Biochar-supported photocatalysts for microplastics removal: mechanisms, material design, and pathways towards real-world applications

Microplastics (MPs) persist in ecosystems, are dispersed over long distances, and readily adsorb hazardous contaminants, posing a critical global threat. Biochar-supported photocatalysts (BSPs) have emerged as next-generation materials that influence the synergistic integration of biochar's physicochemical properties with the oxidative capabilities of semiconductor-based photocatalysis, enhancing the charge-transfer between the components and outperforming their conventional counterparts. In contrast to the previously published reviews, this review provides a comprehensive, mechanism-driven analysis of the degradation performance of MPs using BSP systems, connecting material architecture by integrating data from several disciplines, charge transfer dynamics, and reactive oxygen species (ROS) generation. Biochar's basic high surface area (up to 800 m2 g-1) and rich oxygen/nitrogen containing functional groups enable efficient adsorption of MPs onto the catalyst surface, significantly increasing light-driven reaction efficiency. Rather than discussing the performance of BSP systems in isolation, we adopted a mechanism driven integrated approach towards bridging the critical gap caused by the absence of any comprehensive framework. We summarized experimental findings indicating that BSPs can achieve significant removal of MPs under laboratory-specific conditions, with performance strongly dependent on composite architecture and reactor configuration. Various systems including ZnO/biochar, TiO2/GO/biochar, and multi-heterojunction TiO2/Fe3O4/graphene/biochar composites have demonstrated effective photocatalytic degradation of MPs. These outcomes should be interpreted as indicative of the potential of BSP systems rather than directly comparable performance metrics, since variations in light source, irradiation time, MP polymer type, particle size, catalyst loading, and reactor design can substantially influence the observed removal efficiency. We also highlighted novel design strategies including the fabrication of S-scheme and Z-scheme heterojunctions (e.g., g-C3N4/BiVO4/biochar and TiO2/Fe3O4/biochar) that enhance electron hole separation rates by factors exceeding 5× relative to conventional photocatalysts. BSPs mitigate plastic pollution and achieve carbon-negative profiles via residue valorization. Advanced reactor types such as fluidized, fixed, and membrane-integrated ones show prospects for maximum performance w.r.t. MP removal, despite worries about stability, regeneration, and byproducts.