# Preparation and Modification of Biochar Materials and their Application in Soil Remediation
## 4 Application of Biochar in Soil Remediation  
### Removal of Heavy Metals  
The removal of heavy metals by biochar is mainly reflected in two aspects: One is the adsorption of heavy metals in the pores of biochar to reduce the residual amount in the soil; the other is the ion exchange or redox reaction between the effective components in biochar and heavy metal ions to stabilize the formation of heavy metal precipitates or to reduce toxicity by transforming them into low-valent states. [PERSON] et al. [103] investigated the effect of sheep and earthworm manure biochars on Pb immobilization in a contaminated calcareous soil. The addition of biochars resulted in a significant increase in the Pb content in the residual state, which reduced the Pb activity in the soil. [PERSON] et al. [79] studied the adsorption mechanisms for removal Pb and Cd with dairy manure biochar. The extractable Pb and Cd contents decreased significantly and were converted to the precipitation as carbonate minerals. However, it may also be due to the lack of selective adsorption capability of biochar, which adsorbs nitrogen in the soil, resulting in a decrease in soil nutrients [104]. When the soil pollution is contaminated by complex heavy metals, although biochar reduces the concentration of extractable heavy metals, biochar has different adsorption effects on different heavy metals due to competitive adsorption. [PERSON] et al. [105] showed that straw and bamboo biochar are more effective than Zn in reducing extractable Cu and Pb. [PERSON] et al. [106] also reached a similar conclusion. In the single metal adsorption test, the adsorption capacity of sludge biochar to Zn was the largest, while in the polymetallic adsorption test, the adsorption capacity of Mn, Cu, and Zn decreased, but the adsorption capacity of Cr increased. Table 4 shows the research on removing heavy metals in soil by using biochar in the past two years. \begin{table}
\begin{tabular}{c c c c c} \hline
**Raw Material** & **Tested Soil** & **Pollutant** & **Renentilation Effect** & **Reference** \\ \hline Bamboo, ice straw, and & industrial & Cu & Cu uptake in roots was reduced by 15\%, 33\%, and 26\%, respectively. Rico straw & [107] \\ Chinese walnut shell & contaminated soil & & biochar reduced solubility of Cu and Pb, and Zn & [108] \\ Sewage sludge & Brazil soil & Cd, Pb, and Zn & Biochar reduced the concentration and bioavailable levels of Cd, Pb, and Zn of & [108] \\  & paddy soil near Zn & & Acid-soluble Cd in soils amended with poultry litter biochar was 8\% to 10\% & [109] \\ Poultry litter & and Pb mines & & & lower than in the control polluted soil. \\  & & & Cu concentration in wheat roots was reduced most efficiently to 40\% by & [110] \\ Wheat straw & acid soil & Cd and Cu & biochar. Available Cd and Cu in soil added biochar decreased 18\% and 18.6\%. & \\  & saturated soil, dryland & & The adsorption of Cd on saturated soil increased by 21-41\%, and that on & [111] \\  & soil & & dryland soil increased by 38.5\%. & \\  & & & The addition of biochar to the soil reduced the dissolution rates of Pb and Cu by & \\  & & & 10.0-90\%, and 156\%,-99\%, respectively, and was able to fix Pb and Cu released & [112] \\  & & & by protons and ligands in the soil. & \\ Poultry manure, cow manure, & & Poultry manure decreased.54 mg\(\cdot\)kg\({}^{-1}\) Cr(VI) in acidic soil and 73.93 & \\  & and sheep manure & & mg\(\cdot\)kg\({}^{-1}\) Cr(VI) in alkaline soil. Cow and Sheep manure decreased by 66.61, & \\  & & & 58.67, and 57.81, 68.15 mg\(\cdot\)kg\({}^{-1}\) Cr(VI) in acidic and alkaline soil, respectively. & \\ \hline \end{tabular}
\end{table} 
Table 4: Study on the application of biochar to the remediation of heavy metal pollution in soil. In order to achieve better remediation effect, scholars gradually carry out research on the modification of biochar. Modification refers to the activation of the original biochar through physical and chemical methods, so as to achieve the desired purpose.