**2. Biochar increases soil pH and soil organic carbon content and affects PTMs phytoavailability**

The effects of pH rising in soils are significantly influenced by biochar addition more than by other practices such as liming [28]. **Table 1** shows some of the main characteristics of BCs (pH included) as affected by feedstock sources and pyrolysis temperature. Biochar is superior to lime to remediate PTMs-polluted areas, mainly because acidic conditions can lead to the leaching of metals and threatening of groundwater [28]. Biochars can supply OC and raise soil pH, but lime only increases soil pH. Hence, poultry litter-derived biochar (PLB) proves itself as very effective in immobilizing Cd, even under strong acidic conditions, thus preventing Cd leaching


### **Table 1.**

*Summary of some biochar properties as affected by feedstock sources and pyrolysis temperature.*

*Applications of Biochar for Environmental Safety*

abandoned.

acid soils.

Amending soil with biochar has been practiced for a long time. The high fertility of anthropogenic dark earth soils known as 'Terra Preta de Indio' in the Amazon basin has been related to the high content of charred materials [11–13]. Historically, the source of char in these soils has been considered as a disposal of charcoal from domestic fires and the practice of slash and char agriculture by Pre-Columbian Amazonian Indians [11, 14]. Hence, these soils have remained fertile and rich in biochar derived C stock for hundreds to thousands of years after they were

In addition to the role of biochar in increasing the C sequestration and influencing the reduction of CO2 emissions, biochar has been shown to enhance soil quality and to stabilize PTMs [15]. Biochar has a potential benefit for improving soil fertility [16, 17], improving soil properties such as pH [11–13, 18], cation exchange capacity (CEC) and water holding capacity [19], enhancing plant growth [20], and reducing nutrient leaching losses [21]. The significant amount of calcium (and magnesium) carbonate (Ca/MgCO3) in BCs enables them to function as lime materials providing Ca and Mg to plants and neutralizing acidity when applied to

The role of biochar in improving soil pH, organic carbon (OC), and CEC was also highlighted by [16]. Moreover, biochar can immobilize PTMs (immobilization is the reduction of the potential migration of PTMs to plants, or reduction of phytoavailability) such as cadmium (Cd), lead (Pb), and zinc (Zn) and thereby to reduce the phytoavailability of PTMs (concentration of PTMs in plant parts, or contents of PTMs in soils available to plants) to plants in contaminated soils, notably because it raises the soil pH [18, 22] and increases CEC and OC [23]. Many studies also found biochar application promotes the ability to remove organic contaminants [24, 25]. Because of its porous structure and diverse functional groups [26], biochar has been widely used in the field of agriculture and environmental protection [27] due to its ability to improve soil health and crop yields, and sequestering carbon, immobilizing PTMs and adsorbing organic

For these reasons, studies on biochar land-application have exponentially increased in the last 20 years (**Figure 1**). During the same period (1999–2018), the word 'potentially toxic metal' or 'heavy metal' places itself in the **top 5** within the 25 keywords used in biochar researches, numbering **308** publications [6]. Therefore, this chapter is to provide a summary of the most recent studies on biochar use to improve soil quality and to immobilize the phytoavailability of PTMs to plants of agricultural importance. The main goal is to improve our

*Number of publications (NP) of biochar studies since 1999 (adapted from [6]). RMSE: Root mean square error* 

pollutants such as polycyclic aromatic hydrocarbons (PAHs).

**178**

**Figure 1.**

*value of the exponential model adopted.*

to the groundwater [28]. Besides raising the soil pH, the enhanced OC provided by biochar addition contributes to a decrease in the phytoavailability of PTMs by reducing metal mobility due to bonding metals into more stable fractions [29, 30], such as organic matter-bound and/or highly stable organic complexes which are not readily dissolved by water. The increase of both pH and OC also contributes to a higher CEC, then resulting in a higher PTMs adsorption [31].

