Recent advances in biochar from lignocellulosic and non-lignocellulosic feedstocks for heavy metal remediation

Abstract Heavy metal contamination poses a persistent global ecological challenge due to its toxicity, non-biodegradability, and ability to build up in biological systems. Biochar, a carbon-dense substance obtained through biomass thermal processing, is recognized as a highly effective and environmentally friendly adsorbent for mitigating heavy metal pollution from polluted water and soil. This review evaluates recent innovations in the synthesis and uses of biochar obtained from lignocellulosic as well as non-lignocellulosic sources for the removal of heavy metals. Biomass resources encompass agricultural residues, wood, and animal manure, and industrial by-products, are explored with respect to their influence on biochar yield and physicochemical characteristics. Emphasis is placed on diverse synthesis methods, such as hydrothermal carbonization, microwave-assisted pyrolysis, slow and fast pyrolysis, and ionothermal carbonization, and how these methods tailor surface area, porosity, and surface functionality. The review further addresses natural and man-made origins of heavy metal contamination, highlighting the associated environmental and health risks. The relationships between biochar properties (porosity, functional groups, surface area, and cation exchange capacity) and their adsorption performance are systematically analyzed. Key operational parameters, including solution pH, operating temperature, adsorption time, and concentration of metal ions, are discussed to elucidate their roles in optimizing adsorption efficiency. Finally, the underlying mechanisms governing heavy metal remediation, namely surface complexation, electrostatic attraction, ion exchange, and precipitation are comprehensively reviewed. This article provides an integrated perspective on recent advancements and future directions in biochar engineering for sustainable heavy metal remediation.