**Abstract**

Biochar represents the rich carbon residues that remains after thermally pyrolyzing or liquefying different biomass types in an oxygen-free environment. The availability of animal and agricultural wastes makes the biochar a low-cost product. It is, as a carbon-rich product, resistant to mineralization and decomposition. Biochar can be used as a multifunctional material in many applications in the environmental and agricultural sectors. Recently, a growing interest for the use of biochar in different fields is rising because of its use as a sorbent for organic and nonorganic contaminants from aqueous solutions. In this chapter, recent studies on pyrochar/hydrochar production, characterization, and phosphate adsorption are reviewed and summarized. The remediation technologies for phosphate removal from contaminated water using biochar are also discussed. The effects of reaction temperature and initial solution pH on phosphate adsorption onto biochar are compared. In addition, we highlighted the models that are used for adsorption kinetics and adsorption isotherms.

**Keywords:** biochar, hydrochar, pyrochar, pyrolysis, hydrothermal carbonization, phosphate, filtration

## **1. Introduction**

Phosphorus (P) is an essential nutrient for the growth of plants; however, its excessive release into runoff water can impose a danger on environmental health [1]. Phosphor soluble in water is present under three forms, ortho-, poly-, and organic phosphate. Orthophosphate comprises HPO4 <sup>−</sup>, H2PO4 <sup>2</sup><sup>−</sup>, H3PO4, and PO4 3− which encourages the evolution of aquatic microorganisms and macroorganisms leading to eutrophication. Eutrophication is caused by the presence of excessive phosphate amounts and has adverse impacts on the water ecosystem. In fact, even low amounts of phosphate 0.02 mg/L can cause algae to grow leading to reduced oxygen in water that could kill fish and damage wildlife [2]. The municipal sewage phosphate concentration is in the 4–15 mg-P/L range, while it exceeds 10 mg-P/L in industrial wastewaters. Consequently, phosphate concentration from wastewater needs to be reduced prior to the discharge in water bodies. On the other hand,

phsophorus is a non-renewable source available in limited quantities in nature with Morocco owning approximately 75% of the market [3]. There is an uncertainty about the lifetime of the remaining reserves and their accessibility. Studies have shown that P reserves are limited and will deplete soon with the increasing use of phosphorus as a fertilizer for agronomic production. Thus, it is crucial to develop effective phosphate removal technologies from aqueous solution.

Many technologies have been studied for phosphate removal from industrial and municipal effluents; these have been categorized into chemical, physical, and biological wastewater treatments. Studies have proven the effectiveness of biological and chemical treatments. These treatment methods use plants such as algae or chemicals such as alum, lime, and ferric salts to remove phosphorus from water before discharge into water bodies. Studies have shown that 97% of the total phosphorus (TP) could be removed from aqueous solutions with the use of phosphate biological treatment methods which does not present the risk of adding chemical contaminants to the water. However, the phosphate removal efficiency may be low due to its sensitivity to operation conditions [1]. However, other treatment method costs remain expensive, and some of them necessitate continuous checking of the operating conditions in order to prevent the introduction of new contaminants into the water. Thus, developing a cost-effective technology to prevent eutrophication and manage phosphorus recovery will prevent the shortage of this important element as it can be recycled and reused if it is successfully recovered from wastewater.

Adsorption presents a low-cost and efficient method for phosphorus removal due to its cost-effectiveness, accessibility, and performance. However, the high cost of traditional adsorbents (e.g., active carbon and anion exchange resins) and the difficulty of disposal are problems that limit their use. Thus, researchers study the development possibility of low-cost adsorbents that could come from cheap and available products such as biomass. Biochar has been studied as a prospective adsorbent for its properties including low cost, availability, stability, high porosity, and non-costly preparation and upgrading [2]. However, powder carbon-based biochars were identified to be difficult to separate from aqueous solutions which requires the use of magnetic separation that requires the addition of magnetic iron oxide through chemical co-precipitation [4].