The application of orchard prune-derived biochar (OPBC) to mine tailings reduced phytoavailable (DTPA-extractable) concentrations of Pb, Cd, and Zn [32]. Rice straw-derived biochar reduced Cd concentration in the plant available soil fraction grown in a greenhouse [30]. Biochar addition also showed a potential in reducing Cd and Pb accumulation in ryegrass (*Lolium perenne*) shoots, thus presenting a viable option for safe cultivation in PTMs-polluted soils [29]. Recent studies [22, 33] have found the ability of biochar to reduce the phytoavailability of PTMs (Cu, Zn, Pb, Cd, Mn, and Ni) to ryegrasses. Particularly, the study conducted by [22] elucidated that soil pH and OC increases as a function of biochar application rates played a big role in PTMs (Zn, Pb, and Cd) immobilization, mainly by forming stable (and undissolved) complexes with hydroxyls (OH<sup>−</sup>) (Zn/Pb/Cdx(OH)y) and surface functional groups, respectively. **Figure 2** summarizes their findings and illustrates the importance of biochar application as a soil remediation technique. In another study, the concentrations of Cu, Pb, and Zn decreased as the rates of BCs applied increased, with a significant effect for amendments >1% w/w applied. Especially, the phytoavailability of PTMs decreases gradually with time when the soil is amended by 5% or 10% w/w of biochar [18].

It has been shown that the addition of sugar cane bagasse-derived BC (SCBC) decreased the phytoavailable Pb fraction by 97%, and that the PTM uptake by maize plants decreased with increasing the level of applied BC [34]. The authors attributed such results to an enhancement of soil pH and soil organic carbon (SOC) because of BC addition. In addition, an unpublished work conducted by Antonangelo and Zhang has also revealed how impacting is the increase of soil pH and SOC, as biochar application rates increase, on PTMs (Zn, Pb and Cd) mobility. The study was carried out with no plants and the application rates of poultry litter- (PLB) and switchgrass-derived biochar (SGB) ranged from 0 to 8% w/w

### **Figure 2.**

*Potentially toxic metal (heavy metal) immobilization to ryegrass shoots and roots as a function of biochar application rates. Diagram was created from the work of [22].*

**181**

**Figure 3.**

*w/w of biochar.*

*The Use of Biochar as a Soil Amendment to Reduce Potentially Toxic Metals (PTMs)…*

in multi-metal contaminated soil. Potentially toxic metal phytoavailability was assessed by using two extraction methods (NH4NO3 and DTPTA) after 10 weeks of soil + biochar incubation under laboratory room temperatures. The results of PTMs contents in the extracts and SOC (%) as a function of biochar application rates are summarized in **Figures 3** and **4**, and **Table 2**. The pH increase from 6.5 to 8.0, as shown in **Figure 3**, is a consequence of the increased BCs (SGB and PLB) application rates from 0 to 8%, so is the SOC increase, as shown in **Figure 4**. **Table 2** highlights the significant negative correlations (inverse relationship) of pH and SOC (independent variables) with phytoavailable PTMs in the filtered extracts

The immobilization of Cu, Pb, Cd, and Ni by BCs was attributed to the quantity of surface oxygen-functional groups, which is directly related to the amount of carbon (C) present in the biochar composition [35]. Uchimiya et al. [36] reported that biochar enhanced Cu sorption in a sandy loam soil primarily by cation exchange mechanism, enhanced by the soil C increase. Hence, biochar addition increased the sorption capacity of the soil matrix for both organic and inorganic contaminants [37]. However, BCs may favor the availability of some PTMs such

*Relationship between pH and metal concentrations in NH4NO3 and DTPA extracts after 10 weeks of biochar incubation. Points were plotted from the whole dataset of measurements. One outlier from three replicates (n = 3) was removed, when detected, by using IML and UNIVARIATE (ROBUSTSCALE) procedures of the SAS program. pH ranged from 6.5 to 8.0 as biochars (SGB and PLB) application rates increased from 0 to 8%* 

*DOI: http://dx.doi.org/10.5772/intechopen.92611*

(dependent variables).

