrogen (H). Biochar has a carbon (C) content ranging from 380 to 800g kg−1 and has both alkyl and aromatic structures [7]. Biochar is also composed of inorganic elements including phosphorus (P), calcium (Ca), aluminum (Al), potassium (K), and silicon (Si), whose quantities vary according to the feedstock used [8]. It has been reported that acidic pH can occur during pyrolysis, depending on conditions of production and the raw materials [9]. Other factors can affect the biochar pH, ranging from neutral to alkaline [10]. In general, biochar has a pH between 5 and 12, and its pH tends to increase in response to increased pyrolysis temperature as bionic acid decomposes, and mineral alkali elements increase [11]. Also, the high pH of biochar can be attributed to the functional organic groups present in it, namely hydroxyl-, aldehyde, and ketone- [12]. As a buffer between acid and bases, these functional organic groups influence biochar's hydrophobicity and hydrophilicity as well as its adsorption properties [8]. The functional organic groups have the effect of lowering the negative charge on biochar, and therefore, enhancing its cation exchange capacity (CEC) [13].

Due to its high carbon content, biochar has a complex microstructure with numerous pores, which maximizes its surface area [14]. Biochar's surface area and total pore volume typically range from 8 to 132 m<sup>2</sup> /g and 0.016–0.083 cm3 /g, respectively. Using the right precursor and pyrolysis parameters, biochar can have surface areas and pore volumes as high as 490.8 m2 /g [15] and 0.25 cm3 /g [16]. Following effective post-treatments, such as potassium hydroxide (KOH) activation, the surface area and total pore volume of biochar can be enhanced to 3263 m2 /g and 1.772 cm3 /g, respectively [17], which is comparable to or even greater than commercial activated carbon.

*Biochar Development as a Catalyst and Its Application DOI: http://dx.doi.org/10.5772/intechopen.105439*

Biochar's surface area and porosity are greatly affected by the pyrolysis temperature [16]. Biochar with a higher pyrolysis temperature within a certain temperature range has a greater surface area [12]. As temperature rises in biochar pyrolysis, volatile substances are forced out of the char, causing pores to form a larger surface area [17]. Due to its high porosity/high amount of residual pores and large surface area, biochar can retain a large quantity of water [14, 18–20]. In contrast, a high pyrolysis temperature diminishes the polar functional groups found in biochar, thereby increasing its hydrophobicity [18]. According to the above characteristics, biochar can influence the, pH, soil water-holding capacity, as well as base saturation, and CEC [14]. It is generally possible to modify the properties of biochar by modifying its conditions of preparation [19], as outlined in the next section.

#### **1.2 Biochar production**

To produce biochar from different feedstocks, several approaches have been developed. Torrefaction, pyrolysis, gasification, hydrothermal carbonization (HTC), and flash carbonization are the most prominent thermochemical conversion technologies (**Figure 1**).

#### *1.2.1 Torrefaction*

Torrefaction is a mild pre-treatment consisting of slow heating at 200–300°C, followed by a short retention time before gasification or pyrolysis [20]. Often, the resultant solid product is porous, low density, and carbon-enriched, with low moisture content and O/C ratio, an increase in energy density, and improved grindability, making it easier to store and deliver [21]. Its carbon yield can be affected by temperature, retention times, raw material types, and furnace atmosphere [22]. At 200°C, for example, beech lignin began to degrade, the majority of biomass developed at 230°C and cellulose only degraded over 270°C [22]. Using a pilot process, hardwood and

#### **Figure 1.** *Overview of biochar-based system production and applications.*

switchgrass pellets produced solid yields above 77 wt% [23]. Oil palm fiber pellets were torrefied in an inert atmosphere for 30 min and in an oxidizing atmosphere for 30 min at 275–350°C to yield 43 and 65 wt% biochar, respectively [24].

#### *1.2.2 Pyrolysis*

Pyrogenic carbons are produced by the decomposition of biomass at 300–1200°C without oxygen (or with limited oxygen). During pyrolysis, biochar is produced at temperatures ranging from 300 to 700°C. A pyrolysis process can be classified into slow, fast, intermediate, flash, and vacuum modes [25].

#### *1.2.2.1 Slow pyrolysis*

In slow pyrolysis, the process temperature is lower (400–600°C), the heating rate is slower (~10°C min−1), the vapor residence time is much longer (5–30 minutes), and the holding time is long (hours to days) [25]. Biochar typically yields 20–40 wt%, with yields decreasing with increasing pyrolysis temperature and heating rate [26], however, biochar characteristics are also affected by the procedure and feedstock used [27]. Comparing biochars derived from the wood stem and bagasse with palm kernel shell, paddy straw, and cocopeat, biochar derived from the wood stem and bagasse exhibited a wide range of pores and a high surface area. Biochar develops a significant surface area structure and pore structure at around 500°C [28] with a wide range of mineral compositions and high thermal stability [29].

#### *1.2.2.2 Fast pyrolysis*

Fast pyrolysis refers to the treatment of biomass at high temperatures without oxygen [30]. It is usually necessary to dry and grind the feedstock to facilitate effective heat exchange and conversion. This technique produces high liquid yields (bio-oil) rather than solid char (15–25 wt%) [31]. In contrast to the slow pyrolysis of wheat straw, fast pyrolysis generated biochar with a labile un-pyrolyzed carbohydrate fraction (8.8%) rather than carbonized completely [32]. There was a significant difference in the pH, particle size, and specific surface area for biochars produced using these two methods at 400°C, as well as a significant increase in surface area at 500°C (175.4 m2 g−1), in comparison to 300°C (2.9 m2 g−1) and 400°C (4.8 m2 g−1) [21, 33].

