Cadmium, Cd(II) pollution of soils is a serious environmental and agricultural issue, posing a threat to crop production, environmental quality, food safety, and human health. Therefore, immobilization of Cd(II) in soils is crucial. Biochar-based materials are receiving significant attention as Cd(II) immobilizers, due to their multifunctional surface properties. The remediation/immobilization mechanisms involved are, mainly, surface complexation, chemical reduction, precipitation, ion exchange, π–π interactions, hydrogen bonding, and adsorption. These mechanisms are mostly dependent on biochar surface pore size, oxygen-containing functional groups, pyrolysis temperature used in biochar preparation, biochar feedstock, and soil characteristics. So far, various pristine and modified biochar substrates have been used to remediate heavy metal-contaminated soils. Therefore, in this review paper, we briefly summarize the chemical forms, release sources, and maximum permissible limits of Cd(II) in soil. We also summarize recent scientific findings on the performance of biochar substrates in Cd(II)-contaminated soils to minimize Cd(II) mobility, bioavailability, and potential accumulation in crops. Finally, we identify challenges associated with the use of biochar and suggest areas for future research. The review presents an overview of the knowledge of biochar as a promising amendment for the decontamination of Cd(II)-polluted soils.