### *The Use of Biochar as a Soil Amendment to Reduce Potentially Toxic Metals (PTMs)… DOI: http://dx.doi.org/10.5772/intechopen.92611*

in multi-metal contaminated soil. Potentially toxic metal phytoavailability was assessed by using two extraction methods (NH4NO3 and DTPTA) after 10 weeks of soil + biochar incubation under laboratory room temperatures. The results of PTMs contents in the extracts and SOC (%) as a function of biochar application rates are summarized in **Figures 3** and **4**, and **Table 2**. The pH increase from 6.5 to 8.0, as shown in **Figure 3**, is a consequence of the increased BCs (SGB and PLB) application rates from 0 to 8%, so is the SOC increase, as shown in **Figure 4**. **Table 2** highlights the significant negative correlations (inverse relationship) of pH and SOC (independent variables) with phytoavailable PTMs in the filtered extracts (dependent variables).

The immobilization of Cu, Pb, Cd, and Ni by BCs was attributed to the quantity of surface oxygen-functional groups, which is directly related to the amount of carbon (C) present in the biochar composition [35]. Uchimiya et al. [36] reported that biochar enhanced Cu sorption in a sandy loam soil primarily by cation exchange mechanism, enhanced by the soil C increase. Hence, biochar addition increased the sorption capacity of the soil matrix for both organic and inorganic contaminants [37]. However, BCs may favor the availability of some PTMs such

### **Figure 3.**

*Applications of Biochar for Environmental Safety*

to the groundwater [28]. Besides raising the soil pH, the enhanced OC provided by biochar addition contributes to a decrease in the phytoavailability of PTMs by reducing metal mobility due to bonding metals into more stable fractions [29, 30], such as organic matter-bound and/or highly stable organic complexes which are not readily dissolved by water. The increase of both pH and OC also contributes to a

The application of orchard prune-derived biochar (OPBC) to mine tailings reduced phytoavailable (DTPA-extractable) concentrations of Pb, Cd, and Zn [32]. Rice straw-derived biochar reduced Cd concentration in the plant available soil fraction grown in a greenhouse [30]. Biochar addition also showed a potential in reducing Cd and Pb accumulation in ryegrass (*Lolium perenne*) shoots, thus presenting a viable option for safe cultivation in PTMs-polluted soils [29]. Recent studies [22, 33] have found the ability of biochar to reduce the phytoavailability of PTMs (Cu, Zn, Pb, Cd, Mn, and Ni) to ryegrasses. Particularly, the study conducted by [22] elucidated that soil pH and OC increases as a function of biochar application rates played a big role in PTMs (Zn, Pb, and Cd) immobilization, mainly by forming stable (and undissolved) complexes with hydroxyls (OH<sup>−</sup>) (Zn/Pb/Cdx(OH)y) and surface functional groups, respectively. **Figure 2** summarizes their findings and illustrates the importance of biochar application as a soil remediation technique. In another study, the concentrations of Cu, Pb, and Zn decreased as the rates of BCs applied increased, with a significant effect for amendments >1% w/w applied. Especially, the phytoavailability of PTMs decreases gradually with time when the soil is amended by 5% or 10% w/w of

It has been shown that the addition of sugar cane bagasse-derived BC (SCBC) decreased the phytoavailable Pb fraction by 97%, and that the PTM uptake by maize plants decreased with increasing the level of applied BC [34]. The authors attributed such results to an enhancement of soil pH and soil organic carbon (SOC) because of BC addition. In addition, an unpublished work conducted by Antonangelo and Zhang has also revealed how impacting is the increase of soil pH and SOC, as biochar application rates increase, on PTMs (Zn, Pb and Cd) mobility. The study was carried out with no plants and the application rates of poultry litter- (PLB) and switchgrass-derived biochar (SGB) ranged from 0 to 8% w/w

*Potentially toxic metal (heavy metal) immobilization to ryegrass shoots and roots as a function of biochar* 

*application rates. Diagram was created from the work of [22].*

higher CEC, then resulting in a higher PTMs adsorption [31].