#### *1.2.2.3 Intermediate pyrolysis*

Intermediate pyrolysis produces 15–35 wt% dry and brittle biochar at temperatures between slow and fast pyrolysis, i.e., solid residence durations of 0.5–25 min, vapor residence times of 2–4 s, and moderate temperatures up to 500°C [34]. Utilizing barley straw and wood pellets, a pilot-scale production yielded 30 wt% char with a carbon content of 75 wt% [25]. The process produces 51.7 wt% char from the organic fraction of municipal solid waste as a result of inert fractions in the biomass [35]. **Table 1** illustrates the product yield of pyrolysis processes.

#### *1.2.3 Gasification*

Carbonaceous materials are turned into char, tars, and syngas through gasification at high temperatures (~800°C) in the presence of a gaseous active medium (e.g., carbon


#### **Table 1.**

*Summary of product yield of pyrolysis processes [36].*

dioxide, air, nitrogen, oxygen, steam, or gas mixtures) [37, 38]. During this process, the material is dried, pyrolyzed, partially oxidized, and reduced. Generally, char only makes up 5–10 wt% of the mass of the feedstock [39]. As a by-product of large-scale processes, biochar is produced in large quantities every day. Biochar produced through gasification usually has smaller particles than biochar produced by pyrolysis, lower surface area, and a lower total pore volume [40]. Since the aromatic rings are condensed, gasification chars contain little carbon (20–60 wt%) but are highly stable, preventing microbial mineralization and chemical oxidation; however, their surface chemistry is constrained by their absence of functional groups [34]. Biodiesel generation, catalytic tar decomposition, soil amendment, anode materials for direct carbon fuel cells, and anaerobic digestion additives are just a few of the uses for gasification char [41].

#### *1.2.4 Hydrothermal carbonization*

Biomass can be processed using a thermochemical process called hydrothermal carbonization (HTC). In closed vessels with liquid water and autogenous pressure of 2–10 MPa, the feedstock is heated from 200 to 300°C and hydrochar is produced [42]. The thermal stability of hydrochar is improved by high temperatures (300°C). Wet torrefaction or wet pyrolysis are other terms for HTC [43, 44]. In comparison with biochar, hydrochar contains less carbon, ash, surface area, and a smaller pore volume [39].

#### *1.2.5 Flash carbonization*

Through flash carbonization, biomass can be transformed into biocarbon (i.e., charcoal) rapidly and efficiently, typically by starting and controlling a flash fire at a high temperature within a packed bed (~1 MPa) [45]. The biomass is transformed into gas and charcoal in less than 30 min when the combustion flame flows in the opposite direction of the airflow. Charcoal yields are typically approximately 40 wt% [45].

#### **1.3 Biochar as a promising catalyst**

Biochar can serve as catalyst support. Besides stabilizing and dispersing nanoparticles, biochar can also provide more active sites for catalytic degradation reactions [46]. Biochar's mesoporous structure enhances the proper dispersion of immobilized metal particles while also preventing particle aggregation owing to intra-particle interaction [47]. The incorporation or fixing of metal elements, for example, magnesium (Mn), copper (Cu), cobalt (Co), and iron (Fe) into biochar pores result in no or minimal metal escape into the aqueous phase [48].

As a heterogeneous catalyst or support, biochar offers many advantages including large surface area, lower cost, functional group tailoring, etc., which makes it highly beneficial for many catalytic applications. There are several intrinsic properties of biochar that contribute to its effectiveness as a catalyst [49]. It has a good thermal, stable structure, mechanical stability, and a chemically hierarchical structure that originates from biomass. Biochar-based catalysts have the following distinctive characteristics: (i) heterogeneity, i.e., the reaction mixture can be easily isolated from other reactants; (ii) bifunctionality, i.e., transesterification and esterification are involved; (iii) recyclable; (iv) porous; (v) non-graphițable, i.e., it does not form crystal at high temperatures [50]. Comparing biochar-based catalysts with other solid-based catalysts, biochar has the advantages of being cost-effective, eco-friendly, easy to produce, reusable, and biodegradable.

Furthermore, biochar as a catalyst can be used in many different fields, including agriculture, environment, and energy, for biodiesel production, tar removal, waste management, production of syngas, production of chemicals, and removal of contaminants, etc. [45, 51]. Biochar is an excellent catalyst with several beneficial properties. Biochar, for instance, is catalytically active in cracking tar because of its presence of inorganic elements including Fe and K [47]. A biochar-supported metal catalyst can be synthesized by adsorbing metal precursors on its surface functional groups [52]. Despite this, biochar has some properties that preclude it from functioning as a catalyst, such as poor porosity and low surface area. Considering that biochar contains more functional groups, it must have a large surface area for catalysis. A functional group, such as OH, adsorbs norfloxacin. Adsorption of ammonium is possible through C∙O and ∙OH groups. To endow biochar with specific properties, it is necessary to develop a variety of modification strategies. Furthermore, several processes can be used to activate feedstocks, control synthesis conditions, functionalize materials on the surfaces, form composites with other materials [53], etc.

#### **1.4 Characteristics of biochar based catalyst**

In addition to its properties, biochar's potential for specific applications is dependent on both the biomass source and the conditions of preparation. Biochar, for instance, is suitable as an electrode material because it is electrically conductive and porous [54]. It has been proven that structurally bound nitrogen groups and high porosity biochar make superior supercapacitor electrode materials [55]. However, the intrinsic inorganics, matrix nature, and surface functionality of biochar have a significant influence on its catalytic performance.

