Next-generation Z-scheme and S-scheme heterojunction photocatalysts for microplastics mineralization

Microplastics (MPs) pollution has become a significant environmental issue, signified by the persistence, bioaccumulation, and toxicity of plastic fragments in aquatic ecosystems. Conventional mitigation strategies, including filtration, coagulation–flocculation, pyrolysis, and biodegradation, are limited by high energy consumption, secondary waste generation, and incomplete mineralization. Photocatalysis offers a sustainable alternative by harnessing light-induced charge carriers to generate reactive oxygen species (ROS) that oxidize MPs to CO2 and H2O. However, traditional type-II heterojunctions, while effective for charge separation, often exhibit reduced redox capability, which limits their oxidative efficiency. Z-scheme and S-scheme heterojunctions have been developed to overcome this trade-off by preserving strong oxidative holes and highly reductive electrons. All-solid-state Z-scheme systems achieve this through directional carrier recombination between coupled semiconductors. However, such systems are often hindered by complex architectures, interfacial resistance, and mediator instability. In contrast, S-scheme heterojunctions rely on direct semiconductor contact and internal electric fields to selectively eliminate low-energy carriers, thereby enabling enhanced redox strength, improved visible-light utilization, and superior structural stability. This review critically analyzes recent advances in Z- and S-scheme photocatalysts for MP degradation, with emphasis on band-structure engineering, interfacial charge-transfer behavior, and ROS-driven transformation pathways. Key challenges, including incomplete mineralization, nanoplastic formation, realistic water-matrix effects, scalability, and catalyst cost are critically discussed. In parallel, emerging opportunities, including waste-derived semiconductors, solar-driven operation, and continuous-flow reactor design, are identified. By integrating mechanistic insights with process-level considerations, this review provides a concise roadmap for advancing heterojunction photocatalysis toward practical and scalable MPs remediation.