**180**

**Figure 2.**

biochar [18].

*Relationship between pH and metal concentrations in NH4NO3 and DTPA extracts after 10 weeks of biochar incubation. Points were plotted from the whole dataset of measurements. One outlier from three replicates (n = 3) was removed, when detected, by using IML and UNIVARIATE (ROBUSTSCALE) procedures of the SAS program. pH ranged from 6.5 to 8.0 as biochars (SGB and PLB) application rates increased from 0 to 8% w/w of biochar.*

### **Figure 4.**

*Soil organic carbon (OC%) changes due to biochar (SGB and PLB) application rates after 10 weeks of incubation. Bars represent the standard deviation of the mean (n = 3) results are significant at* P *< 0.01 (Tukey test).*


*\*: P < 0.05; \*\*: P < 0.01; \*\*\*: P < 0.001; NS: non-significant (P > 0.05). The R-values were calculated from the whole dataset of measurements. SGB + PLB: two biochar treated soils. One outlier from three replicates (n = 3) was removed, when detected, by using IML and UNIVARIATE (ROBUSTSCALE) procedures of SAS program.*

### **Table 2.**

*Pearson correlation coefficient (R) between metal extracted from NH4NO3 or DTPA and soil attributes (pH and organic carbon-OC) after 10 weeks of soil+biochar (SGB and PLB) incubation.*

as arsenic (As). The addition of sugar cane bagasse-derived BC (SCBC) stabilized Pb but accelerated the desorption of arsenic (As); consequently, increased its availability [38]. That is probably a result of charge repulsion between the negatively charged SCBC and the arsenate anion (AsO4 <sup>3</sup><sup>−</sup>).

### **3. The effect of biochar feedstock sources on PTMs phytoavailability**

While investigating the effects of chicken manure (CMB) and greenwaste biochar (GWB), both produced at 550°C (pyrolysis temperature), on the immobilization and phytoavailability of Cd, Cu and Pb in metal-spiked and multimetal contaminated soils, [39] found that both BCs significantly decreased Cd and Pb mobility, mostly by modifying those PTMs from the easily exchangeable soil fraction to less available organic bond fraction. Additionally, they reported

**183**

**Figure 5.**

*The Use of Biochar as a Soil Amendment to Reduce Potentially Toxic Metals (PTMs)…*

that the application of the two feedstock-derived BCs increased the root and shoot dry biomass and decreased the accumulation of Cd, Cu, and Pb in Indian mustard (*Brassica juncea*), thus illustrating the role of biochar in reducing metal phytoavailability while supplying plant nutrients, regardless of the feedstock source. However, according to the authors, the CMB was more effective in metal

A significant decrease in the transfer factor values (TF) of PTMs (Zn, Pb, and Cd) from ryegrass roots to ryegrass shoots when evaluating PLB and SGB additions to a multi-metal contaminated soil was found by [22], and that the PLB was more efficient in such reduction than SGB. This was probably a consequence of their higher pH, CEC, specific surface area (SSA), and stronger buffering capacity as reported by [23], which resulted in the higher efficiency of PLB in decreasing PTMs uptake, as highlighted by the higher decrease of the bioconcentration factor (BCF = [PTMs in shoots/PTMs concentration in soil]) as PLB application rates increased. **Figure 5** (and **Figure 3**) briefly summarizes such findings and highlights the PTMs immobilization