#### *1.4.1 Bulk element and inorganics*

The carbon content of activated carbon from coal is approximately 80–95%; however, that content is lower for biochar (45–60 wt%) than carbon black (98%) [5]. Biochar also contains substantial amounts of hydrogen and oxygen. Another characteristic of biochar is that it contains small amounts of inorganic elements like potassium, sodium, calcium, magnesium, sodium, iron, and calcium. The nature of raw

#### *Biochar Development as a Catalyst and Its Application DOI: http://dx.doi.org/10.5772/intechopen.105439*

biomass greatly affects the amount and composition of inorganics. Woody biomass, as well as herbaceous and hydrophyte biomass, usually have a much lower inorganic content than biochar made from these sources [56, 57].

The inorganic components of biochar are crucial to many of the biochar's catalytic applications [47], including tar cracking [58], methane decomposition, and bio-oil upgrading [59].

#### *1.4.2 Chemistry of biochar matrix*

Amorphous crystalline sheets of high-conjugated aromatics make up most of the biochar matrix. As shown in **Figure 2**, these aromatic sheets are crosslinked randomly. In response to rising processing temperatures, biochar crystallites increase in size, and order is created throughout the entire structure [62]. The aromatic structure of biochar may also contain heteroatoms, including N, P, and S. These heteroatoms have a different electronegativity from the aromatic C, which results in biochar's chemical heterogeneity. This plays a key role in catalytic applications [58].

#### *1.4.3 Surface functional groups*

Comparing biochar to other carbon materials including (activated carbon and carbon black), **Figure 3** shows that it typically contains large numbers of surface functional groups. Biochar can be functionalized using its surface functional groups. Moreover, biochar has been shown to facilitate the loading of metal precursors onto metal catalysts as part of the synthesis of a metal catalyst supported by biochar [52]. Biochar-based catalysts can also work better for certain reactions if they contain some surface functional groups. Biochar-based solid-acid catalysts are typical examples. Kitano, Yamaguchi [63] demonstrated that sulfonated carbon is more effective at hydrolyzing cellohexaose, than sulfonic acid (SO3H)—bearing resins. Adsorption sites, in this case, were found in the carboxylic acid (COOH) and hydroxyl (OH) groups of phenolic groups in the carbon material. Researchers found that the combination of functional groups on biochar-based solid acids was efficient for hydrolyzing cellulose and 1,4-glucan.

**Figure 3.** *A porous biochar model with multiple functional groups (adapted from Yang et al [61]).*

#### **2. Preparation of biochar-based catalyst**

Biochar has been activated and functionalized in various ways to adjust its physicochemical properties, leading to enhanced reactivity in a range of processes and applications [48]. Impregnation and physical or chemical activation are the most popular methods. *In-situ* or post-synthesis methods are employed in such modifications. Biochar-based catalysts have the potential to be a feasible alternative to metalbased catalysts and carbon catalysts driven by fossil fuels. **Table 2** lists the types of biomass used to make biochar-based catalysts.

#### **2.1 Impregnation**

This technique involves mixing feedstock and metallic precursors (in-situ) into biochar structures to incorporate active metallic species into them [64]. With the use of biochar, lignin magnetite pellets were synthesized into zero-valent iron at 900°C [64]. It was possible to remove trichloroethylene by both adsorptive and degradative mechanisms due to the macro-porosity developed. Rice straw biochar was impregnated with cobalt nitrate (Co(NO3)2), then hydrothermally treated and calcined to produce the composite [65]. In comparison to pure biochar (43.0 m2 g−1, 0.081 cm3 g−1) and cobalt (II, III) oxide (CO3O4 (37.0 m2 g−1, 0.184 cm3 g−1), the composite showed greater SBET (62.7 m2 g−1) and total pore volume (0.207 cm3 g−1). The catalyst was shown to be effective for oxidatively degrading ofloxacin (over 90% removal in 10 min) using peroxymonosulfate (PMS). An X-ray photoelectron spectrometer (XPS) study revealed that the rich mesoporous support contains many CO∙OH groups, which are important for activation. The obtained pristine biochar may also contain metal species varying in amounts and characteristics, depending on the biomass source. Despite this, impregnation typically produces composites rather than carbonaceous biochar, so one could compare biochar with impregnated composites and exhausted catalysts.

#### **2.2 Physical activation**

A physical activation process involves exposing the pyrolyzed biochar materials to a streamflow control or carbon dioxide or a mixture of both when temperatures


**Table 2.**

*Production methods and feedstocks for biochars and biochar-based catalysts.*

exceed 700°C. Gaseous activation agents, depending on the degree of C∙H2O and/ or C∙CO2 gasification that occurs at such high temperatures, are capable of partially eroding carbon atoms in the as-prepared biochar matrix [66]. By physically activating the carbonized material, most of the reactive carbon parts can be eliminated and the enclosed pores in the biochar matrix can be opened and interconnected [67]. Consequently, the surface area of biochar increases significantly, resulting in an improved micropore structure and a lower mesopore content [68]. **Figure 4** illustrates the process for producing biochar-based catalysts.

