A review of pristine and modified biochar immobilizing typical heavy metals in soil: Applications and challenges

In recent years, the application of biochar in the remediation of heavy metals (HMs) contaminated soil has received tremendous attention globally. We reviewed the latest research on the immobilization of soil HMs by biochar almost in the last 5 years (until 2021). The methods, effects and mechanisms of biochar and modified biochar on the immobilization of typical HMs in soil have been systematically summarized. In general, the HMs contaminating the soil can be categorized into two groups, the oxy-anionic HMs (As and Cr) and the cationic HMs (Pb, Cd, etc.). Reduction and precipitation of oxy-anionic HMs by biochar/modified biochar are the dominant mechanism for reducing HMs toxicity. Pristine biochar can effectively immobilize cationic HMs. The commonly applied modification method is to add substances that can precipitate HMs to the biochar. In addition, we assessed the risks of biochar applications. For instance, biochar may cause the leaching of certain HMs; biochar aging; co-transportation of biochar nanoparticles with HMs. Future work should focus on the artificial/intelligent design of biochar to make it suitable for remediation of multiple HMs contaminated soil.

Automated summary

In recent years, the use of biochar for remediating heavy metal-contaminated soil has gained considerable global attention. This review summarizes the latest research on the immobilization of soil heavy metals using biochar in the past five years (until 2021). The methods, effects, and mechanisms of biochar and modified biochar in immobilizing typical heavy metals in soil are systematically presented. The reduction and precipitation of oxy-anionic heavy metals by biochar/modified biochar are the dominant mechanisms in reducing their toxicity. Pristine biochar can effectively immobilize cationic heavy metals. Commonly applied modification methods involve adding substances capable of precipitating heavy metals to the biochar. Additionally, the risks associated with biochar applications are assessed, such as leaching of certain heavy metals, biochar aging, and co-transportation of biochar nanoparticles with heavy metals. Future research should focus on the artificial/intelligent design of biochar for the remediation of multi-heavy metal-contaminated soil.