Functionalized biochar–metal oxide nanocomposites for the catalytic degradation of recalcitrant pollutants in wastewater treatment

Modified biochar (BC) nano-compounds are very promising candidates for the accelerated degradation of reoccurring contaminants in wastewater treatment. The surface activity, structural repair, and dopant chemistry of biochar determine its adsorption ability and catalytic behaviour. Metal oxides such as Fe₃O₄ and FeS₂ offer fast electron-transfer pathways that accelerate the redox-mediated transformations of heavy metal ions, while surface-engineered functional groups, particularly phosphorus moieties and EDTA ligands, provide tunable coordination environments that significantly enhance selectivity toward Pb²⁺ and Ni²⁺. Chemical activators such as ZnCl₂ and KOH introduce hierarchical porosity and widen the distribution of adsorption sites, thereby improving the reactant accessibility and charge-transfer efficiency. Layered double hydroxides (Ni-Fe and Mg-Al) exhibit structurally defined anion-exchange galleries that preferentially interact with oxyanions (PO₄ ^3- and NO₃ ^-), whereas heteroatom-doped biochars (S/N-BC) generate polarized electron densities favourable for binding soft metal ions such as Hg²⁺ and Cu²⁺. Additionally, carbonaceous nanostructures (CNTs and graphene oxide) enable π-π stacking and delocalized electron mediation for the sensitive detection and transformation of aromatic dye molecules. From a materials chemistry perspective, the innovation lies in tailoring these nano-hybrids at the atomic-to-nano scale, optimizing their mechanical stability, interfacial charge dynamics, and metal-support interactions while comprehensively evaluating their structural persistence and catalytic fidelity under realistic aqueous reaction conditions. AI-directed designs aid in performance optimization. Finally, identifying research gaps, optimizing benefit circulation, and conducting comprehensive life cycle assessment (LCA) are crucial for the effective implementation of these concepts. In this review, these aspects are systematically discussed and highlighted to guide future research directions for the next generation of studies.