Activated biochars differ significantly from one another in terms of a specific area, pore size distribution, and porosity based on the type of biomass, reaction parameters, and activating gas [66]. Lima et al. [62] for example, evaluated the effects of steam activation on the surface areas and porosities of different biochars, as well as their metal ion adsorptive capabilities. They found that steam-activating biochars at 800°C for 45 minutes dramatically increased the surface area and micropore volume from less than (5 m<sup>2</sup> g−1) to (136–793) m2 g−1. In addition, due to the increased porosity and surface area, these biochars were able to improve their metal ion adsorption performance to varying degrees after activation [69]. In addition, Kołtowski et al. [60] utilized steam and CO2 to activate biochar produced from the slow pyrolysis of willow. Their findings revealed that both steam and carbon dioxide activation considerably increased the porosity and surface area of biochar. Additionally, steamactivated biochar (840.6 m<sup>2</sup> g−1) and CO2-activated biochar (512.0 m2 g−1) showed significantly larger surface areas than those of unactivated biochar (11.4 m<sup>2</sup> g−1). In contrast with the CO2-activated biochar, steam-activated biochar was found to have higher specific surface areas and pores [60].

#### **2.3 Chemical treatment**

Chemical activation involves mixing freshly prepared biochar with activation agents (e.g., KOH, ZnCl2, K2CO3, H2SO4, H3PO4, etc.). The biochar is subsequently heated at high temperatures in an inert gas flow [70]. While the mechanism for chemical activation is still unclear, chemical activation is more corrosive than physical activation [71]. However, high temperatures can significantly enhance the corrosion properties of chemical activation substances. Aside from removing some carbon atoms from the biochar matrix, these chemicals might suppress tar formation and/or

#### **Figure 4.**

*Method involved in producing biochar-based catalyst.*

facilitate the formation of volatile compounds [67]. It was reported by Liu et al. [59], that chemical erosion and physical activation lead to large surfaces and high porosities in KOH-activated biochar, and metallic K intercalation. Chemical activation generally results in a higher activation efficiency than physical activation, and chemical activation may be performed at a relatively lower temperature, resulting in a more porous and higher surface area biochar [43]. Although chemical activation leaves biochar with improved surface area and porosity, it is usually necessary to wash it to remove impregnating agents and salts [50]. The use of chemical activation is, therefore, affected to some extent by several factors, including corrosion of equipment, chemical recycling, secondary pollution, etc. [43].

Several factors affecting the chemically activated biochar, including the temperature of activation, feedstock type, the type, and concentration of the activating agent, etc., are significant [66]. Biochar impregnated with KOH solution has been investigated by Dehkhoda et al. [65] to determine how activation temperature (685–700°C) influenced the electrosorption performance, porosity, and surface area. In their study, there was an increase in the surface area of the biochar from (1.66 m2 g−1) to (614– 990 m2 g−1), as well as its porosity, which increased from negligible to 0.6–0.9 m3 g−1. Additionally, as the temperature rises, a decrease in biochar surface area is observed, by collapsing and burning off the micropore walls or causing the formation of graphite-like structures in the matrix. Since biochar activated at 675°C contains more micropores and oxygen-containing functional groups, its overall electrosorption capacitance was more than twice as high as that of activated biochar at 1000°C [68].

#### **3. Application of biochar catalyst**

The growing discovery of biochar as a diverse material for catalytic activities has prompted preliminary study into the catalytic potential of biochar as well as applications in different processes.

#### **3.1 Biodiesel production on biochar catalysts**

It has been demonstrated that biodiesel can be used as a renewable alternative to traditional petrochemical-derived diesel [67, 72]. The application of traditional catalysts in the synthesis of biodiesel from biomass (vegetable oils) has been extensively explored. However, the manufacture of such catalysts necessitates the use of costly metal precursors. Because of their low cost and versatility, sulfonated biochars have been utilized to produce biodiesel. It has been demonstrated that sulfonated

#### *Biochar Development as a Catalyst and Its Application DOI: http://dx.doi.org/10.5772/intechopen.105439*

biochar can produce the maximum productivity (88%) of biodiesel products from vegetable oil in the esterification of FFAs (free fatty acids) and transesterification of TGs (triglycerides) carried out simultaneously at 100°C for 15 h [72–74]. It was observed that after five recycles of the catalyst, the output of methyl esters reduced from 88% to 80%, due to the leaching of ∙SO3H functional groups [74]. Using a biochar catalyst made from palm kernel shells to transesterify sunflower oil, Kostić et al. [69] investigated the catalytic activity. With the deposition of 3 wt% catalysts into a reaction, the production of methyl esters was 99% at 65°C [75]. The solid acid/ base biochar catalysts mentioned above resulted in a significant synthesis of biodiesel from a variety of edible oils. In contrast, both catalysts exhibited signs of deactivation after many re-uses in the laboratory. While transesterification was taking place, the base catalyst was contaminated by undesired secondary products formed by CaO and the feed oil interactions [75]. The ester output (from TGs and FFAs) is comparable to that obtained from non-biochar catalysts. However, to make biochar catalysts for biodiesel generation more realistic, the stability of biochar catalysts must be increased to prevent the need for post-treatment processes to remove S or Ca from the catalyst [75]. The biodiesel production efficiency of different biochar and non-biochar-based catalysts is shown in **Table 3**.


