Recent advances and challenges in advanced oxidation processes for degradation of nano- and microplastics in water: a critical review

This review consolidates recent advances in advanced oxidation processes (AOPs) for nanoplastic and microplastic (NMP) degradation, focusing on four major approaches: ozonation, photocatalysis, Fenton-based systems, and electrochemical oxidation. It critically examines how NMP characteristics, operational conditions, material design, and technological advancements influence system performance and degradation pathways. Ozonation of NMPs was enhanced through UV, ultrasound, or catalysts. Photocatalysis offers a sustainable route for degrading NMPs. Efforts such as heterojunction engineering, metal doping, and immobilization on various supports have improved activity and reusability, while multifunctional designs now allow simultaneous pollutant removal or hydrogen production. Fenton-based processes, particularly photo- and heterogeneous systems, extend operational flexibility and reduce iron leaching, but their NMP removal performance remains inconsistent, and emerging variants such as self-Fenton and bio-Fenton are still hampered by high energy requirements, slow kinetics, and scalability issues. Electrochemical oxidation of NMPs provides direct and indirect oxidative routes, with advances in electrode design, ranging from layered double hydroxides to doped-metal oxides, improving radical generation and durability. Hybrid electrochemical systems that combine sonication, oxidants, and membranes show further promise, though unresolved challenges include electrode leakage, reliance on indirect performance metrics, harsh operational conditions, and high energy use. Across all methods, integrated systems and advanced analytical tools are increasingly applied to enhance efficiency and clarify mechanisms. This review also highlights remaining knowledge gaps and proposes future directions, including the adoption of advanced analysis, standardized evaluation, and the development of scalable, cost-effective designs to ensure the safe and practical deployment of AOPs for NMP remediation.