Modified biochar as a green adsorbent for removal of hexavalent chromium from various environmental matrices: Mechanisms, methods, and prospects

• Cr(VI) is a persistent pollutant due to its toxicity and carcinogenicity. • Engineering approaches of biochar improves Cr(VI) removal. • Complexation, electrostatic interactions, and precipitation are the key mechanisms. • Co-existing ions and pH of the solution influences Cr(VI) removal by biochar. • Biochar can be effectively regenerated without much loss of removal efficiency. Hexavalent chromium (Cr(VI)) contamination has gained interest from regulatory authorities due to its high toxicity and carcinogenicity. Different approaches have been applied for the remediation of Cr(VI) from aquatic media, with adsorption being the most common method. Biochar has gained popularity in recent years as a cost-effective and greener adsorbent for remediation of heavy metals (HMs) via different mechanisms, including complexation, electrostatic or ionic interactions, and precipitation. Owing to its facile synthesis methods and excellent physico-chemical properties, biochar has been applied extensively for the removal of Cr(VI) from different matrices. However, very few studies have targeted the elimination of Cr(VI) from real polluted environmental matrices, such as soil or water. Therefore, this review comprehensively discussed the potential application of modified biochar in remediation of Cr(VI) from different environmental matrices, starting with its toxicology impacts on flora and fauna. Furthermore, this review has provided a comprehensive state-of-the-art analysis of biochar tailoring technology, modification methods, various adsorption mechanisms, and factors which influence the adsorption of Cr(VI) from different environmental matrices, along with regeneration and reuse of spent adsorbents. Finally, current work focuses on the major gaps in the literature and areas in which further research should be pursued in addition to the effective application of biochar as adsorbent for the remediation of Cr(VI).