From wastewater treatment to energy devices: A comprehensive review on biochar utilization and reuse pathways.

In current years, the elimination of heavy metals from wastewater systems has become a global concern due to persistent adverse effects on ecosystems and human health. Adsorption technique has gained much attention for the removal of heavy metals from aquatic environments because of advantages like great efficiency, cost-effectiveness and simple operation. Biochar, a carbonaceous material, has been extensively applied for removing heavy metals from aqueous systems due to its low cost and eco-friendly nature. However, numerous existing studies are focusing on elimination of heavy metals from wastewater by using biochar as an adsorbent, but very few emphasize on re-utilization of spent biochar, which is a crucial concern. This is because if spent biochar is discharged or landfilled untreated into the environment can increase the risk of secondary pollution thus harming the ecosystem and mankind. Therefore, the present paper critically reviews the recent applications of biochar in removing heavy metals from wastewater along with potential electrochemical applications of metals loaded biochar in developing super-capacitors or other energy storage devices. The review also comprehensively discussed the characteristics of different biomass feedstock, methods used for biochar production and various physical, chemical and biological biochar engineering techniques. The key mechanisms involved in adsorption of heavy metals from aquatic systems through biochar have been reviewed. The paper also highlighted technical barriers, benefits-cost analysis, market status, future directions and scope of biochar as an adsorbent. Hence, the review will assist the successful development of highly efficient biochar-based adsorption techniques for eliminating heavy metals from aqueous environments.