Enhancing water remediation using biochar-hydrogel composites: the critical role of surface functional groups

Biochar–hydrogel composites have emerged as advanced materials that synergistically integrate the high surface area and tunable surface chemistry of biochar with the hydrophilic, crosslinked structure of hydrogels, resulting in enhanced performance for water and wastewater treatment. This review systematically examines the critical role of surface functional groups, such as carboxyl, hydroxyl, phenolic, amine, thiol, and phosphate moieties, showing the adsorption mechanisms, kinetics, and reusability of biochar–hydrogel composites. Functionalization approaches, including in-situ modification during synthesis and post-synthetic chemical treatments, are shown to tailor the density and diversity of these groups, thereby improving pollutant affinity and selectivity. The interplay between surface charge, hydrophilicity, and hierarchical porosity enables multifaceted adsorption via electrostatic attraction, hydrogen bonding, π-π interactions, and redox processes. Kinetic and thermodynamic analyses reveal that chemisorption, governed by surface functional groups, is often the dominant adsorption mechanism, while regeneration studies highlight the importance of matching functional group chemistry with proper eluents to maintain reusability. Despite significant advances, challenges remain in optimizing functionalization protocols, balancing cost and scalability, and achieving homogeneous biochar dispersion within hydrogels. By providing a comprehensive synthesis of recent advances and identifying knowledge gaps, mostly regarding the optimization of functional group engineering, this work offers critical insights for the development of next-generation biochar-hydrogel composites as sustainable, high-performance adsorbents in water and wastewater treatment.