When comparing other soil amendments or feedstocks (raw material) with a feedstock-derived biochar, [40] showed that mussel shells, cow manure, and oak wood biochar application reduced Pb phytoavailability and phytoavailability in a highly contaminated military shooting range soil in Korea. Their study also showed increases in germination percentage and root elongation of lettuce (*Lactuca sativa*) in soil treated with the tested amendments, indicating a reduction of Pb accessibility. Outstandingly, biochar was more effective in decreasing Pb availability than the other tested soil amendments [40]. The application of BCs derived from animal wastes (pig manure biochar, and PLB) reduced the mobility of Cu, Cd, Pb and Zn from 28 to 69%, 77 to 100%, 94 to 99%, and 15 to 97%, respectively, in a

*Bioconcentration factor (BCF) and transfer factor (TF) of potentially toxic metals (Zn, Pb, and Cd) from ryegrass roots to ryegrass shoots as a function of biochar application rates. Biochars were either derived from* 

*switchgrass (SGB) or poultry litter (PLB) feedstocks. Graphs were modified from [22].*

*DOI: http://dx.doi.org/10.5772/intechopen.92611*

multi-metal contaminated soil [41].

immobilization and plant growth than the GWB.

as a function of two feedstocks derived-biochar application rates.

### *The Use of Biochar as a Soil Amendment to Reduce Potentially Toxic Metals (PTMs)… DOI: http://dx.doi.org/10.5772/intechopen.92611*

that the application of the two feedstock-derived BCs increased the root and shoot dry biomass and decreased the accumulation of Cd, Cu, and Pb in Indian mustard (*Brassica juncea*), thus illustrating the role of biochar in reducing metal phytoavailability while supplying plant nutrients, regardless of the feedstock source. However, according to the authors, the CMB was more effective in metal immobilization and plant growth than the GWB.

A significant decrease in the transfer factor values (TF) of PTMs (Zn, Pb, and Cd) from ryegrass roots to ryegrass shoots when evaluating PLB and SGB additions to a multi-metal contaminated soil was found by [22], and that the PLB was more efficient in such reduction than SGB. This was probably a consequence of their higher pH, CEC, specific surface area (SSA), and stronger buffering capacity as reported by [23], which resulted in the higher efficiency of PLB in decreasing PTMs uptake, as highlighted by the higher decrease of the bioconcentration factor (BCF = [PTMs in shoots/PTMs concentration in soil]) as PLB application rates increased. **Figure 5** (and **Figure 3**) briefly summarizes such findings and highlights the PTMs immobilization as a function of two feedstocks derived-biochar application rates.

When comparing other soil amendments or feedstocks (raw material) with a feedstock-derived biochar, [40] showed that mussel shells, cow manure, and oak wood biochar application reduced Pb phytoavailability and phytoavailability in a highly contaminated military shooting range soil in Korea. Their study also showed increases in germination percentage and root elongation of lettuce (*Lactuca sativa*) in soil treated with the tested amendments, indicating a reduction of Pb accessibility. Outstandingly, biochar was more effective in decreasing Pb availability than the other tested soil amendments [40]. The application of BCs derived from animal wastes (pig manure biochar, and PLB) reduced the mobility of Cu, Cd, Pb and Zn from 28 to 69%, 77 to 100%, 94 to 99%, and 15 to 97%, respectively, in a multi-metal contaminated soil [41].

### **Figure 5.**

*Applications of Biochar for Environmental Safety*

as arsenic (As). The addition of sugar cane bagasse-derived BC (SCBC) stabilized Pb but accelerated the desorption of arsenic (As); consequently, increased its availability [38]. That is probably a result of charge repulsion between the

*Pearson correlation coefficient (R) between metal extracted from NH4NO3 or DTPA and soil attributes* 

*(pH and organic carbon-OC) after 10 weeks of soil+biochar (SGB and PLB) incubation.*

**3. The effect of biochar feedstock sources on PTMs phytoavailability**

While investigating the effects of chicken manure (CMB) and greenwaste

immobilization and phytoavailability of Cd, Cu and Pb in metal-spiked and multimetal contaminated soils, [39] found that both BCs significantly decreased Cd and Pb mobility, mostly by modifying those PTMs from the easily exchangeable soil fraction to less available organic bond fraction. Additionally, they reported

biochar (GWB), both produced at 550°C (pyrolysis temperature), on the

<sup>3</sup><sup>−</sup>).

