Modified TiO₂ and ZnO photocatalysts for microplastic degradation: mechanisms, challenges, and recent advances

Microplastics have emerged as persistent environmental pollutants, ubiquitously present in aquatic and terrestrial ecosystems. Their minute size facilitates entry into food webs, where they disrupt physiological functions and trigger oxidative stress and inflammatory responses in living organisms. Furthermore, microplastics act as vectors for toxic chemicals and pathogenic microorganisms, intensifying their ecological and health-related hazards. Due to their stable and highly polymerized structure, natural degradation of these materials remains extremely limited. Photocatalysis has gained considerable attention as a sustainable and eco-friendly approach for the degradation of microplastics. This review comprehensively summarizes research progress from 2015 to 2025, focusing on photocatalytic mechanisms, degradation pathways, and efficiency trends, particularly for polyethylene, polypropylene, and polystyrene. Among various photocatalysts, titanium dioxide (TiO₂) and zinc oxide (ZnO) have been most extensively investigated owing to their stability, strong oxidizing potential, and photochemical activity. Under ultraviolet irradiation, these semiconductors generate reactive oxygen species (ROS), including superoxide anions (·O₂⁻) and hydroxyl radicals (·OH), which initiate oxidative scission of polymer chains, ultimately leading to mineralization into CO₂ and H₂O. Recent advances have concentrated on improving photocatalytic performance through structural and compositional modifications aimed at enhancing visible-light absorption, reducing electron-hole recombination, and increasing surface reactivity. This review provides a detailed overview of these advancements and offers a comparative evaluation of modified TiO₂ and ZnO photocatalysts, emphasizing their potential for efficient and sustainable mitigation of microplastic pollution.