• Chemical modification enhances biochar properties (surface area, pore volume & size). • Cationic/anionic metal adsorption mechanisms on modified biochar are presented. • Regeneration & reuse of spent modified biochar via desorption cycles was explored. • Field validation, green synthesis, and spent adsorbent management are key priorities. Widespread discharge of heavy metals (HMs) into aquatic environments due to industrial and agricultural practices poses a significant risk to the ecosystem and public health. Biochar is a kind of carbon-rich porous materials derived from pyrolyzing biomass, which is recognized as a promising and cost-effective adsorbent. However, the limited adsorption capacities of pristine biochar, especially towards anionic metal species, restrict its practical usage. Strategies of chemical modification, including the use of acids, alkalis, oxidants, and metal impregnation, have been extensively explored to enhance the physicochemical properties of pristine biochar and improve its adsorption performance. This review provides a comprehensive analysis of these modification techniques and systematically compares heavy metal adsorption performance of pristine and modified biochar for both cationic (e.g., Pb²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Hg⁰) and anionic (e.g., Cr(VI), As(V), Sb(III)) species, elucidating fundamental mechanisms such as precipitation, complexation, ion exchange, and redox reactions. The review also addresses the environmental risks and secondary pollution potential of chemical modification processes, including waste liquid discharge containing residual modifiers and leaching of toxic substances from modified biochar. Reported data indicate that the modified biochar has excellent regeneration capabilities, maintaining 80-100% recovery in repeated cycles with diverse regeneration agents. Consequently, the review also outlined the regeneration potential and reusability of the spent modified biochar, as well as its various disposal and resource utilization pathways . Economic analysis and life cycle assessment were included, and the critical challenges, including variabilities, lack of real-life and long-term studies, and scaling difficulties, were addressed. Future perspectives include standardization, field validation, green modification strategies, advanced characterization with in-situ techniques, and multifunctional biochars to translate laboratory research into sustainable field-level applications.
Abdelrhman et al. (Fri,) studied this question.