#### **Table 3.**

*A comparison of biochar and non-biochar-based catalysts for biodiesel production.*

#### **3.2 Biomass hydrolysis on biochar catalysts**

Biochar catalysis has been applied in biomass hydrolysis. The fact that most biochar-based catalysts are more effective than commercially available and traditional catalysts has long been recognized. According to Ormsby et al. [75], pinewood chips and peanut hulls that were sulfonated with H2SO4 were used as the raw materials for biochar. When used to hydrolyze xylan, the sulfonated pine chip-biochar catalyst demonstrated an 85% transformation rate in 2 h at 393 K. On the other hand, while having a greater surface area (1391 m2 g−1) than the biochar catalyst (365 m2 g−1), industrial activated carbon only achieved a 57% transformation in 24 hours [84]. Furthermore, biochar catalysts showed greater starting process rates for the hydrolysis of cellobiose and xylan when compared to other catalysts (activated carbon and Amberlyst-15) [84], indicating that they were more efficient than the other two catalysts. Moreover, the hydrolysis of maize stover, switchgrass, and prairie cordgrass

biomass was accomplished using a corn stover-biochar mixture [85]. Compared to a traditional homogeneous H2SO4 catalyst, the catalyst exhibited a stronger preference for glucose and xylose, confirming its superior efficiency in biomass hydrolysis. The existence of sulfonated corn stover-based biochar increased the production of glucose and xylose from lignocellulosic biomass [48]. The glucose output was 8–10% and the xylose yield was 23–41% when compared to the equivalent polysaccharide [85]. The findings were equivalent to those obtained from the hydrolysis of model substances using a similar catalyst: cellulose yielded 3% glucose and xylan yielded 40% xylose. This indicated that the biochar was able to sustain good efficiency even when exposed to contaminants and a complex matrix of biomass materials. The performance of different biochar based catalysts for hydrolysis is shown in **Table 4**.


*Hydroxymethyl furfural = HMF, reducing sugars = RSs, TON = turnover number, temperature = Temp, time = T, catalyst amount = Ccata, feedstocks concentration = CF, imidazolium chloride = IL-Cu; anhydrous MeCN = anhydrous acetonitrile. Source: Adapted from Shan et al. [90].*

#### **Table 4.** *Biochar catalyst for hydrolysis.*

#### **3.3 Production of biogas**

#### *3.3.1 Tar reforming (syngas synthesis)*

Tar reforming is the process of converting the hydrocarbon combination that is inevitably generated following the gasification and pyrolysis of biomass into useful syngas (combination of CO and H2). Syngas is a multipurpose intermediate and/ or beginning raw material for the synthesis of fuels and chemicals. As a result of this fact, several studies have investigated the potential involvement of biochar catalysts in the generation of syngas in recent years [58]. Biochar comprises catalytic centers that are similar to those found in traditional catalysts, such as dolomites (MgCO3·CaCO3), olivine ((Mg2+, Fe2+)2SiO4), and Ni- and alkali metal-based catalysts, could be efficient for tar reforming [78]. The switchgrass biochar that had been activated by KOH demonstrated the highest efficacy, with around 90% elimination of toluene. This was likely owing to the increased surface area of the switchgrass biochar. Iron calcined biochar [79] and nickel nanoparticle-embedded biochar [80] have also been shown to be efficient. Ren et al. [58] noted that the application of a biochar catalyst improved the quantity of syngas produced during biomass pyrolysis. At 480°C, it was discovered that the syngas output increased from 15 wt% to 46 wt% in the absence and presence of biochar catalyst respectively. According to Ren et al. [58, 81], the hydrogen content in syngas rose significantly with the addition of the biochar catalyst (27 vol%), in contrast to when the catalyst was not employed. A current investigation shows that biochar can be applied in the dry reforming process [82]. The dry reforming of CH4 was carried out on a tungsten carbide [83] Based on a biochar (WC-biochar) catalyst. As the CH4/CO2 ratio rose, the CH4 transformation reduced, while the CO2 transformation improved. Increases in the CH4/CO2 ratio and temperature resulted in greater H2 production, and the WC-biochar catalyst remained stable for 500 hours after being introduced into the system [82].

#### *3.3.2 Tar elimination*

The gasification of biomass is a viable sustainable energy pathway since it has the potential to enhance the generation of large quantities of syngas. A consequence of its synthesis, however, is the formation of condensable hydrocarbons (tar). Tars can accumulate in pipelines throughout a system, causing them to become clogged and potentially inhibiting downstream operations [84]. To commercialize biomass gasification for syngas generation, the elimination and/or mitigation of tar is a vital first stage in the procedure [85, 90]. In reality, catalytic tar cracking was carried out at 823–1173 K, with dolomite, olivine, and base metals including nickel [78], serving as catalysts. These conventional tar cracking catalysts, on the other hand, were susceptible to deactivation as a result of coking and contamination [91]. It has been attempted numerous times to degrade tars using a secondary reactor containing noble metal catalysts (e.g., platinum, palladium, and rhodium) [92], but the restoration of the catalyst has remained a difficult process. The introduction of an affordable catalyst for tar breakdown is therefore preferable in this situation. In this regard, biochar was found to be superior to traditional catalysts when used as a catalyst to remove tar [93]. The tar removal efficiency of biochar catalysts is summarized in **Figure 5**. The majority of investigations have relied on model processes of tar disintegration with toluene, naphthalene, and phenol. Moreover, the biochar-based metal catalysts (e.g., Nickel and Iron) outperformed the typical mineral catalysts in terms of tar removal

#### **Figure 5.**

*Evaluation of tar elimination using biochar-based catalysts at 973–1173 K (adapted from Lee et al. [84] with modifications).*

efficiency. For example, a catalyst constituted of a combination of NiO and woodbiochar eliminated 97% of the genuine tars formed during sawdust gasification, resulting in an improvement in syngas synthesis attributed to the catalytic reformation of the tars [94]. According to Shen et al. [79], bimetallic catalysts based on rice husk-biochar generated seven times fewer tars in the biomass combustion process than monometallic catalysts and raw biochars during the pyrolysis of biomass. The NiO-biochar catalyst combination remained stable for an 8-h time in the stream (TOS). One of the limitations linked to biochar and metal-biochar catalysts for tar reduction is the process temperature, as tar elimination occurs at >973 K. At reduced temperatures (i.e., 843 K) with the typical nickel catalyst, tar removal can be commenced [92], however, biochar is not yet efficient at these lower temperatures [95]. To overcome these restrictions and broaden the scope of biochar's application as a catalyst, future work must concentrate on overcoming these constraints.

