Recent advances in synthesis and application of modified biochar for mitigation of heavy metals from soil and water: a critical review

Heavy metal pollution continues to threaten human health and the environment seriously. For the removal of heavy metals, many tests have been conducted so far but modified biochar has gained popularity for soil and water remediation due to its desirable physio-chemical surface properties. The current advances in the removal of heavy metals from both soil and water have been reviewed thoroughly. The ability of biochar to remove contaminants is dependent on its surface groups, ion exchange capacity and pore size distribution. The physical structure and molecular constitution of biochar could be crucial when added to water and soil. Various types of feedstock have been used to synthesize biochar including agriculture and forest wastes, industry-based byproducts and non-conventional waste materials. The modification strategies of biochar encompassing organic and non-metal modification, the magnetization of biochar, as well as the regeneration of modified biochar and adsorption of different heavy metal ions comprising Cr, Cd, Cu, Hg and Pb have been comprehensively reviewed. The applications, challenges and future research directions of biochar for the removal of heavy metals from wastewater and soil are also outlined in this review.