**SGB PLB SGB+PLB SGB PLB SGB+PLB**

Zn (NH4NO3) Zn (DTPA)

Pb (NH4NO3) Pb (DTPA)

Cd (NH4NO3) Cd (DTPA)

pH −0.75\*\* −0.82\*\*\* −0.81\*\*\* −0.15NS −0.76\*\*\* −0.55\*\* OC −0.79\*\* −0.74\*\* −0.43\* −0.31NS −0.60\* −0.47\*

*Soil organic carbon (OC%) changes due to biochar (SGB and PLB) application rates after 10 weeks of incubation. Bars represent the standard deviation of the mean (n = 3) results are significant at* P *< 0.01* 

pH −0.55\* −0.05NS −0.16NS 0.03NS −0.63\* −0.44\* OC −0.64\* −0.11NS −0.46\* −0.12NS −0.77\*\* 0.10NS

pH −0.91\*\*\* −0.82\*\*\* −0.72\*\*\* 0.04NS −0.83\*\*\* −0.66\*\*\* OC −0.91\*\*\* −0.91\*\*\* −0.01NS −0.11NS −0.92\*\*\* 0.06NS *\*: P < 0.05; \*\*: P < 0.01; \*\*\*: P < 0.001; NS: non-significant (P > 0.05). The R-values were calculated from the whole dataset of measurements. SGB + PLB: two biochar treated soils. One outlier from three replicates (n = 3) was removed, when detected, by using IML and UNIVARIATE (ROBUSTSCALE) procedures of SAS program.*

negatively charged SCBC and the arsenate anion (AsO4

**182**

**Soil attribute**

*(Tukey test).*

**Figure 4.**

**Table 2.**

*Bioconcentration factor (BCF) and transfer factor (TF) of potentially toxic metals (Zn, Pb, and Cd) from ryegrass roots to ryegrass shoots as a function of biochar application rates. Biochars were either derived from switchgrass (SGB) or poultry litter (PLB) feedstocks. Graphs were modified from [22].*

A study conducted by [42] observed that the addition of hardwood-derived biochar (HWB) to a PTMs contaminated mine soil reduced pore water solubility of Pb concentrations and ryegrass Pb levels. On the other hand, the combination of biochar with greenwaste compost (GWC) was more effective in reducing Pb in soil pore water and uptake by ryegrass. However, the biochar itself was more effective in reducing pore water Cu than GWC. Additionally, [43] reported that the addition of HWB and GWC to a multi-element contaminated soil significantly reduced concentrations of Cd and Zn in pore water during a 60 days exposure to field conditions and reduced phytoavailability of these elements resulting in increased shoot emergence of ryegrass. In contrast, concentrations of Cu and As in pore water increased with amendment applications [38, 43]. In a laboratory column study, [44] reported that HWB reduced the concentrations of Cd and Zn in leachate obtained from a multi-metal polluted soil with evidence of surface retention of both metals on biochar.

The work of [45] compared the impacts of broiler litter-derived biochar and pecan shell-derived steam activated carbon amendments on PTMs (Cu, Ni, and Cd) immobilization and the effects of oxidation on mineral retention in synthetic rainwater leaching experiments. Conversely, their study found that biochar was most effective in immobilizing Cu, whereas activated carbon immobilized Ni and Cd to a larger extent than biochar.

Contrarily, some BCs might only slightly decrease or even significantly increase extractable PTMs depending on the feedstock and pyrolytic temperature [33, 42]. Overall, the influence of biochar on PTMs extractability varies depending on the feedstock, application rate, and BCs particle size [46]. Generally speaking, biochar is a promising tool to reduce the mobility of PTMs in mining areas [22].