#### **3.4 Wastewater treatment**

Due to its ability to remedy environmental pollutants, biochars are becoming highly significant for enhancing environmental quality in the world today [96]. Wastewater, which is a result of household, commercial, and agricultural operations, has long been a global concern since it affects everyone. Biochars offer a significant deal of promise for use in wastewater remediation applications. Biochar's applications in the cleanup of different wastewaters are the primary focus of this section.

#### *3.4.1 Industrial wastewater remediation*

Industrial wastewater originates from a variety of sources. In addition, heavy metals and organic contaminants are the most prevalent contaminants in industrial wastewater. It has been demonstrated that biochars can be used in the treatment of industrial effluent. It is possible to cast membranes, beads, and solutions from a biochar-chitosan combination that has been cross-linked. It has the potential to be used efficiently as an adsorbent for the adsorption of heavy metals in industrial wastewater. The amount of

#### *Biochar Development as a Catalyst and Its Application DOI: http://dx.doi.org/10.5772/intechopen.105439*

chitosan and biochar used in the adsorption of Cu, Pb, As, Cd and other heavy metals in industrial wastewater would depend on the ratio of the two materials [97]. Gliricidia biochar has shown promise in the elimination of crystal violet (CV) from aquatic environments in dye-based industries. A biochar's pH value, surface area, and pore volume are all important factors to consider throughout the CV sorption process [98]. Biochar made from bagasse was employed to absorb lead from the effluent of the battery production sector. The maximal adsorption ability can attain 13 mg/g, and the adsorptive activity is dependent on the moderate pH value, contact time, and concentration [99]. So far, the majority of the trials on the utilization of biochar in the clean-up of contaminants from industrial wastewater have been carried out in a laboratory environment; however, additional study and deployment in the actual situation are required.

#### *3.4.2 Treatment of municipal wastewater*

Biochar can be employed alone or in combination with other techniques for municipal wastewater treatment, resulting in the retrieval of labile nitrogen and phosphorus [100]. Engineered biochar containing aluminum oxyhydroxides (AlOOH) was used to recover and restore phosphorus from tertiary remediated wastewater [101]. The adsorption strategy of phosphorus is mostly based on electrostatic interaction. Phosphorus adsorbed on manufactured biochar has the potential to be used as a slow-release fertilizer for agricultural activities. Biochar generated from digested sludge was employed as an adsorbent for the elimination of NH4 from municipal wastewater. Biochar produced at 723 K has the maximum NH4 reduction capability due to its increased functional group density and surface area, and the procedure is governed by chemisorption [102]. This shows that biochar derived from waste sludge can be utilized to ozonate refinery effluent and achieve a significant reduction rate of total organic carbon (TOC) [103].

#### *3.4.3 Wastewater treatment in the agricultural sector*

Because of the rapid development of the agriculture sector, agricultural pollution is getting extremely serious. As a result, pesticides and toxic heavy metals are released into croplands in large quantities, the situation is becoming increasingly worrisome [104, 105]. The use of biochar and its modified forms in the remediation of agricultural wastewater pollution has been investigated. Pesticides such as atrazine and pentachlorophenol are two of the most often used in agriculture. Adsorption of atrazine and imidacloprid from agricultural wastewater by rice straw biochar and phosphoric acid-modified rice straw biochars is much higher than that of adjusted rice straw biochar [106]. Corn straw and soybean biochars both exhibit strong atrazine reduction potentials, with the adsorption efficiency owing mostly to the pH value and pore volume of the biochars [107]. Steam-activated biochar is efficient at eliminating sulfamethazine, and the rate at which it absorbs the substance is reliant on the pH value [108]. The presence of hazardous heavy metals in agricultural wastewater is yet another widespread issue.

#### **4. Emerging advances in the applications of biochar catalyst**

Recent advancements in the use of biochar for processes other than agriculture have been linked to biochar's various properties. Among other characteristics that are suitable for electrode materials, biochar has high porosity and high electrical conductivity [54]. It is preferred to use biochar with structurally bound nitrogen groups and high porosity as electrode materials for supercapacitors [55]. During catalysis, surface functionality, matrix nature, and intrinsic inorganic components are all important factors [49]. Unlike activated carbon derived from coal, biochar has a considerable amount of other organics present in it based on the biomass feedstock. These organics aid its compatibility, utilization, and effectiveness for varying applications than activated carbon.

There are several advantages to using biochar as a catalyst or catalyst support. Firstly, since biomass resources are sustainable and synthesis techniques have been developed, the process for producing biochar is simple and inexpensive. Secondly, the physicochemical properties of biochar can be easily tuned through a variety of methods. As a third consideration, biochar may be of interest in catalytic applications because of its surface functional groups, a hierarchical structure derived from the biomass matrix, and the presence of inorganic species [48]. Additionally, active metals and biochar support may, in some cases, have synergistic effects on catalysis [49].

