Asian Journal of Geological Research

Incidence of heavy metal contamination of water and soil is a chronic environmental issue of concern all over the world, due to the non-biodegradability, high level of toxicity and bio-accumulative strength, and of these metals such as lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg) and chromium (Cr) to ecological systems and human health in large scale as manifested in affected mining centres in China, ground water systems in India, industrial regions in Eastern Europe and artisanal mining regions in the rest of the Sub-Sahara Africa. The existing methods of remediation, however effective under controlled conditions, are often limited by high cost of operations, secondary pollution, low scalability, and unsustainability in the long run, particularly in low resource areas, where alternative forms of treatment are still economically out of reach due to the use of conventional methods. Biochar is a low-cost, plentiful, eco-friendly, and multifunctional material with a multifunctional role that has increasingly attracted attention as an adsorbent in the remediation of heavy metals with reported field and laboratory applications in biochar; rice-husk biochar to remove Cd in China, corn-stover biochar to remove Pb in the United States, and an agricultural waste-derived biochar to remediate contaminated soils in India and Brazil. Therefore, available reviews considerably demonstrate the efficacy of adsorption, and less significant analysis was done on regeneration possibilities, large scale usage, economic viability and the problem of secondary pollutants to attain environmental safety.

The aims of the review are to critically evaluate the latest trends in the biochar application to the removal of heavy metals by comparing the methods of its production, the adsorption processes and the efficacy of the biochar adsorbents with or without modifications, taking into consideration the limitations of the scalability and the efficiency of the regeneration process, based on the global experimental and pilot-scale research conducted so far in Asia, Europe, and Africa as well. As a contrast to the earlier reviews, this study explicitly evaluates performance trade-offs against the problems of physicochemical modification, complexity of production, and real-world implementation. The present peer-reviewed findings are incorporated in this review, which include conditions of pyrolysis, biomass precursor selection, surface modification, and adsorption processes that govern the removal of heavy metals, and case studies in this have included rice husk, coconut shell, bamboo, and palm kernel shell biochars used in the removal of heavy metal in contaminated water systems and soils across the globe.

Analysis indicates that the surface-modified biochars usually have superior adsorption capacities and selectivity to the unmodified biochar, due to the increase of surface area, functionalities, and surface charge properties, and reported adsorption capacities of 150300 mg/g in Pb-Cd- modified biochar derived with agricultural wastes in China, Malaysia and the United States. The predominant adsorption processes include surface complexation, ion-exchange, precipitation as well as electrostatic interactions. Increased cost of production, complicated procedure, ineffective regeneration and secondary pollution problems usually go hand in hand with these improvements. Dominating adsorption processes include ion-exchange, surface complexation, precipitation and electrostatic. Although with these performance benefits, the efficiency is limited by increased complexity of the process, high production costs, difficulties with effective regeneration, and possible secondary pollution issues as experienced in pilot-scale remediation experiments in the industrial wastewater treatment systems. In addition, there are still significant obstacles in the form of cost-of-production, regeneration unsteadiness, and economic viability in the case of large-scale operation.

Comprehensively, this review describes the significant methodology that explains performance trade-offs between modified and unmodified biochars, defines the outstanding technical and economic challenges limiting their universal applications. The researches highlight the need in futuristic studies on regeneration strategies, and development of cost-effective production pathways to realize the sustainable and practical implementation of biochar in remediation of heavy metals, especially in developing the world where low-cost biomass resources, including palm kernel shell, rice husk, and crop residues, are more accessible.