Biochar has many advantages over traditional adsorbents such as being environmentally friendly, recyclable, low cost, easy to prepare, and having a high porosity. Consequently, researchers have studied the potential application of biochar as a phosphate adsorbent from wastewater [5] and its use as a fertilizer for soil quality enhancement. However, non-modified biochar with a surface charged negatively has a limited adsorption capacity for anionic pollutants which requires impregnating some metals on the biochar surface (Fe, Mg, Al, etc.) [2]. The preparation process of magnetic adsorbents is considered complicated and costly due to the complicated steps involved starting by the precursor preparation, oxidation of activated carbon, and then iron grafting [6]. For this reason, biochar use as a phosphate adsorbent from wastewater needs to be examined. Factors affecting the effectiveness of the anionic pollutant adsorption such as the functional site amount, affinity, dispersion, and surface accessibility need to be considered [7]. Some researchers studied the phosphate adsorption capacity of modified biochar. Junk et al. prepared magnetic biochar by magnesium ferrite impregnation through co-precipitation of Mg and Fe and pyrolysis and found to have an adsorption capacity of 487.99 mg P/g where P is the symbol of phosphorus [8, 9]. Junk et al. adsorption rate is considered among the highest when compared to other biochars adsorption rates 41.16 mg P/g, 125.40 mg P/g, 135 mg P/g [13, 17, 40]; this is shown later in Section 4 [7].

Other techniques are also employed to improve biochar surface characteristics for better adsorption. These treatment methods can be divided into gas phase

**73**

in **Figure 1**.

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater*

P is a vital nutrient for the growth of aquatic organisms, plants, and animals [12, 13]. It also plays an essential role as a limiting nutrient, preventing the development of algae and aquatic plants in the water ecosystem [14]. TP amount needs to be less than 0.03 mg/L, while phosphate level between 0.005 and 0.05 mg/L as an excessive amount of phosphate in water bodies causes eutrophication. Eutrophication leads to the destruction of water ecosystem including rivers, lakes, and seas causing harm to human health and an increase to the water treatment cost [15]. Natural phosphate reserves are limited and will be depleted in 50–100 years [16]. Thus, phosphorus recovery from phosphate-rich wastewater can simultaneously solve the

Biomass HTC is performed at temperatures ranging from 160–800°C. It is categorized into low-temperature reactions (lower than 300°C) and high-temperature reactions (300–800°C). During high-temperature HTC, the principal reaction is gasification, and the products are gases including hydrogen and methane [18] while carbonization takes place at low temperatures. Gasification favors the production of

Hydrogen formation is endothermic and methane formation is exothermic. Therefore, at high temperatures hydrogen formation exceeds methane as illustrated

Hydrothermal carbonization is used for high moisture content biomass contrarily to pyrolysis and gasification that deals with low moisture content biomass [19]. Many factors affect the hydrochar characteristics. These include the waterto-biomass ratio, reaction temperature, residence time, and pressure. Hydrochar is defined as the product of HTC reaction and has a 40–70 wt% yield. Hydrochars have lower yields but high higher heating value (HHV) than low-temperature pyrolysis (LTP) pyrochars [20]. The ultimate analysis confirmed these findings as it shows hydrochars having high carbon contents and low oxygen contents than pyrochars. Pyrochars have higher yields than hydrochars and thus higher-energy yields despite their lower HHVs. This indicates that biomass experienced a deep carbonization and decomposition in the LTP process. Oliveira et al. [21] showed that deeper carbonization of biochar can be achieved by combining different

C6 H12 O6 + 6H2O 6CO2 + 12 H2 (1)

C6 H12 O63 CH4 + 3CO2 (2)

phosphorus depletion issue and improve water quality [17].

**3.1 Biochar production from hydrothermal carbonization**

**3. Biochar production through pyrolysis and hydrothermal** 

and liquid phase. Gas phase uses steam or carbon dioxide for achieving physical activation, whereas liquid phase uses chemicals [10]. Both liquid phase and gas phase treatment methods use activation temperatures higher than 450°C. Chemical activation has advantages over physical activation due to its less cost and time along

*DOI: http://dx.doi.org/10.5772/intechopen.92612*

with higher char porosity [11].

**2. Phosphate in water bodies**

**carbonization (HTC)**

methane and hydrogen through

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater DOI: http://dx.doi.org/10.5772/intechopen.92612*

and liquid phase. Gas phase uses steam or carbon dioxide for achieving physical activation, whereas liquid phase uses chemicals [10]. Both liquid phase and gas phase treatment methods use activation temperatures higher than 450°C. Chemical activation has advantages over physical activation due to its less cost and time along with higher char porosity [11].