#### **4.1 Energy storage and conversion**

Due to excess energy generation, energy storage is becoming more popular in some developed countries, and stored energy can also be used as a backup in the event of an emergency. The increased use of electric vehicles necessitates the continuous development of batteries with greater energy storage capacity. Despite continuous battery development, there are times when an unplanned situation may occur in electric vehicles. To alleviate such a situation, supercapacitors, which are energy storage devices primarily made of carbon materials, have been applied as continuous power sources in digital communications systems and electric vehicles. Because of its wide availability and low environmental impact, carbon materials with a high surface area and a rich porous structure are the primary raw materials for making super-capacitors [98]. It is crucial to the development of the supercapacitor industry to produce attractive, high-quality carbon materials at a reasonable price [99].

The utilization of biochar as material for supercapacitors has been tested by researchers with incredible results obtained. Biochar is made from paper cardboard and woody biomass. Based on the pyrolysis of woody biomass, the biochar supercapacitor electrodes exhibited a potential window of about 1.3 V, and fast chargingdischarging behaviors with about 14 F/g gravimetric capacitance [100]. The authors also enhanced the performance of woody biochar by activating it with nitric acid. According to the researchers, the nitric acid treatment helped increased the capacitance from 14 to 115 F/g with 5000 usage cycles [100]. Likewise, Liu et al. [98] also created a high-performance supercapacitor out of biochar-derived carbon monolith, which was created by pyrolyzing poplar wood at 900°C for 6 h and then surface-modifying with nitric acid. The supercapacitor was discovered to have a highly consistent structure as well as a high porosity. The maximum specific capacitance was high (234 F/g) and cyclic stability was excellent [98, 99].

With the recent development of direct carbon fuel cell (DCFC) which converts carbonaceous material directly into electricity. The DCFC directly oxidizes solid carbon to produce electricity by using the chemical energy contained therein. Fuel utilization can reach nearly 100% if fuel feed and product gases are separated easily. The use of biochar as an energy source for this fuel cell has shown tremendous results. In a study by Kacprzak et al. [101], nine different carbonaceous fuels were tested, including

commercial graphite, a carbon black, two commercial types of hard coal, and four biochars made by the authors, and one commercial biochar. At 0.5 V, commercial biochar had the second-highest current density (64.22 mA/cm2 ) and the third-highest power density (32.8 mW/cm2 ). Biochar produced in the laboratory had a high current density (36–44.6 mA/cm2 ) and power density (18–22.4 mW/cm2 ) [102].

#### **4.2 Challenges and prospects of biochar-based catalyst applications**

The use of biochar just as any other material has some limitations in its application for energy storage, conversion, and electrocatalyst. In terms of energy storage, the performance efficiency of tested biochar is still low when compared to its counterparts, though the biochar is easy to access and economical. Likewise, in the use of biochar in DCFC, it has been reported that upon consumption of the carbon content, the ash content present in biochar blocks the active surface area thereby impeding the effectiveness of the whole process [102]. In terms of reusability as a catalyst, further work still needs to be done as biochar from some feedstocks is reusable after the second attempt. For electrochemical oxidation of fuel, an ideal anode should have a large surface area, high porosity, and a continuous frame to ensure mechanical strength. Boosting the DCFC's power output and durability is therefore possible by improving its anode material [99].

Along with biochar's widespread use in wastewater remediation, scientists should consider its possible adverse impact on the ecosystem. To effectively employ biochar, one of the most significant features that must be considered is its capacity to maintain its stability throughout time. The aromaticity and extent of aromatic condensation of biochar are two factors that influence the stability of biochar [103]. When biochar is employed for wastewater detoxification, the possible emission of carbon from the biochar can cause the carbon concentration of the solution to be treated to rise. Moreover, the discharge of heavy metals from biochar formed from sludge is a possibility, particularly for biochar generated from sludge. Huang et al. [105] demonstrated that the dissolution of organic materials from biochar into an aqueous solution is caused by the biochar's instabilities. In addition, it was discovered that the stability of the biochar deteriorated after multiple cycles when it was employed as a support for a catalyst. This can be attributed to variations in the carbon framework of the biochar. It is usually acknowledged that the stability of biochar relies on the type of the starting feedstock as well as the experimental settings utilized during its thermal transformation. As a result, it is required to establish a relationship between these two factors and the stability of the biochar. Another significant element to consider is the renewal and restoration of biochar after it has been utilized. The adsorption procedure is characterized by the transition of pollution from the liquid stage to the solid material/adsorbent phase in most cases. As a result, it is critical to transforming the hazardous pollutants that are bonded to biochar into non-toxic conditions to control them effectively [101].

#### **5. Challenges, prospects, and future perspectives**

#### **5.1 Challenges and prospects of effective application of biochar-based catalyst**

The use of biochar-based catalysts can be beneficial in several catalytic processes, including biodiesel production, bio-oil up-gradation, reforming, and

various organic reactions involving specialty or functional chemicals. These are currently in their infancy and must be scaled up. Biochar production systems must be set up on an industrial scale to enable the scaling up of these processes. The biggest barriers to scaling up biochar production are multiple competing end-users, as well as the collection and transportation of raw materials to the facilities that manufacture biochar. Homagain [107] studied the sensitivity of transportation distance and distinct carbon offset values and found that the system is financially viable at 200 km with good biomass availability. Furthermore, the seasonal biomass production cycle makes it difficult to maintain a steady supply of sustainable and reliable fuel.

The moisture content and particle size are other critical parameters in the synthesis of biochar. The biochar production method requires a lot of energy to process feedstocks with a high moisture content or large particle size. During biochar production, it is necessary to pre-process feedstock by drying and reducing its size. The heat resistance of feedstocks, on the other hand, limits heat transfer during biochar formation. Due to temperature differences, this phenomenon causes unconverted feedstock to accumulate on the inner walls of reactors, posing a significant barrier to the widespread production of uniform biochar [103].

