**4. The effects of biochar conversion processes on PTMs phytoavailability**

Biochars are effective in the immobilization of PTMs and this effect varies depending on biochar nature and pyrolysis conditions. [47] investigated the impact of pyrolysis temperature on BCs ability to stabilize PTMs in Small Arms Range soil using broiler litter BCs produced at 350 and 650°C. They found that both BCs were effective in stabilizing Pb and Cu at application rates of ≤5% without releasing Sb. In other experiments, [48, 49] suggested using BCs prepared at high temperatures, 650–800°C, for remediation purposes. Additionally, the uptake of PTMs by ryegrass planted in biochar-treated soils generally decreased with increasing pyrolytic temperature [33]. It was also pointed out by [50] that two different feedstocks-derived BCs were more effective in chromium (Cr) adsorption when pyrolyzed at higher temperatures. However, low-temperature biochar was more effective in stabilizing Pb than high-temperature biochar [47]. Such a result was attributed to the higher soluble P concentration of low-temperature biochar, which resulted in a greater Pb immobilization by the formation of lead-phosphate precipitates. In similar experiments using oxidized and unoxidized plant-derived BCs, [49] observed that oxidized BCs rich in carboxyl functional groups had greater ability for Pb, Cu, and Zn immobilization than unoxidized BCs. Therefore, the effect of BCs on the mobility of PTMs in soil is not only a function of the pyrolysis temperature, but also the feedstock used, as previously mentioned, soil properties, and surface functional groups. Indeed, the ability of BCs in reducing the phytoavailability of PTMs in soil depends on its surface functional groups, specific surface area, and porosity [23].

**185**

**Figure 6.**

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

**Figure 6** was adapted from the recent work of [51] and clearly shows that simply increasing pyrolysis temperature of BCs to enhance PTMs immobilization is not a pragmatism. The recovery efficiency (RE, in %) of PTMs (in a multi-metal contamination scheme: Pb + Cu + Zn) from soils amended with mesquite BCs (MBC) pyrolyzed at four different temperatures (300, 400, 500, and 600°C), have decreased as the initial concentration of added PTMs increased (**Figure 6**). However, MBC pyrolyzed at the highest temperature (600°C) has shown an overall higher RE% of Pb, Cu, and Zn (**Figure 6**). The authors also emphasized that there was a competitive adsorption among the PTMs into BCs exchangeable sites with a preferable affinity for Pb sorption. That would probably favor other PTMs to be phytoavailable in the medium. According to [22, 23, 51], surface functional groups responsible for metals retention are prone to change when pyrolysis temperature increases, which changes metals' sorption effectiveness to the same extent. Phenolic groups (OH) decrease followed by the increase of aromatic carbon contents (C=C stretching) in produced BCs are attributed to the depolymerization and dehydration of materials as pyrolysis temperature increases, then resulting in the formation of C=C double bonds, carbonyl, and carboxylic functional groups [23, 51–53]. Those functional groups are also responsible for PTMs adsorption and

Biomass gasification has also been demonstrated as an alternative method of pyrolysis to produce BCs [54, 55], although to a much lesser extent. It is a

*Lead (Pb), copper (Cu), and zinc (Zn) removal efficiency (RE%) (PTM adsorption) in a multi-metal contaminated soil amended with biochars derived from mesquite and pyrolyzed at 300°C (MBC 300), 400°C (MBC 400), 500°C (MBC 500), and 600°C (MBC 600). The graph is modified from [51] published work.*

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

complexation.

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

**Figure 6** was adapted from the recent work of [51] and clearly shows that simply increasing pyrolysis temperature of BCs to enhance PTMs immobilization is not a pragmatism. The recovery efficiency (RE, in %) of PTMs (in a multi-metal contamination scheme: Pb + Cu + Zn) from soils amended with mesquite BCs (MBC) pyrolyzed at four different temperatures (300, 400, 500, and 600°C), have decreased as the initial concentration of added PTMs increased (**Figure 6**). However, MBC pyrolyzed at the highest temperature (600°C) has shown an overall higher RE% of Pb, Cu, and Zn (**Figure 6**). The authors also emphasized that there was a competitive adsorption among the PTMs into BCs exchangeable sites with a preferable affinity for Pb sorption. That would probably favor other PTMs to be phytoavailable in the medium. According to [22, 23, 51], surface functional groups responsible for metals retention are prone to change when pyrolysis temperature increases, which changes metals' sorption effectiveness to the same extent. Phenolic groups (OH) decrease followed by the increase of aromatic carbon contents (C=C stretching) in produced BCs are attributed to the depolymerization and dehydration of materials as pyrolysis temperature increases, then resulting in the formation of C=C double bonds, carbonyl, and carboxylic functional groups [23, 51–53]. Those functional groups are also responsible for PTMs adsorption and complexation.

Biomass gasification has also been demonstrated as an alternative method of pyrolysis to produce BCs [54, 55], although to a much lesser extent. It is a

### **Figure 6.**

*Applications of Biochar for Environmental Safety*

on biochar.

Cd to a larger extent than biochar.

**phytoavailability**

surface area, and porosity [23].

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

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

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].

Biochars are effective in the immobilization of PTMs and this effect varies depending on biochar nature and pyrolysis conditions. [47] investigated the impact of pyrolysis temperature on BCs ability to stabilize PTMs in Small Arms Range soil using broiler litter BCs produced at 350 and 650°C. They found that both BCs were effective in stabilizing Pb and Cu at application rates of ≤5% without releasing Sb. In other experiments, [48, 49] suggested using BCs prepared at high temperatures, 650–800°C, for remediation purposes. Additionally, the uptake of PTMs by ryegrass planted in biochar-treated soils generally decreased with increasing pyrolytic temperature [33]. It was also pointed out by [50] that two different feedstocks-derived BCs were more effective in chromium (Cr) adsorption when pyrolyzed at higher temperatures. However, low-temperature biochar was more effective in stabilizing Pb than high-temperature biochar [47]. Such a result was attributed to the higher soluble P concentration of low-temperature biochar, which resulted in a greater Pb immobilization by the formation of lead-phosphate precipitates. In similar experiments using oxidized and unoxidized plant-derived BCs, [49] observed that oxidized BCs rich in carboxyl functional groups had greater ability for Pb, Cu, and Zn immobilization than unoxidized BCs. Therefore, the effect of BCs on the mobility of PTMs in soil is not only a function of the pyrolysis temperature, but also the feedstock used, as previously mentioned, soil properties, and surface functional groups. Indeed, the ability of BCs in reducing the phytoavailability of PTMs in soil depends on its surface functional groups, specific

**4. The effects of biochar conversion processes on PTMs** 

**184**

*Lead (Pb), copper (Cu), and zinc (Zn) removal efficiency (RE%) (PTM adsorption) in a multi-metal contaminated soil amended with biochars derived from mesquite and pyrolyzed at 300°C (MBC 300), 400°C (MBC 400), 500°C (MBC 500), and 600°C (MBC 600). The graph is modified from [51] published work.*

technology that uses a controlled process involving heat, steam, and oxygen to convert biomass to hydrogen (and other products), in the absence of combustion. A recent study of [55] have demonstrated that the SSA, CEC, and basic functional groups of the pine woodchips-derived BCs (PWC) increased as the rate of airflow increased during the BCs conversion process. Therefore, such improved properties would favor PTMs immobilization in contaminated soils if a proper rate of PWCs were applied. More studies on different gasification processes of applied BCs affecting PTMs mobility in soils are encouraged to broaden the BCs options for remediation purposes.