Biochar's properties can also be difficult to fine-tune once it has been produced to achieve the required transformation. Following the proper design of biochar-based catalysts, the resulting materials will have real-world applications and will be able to replace catalysts that are expensive, non-renewable, and harmful to the environment. These conditions can be met by conducting mechanistic investigations during the char activation/synthesis/loading of necessary metals and catalytic processes. It is critical to comprehend two key factors in the catalytic process. The first is the interaction between biochar's physicochemical properties and its catalytic activity. The second step is to tune physicochemical parameters during the char production and activation process based on catalytic activity. Regarding this, the investigation of high surface area, active sites, and optimal pores is critical to managing the combined impacts of important production process variables (e.g., reagent gas, duration, heating rate, and temperature) and activation process variables (e.g., chemical, and physical). Just a few experiments have been conducted to control the physicochemical parameters of biochar for catalytic applications. However, the biorefinery of the future will require a single-step method for producing biochar with effective porous structure and functionality that is closely related to the production of biochemicals, biogas, and biofuels.

#### **5.2 Future perspectives**

Although biochar has many applications, biochar-based catalysts are still in the very early stages of development. Therefore, it is imperative to develop a method that can maximize catalytic activity. Researchers are currently exploring the modifications that can be carried out on biochar-based catalysts to apply them in future fields such as catalysis, environmental pollution, energy storage and conservation, and even chromatography.

Laboratory research is still underway for biochar-based catalysts. A purposedriven synthesis and modification will be necessary for the future of an industrial application. Mechanistic studies may help to achieve this. A first step would be to investigate how biochar's catalytic properties relate to its physicochemical properties. To accomplish this, advanced characterization techniques of catalytic

#### *Biochar Development as a Catalyst and Its Application DOI: http://dx.doi.org/10.5772/intechopen.105439*

materials can be combined with theoretical modeling of the mechanisms involved. Second, it is critical to determine how biochar's properties are affected by synthesis conditions and feedstock. It is extremely difficult to work with biomass because of its complex composition and complex formation mechanism. The application of advanced characterization techniques, such as pyrolysis/gas chromatography/mass spectrometry (Py/GC/MS), and thermogravimetric analysis/Fourier-transform infrared spectroscopy/mass spectrometry (TGA/FTIR/MS), is potentially vital for the future.

In terms of process optimization, the role of catalysts in biochar synthesis must be given much more thought. The presence of some inorganic species in biomass feedstock can catalyze pyrolysis. However, their autocatalysis is not enough to ignite the process. A catalyst must achieve at least one of the following goals: (1) to reduce reaction temperature or residence time so that biochar can be produced more efficiently; (2) to make biochar with desirable properties in a single step instead of having modification and synthesis done separately. In the future, we may be able to produce biochar-supported catalysts directly from biomass using catalysts that can produce effective functional groups and porous structures in a single step. A biomass refinery would also be able to produce biofuels and biochemicals in close coordination with manufacturing biochar-based catalysts, allowing for a more integrated and environmentally sustainable process for using biomass.

Biochars intended for use as catalysts require a functionalization and/or activation process because of their limited porosity, surface area, and surface functional groups. According to the activation technique, biochar can have varying physicochemical properties, such as surface area or porosity. Activated biochar can be endowed with specialized properties via the addition of functional groups or substances, such as selectivity, catalysis, and selective adsorption. Although biochars vary significantly according to the type of biomass they are produced from, as well as their production conditions and functionalization or activation. Future research should focus on the production of biochar with stable properties on an industrial scale.

#### **6. Conclusion**

The use of biochar-based catalysts in environmental applications has excellent catalytic properties. Recent achievements of biochar catalyst preparation procedures, as well as their performance, were examined from a range of applications. Additionally, the catalytic properties of biochar were examined further by its production and activation methods. Through various chemical and/or physical treatments, biochar can be modified in terms of morphology and surface functionality. Therefore, biochar has a strong potential for replacing costly and non-renewable conventional catalysts.

It has been demonstrated that biochar-derived catalysts are effective in a variety of reactions, including the production of biodiesel from biomass, removal of tars from bio-oil and syngas, and production of syngas. However, biochar catalyst properties (including surface functionality, surface area, porosity, and acidity) vary widely with biomass origin, biochar synthesis conditions, and pre/post-treatment. Yet, there is limited information about how biochar's properties can be controlled to enable its catalytic applications. Therefore, further research is needed to develop the catalytic properties of biochar to design active, stable, and selective biochar catalysts. Also, if biochar is to be considered as an industrial heterogeneous catalyst, the development

of a method that allows for the manufacture of biochar on an industrial scale is extremely desirable. For large-scale production, it is also challenging to secure stable sources of raw biochar materials. To meet these challenges, biochar catalysts must be stimulated and facilitated to be used in real-world applications to replace costly, non-environmentally benign catalysts, which have been used for a wide range of applications until now.

### **Author details**

Stephen Okiemute Akpasi1 \*, Ifeanyi Michael Smarte Anekwe2 , Jeremiah Adedeji3 and Sammy Lewis Kiambi1

1 Department of Chemical Engineering, Durban University of Technology, Durban, South Africa

2 School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa

3 Department of Chemical Engineering, School of Engineering, University of KwaZulu-Natal, Durban, South Africa

\*Address all correspondence to: stephenakpasi48@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Biochar Development as a Catalyst and Its Application DOI: http://dx.doi.org/10.5772/intechopen.105439*

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Section 4
