Aging of Microplastics and Nanoplastics Induced by Advanced Oxidation Processes in Wastewater Treatment and Their Biological Toxicity: A Systematic Review

Microplastics and nanoplastics (MNPs), as emerging contaminants, have garnered growing interest for their persistence and biological toxicity. Wastewater treatment plants (WWTPs) are significant convergence points for MNPs, where they undergo complex aging, particularly during advanced oxidation processes (AOPs), leading to different environmental fate and behavior. This study aims to discuss the aging of MNPs in wastewater treatment induced by AOPs and evaluate their biological risks. This review was conducted in accordance with the 2020 PRISMA guidelines. We searched three electronic databases-Scopus, Science Direct, and Web of Science-for relevant articles published between the year 2000 and March 2026. A total of 39 studies met the inclusion criteria and a narrative synthesis was conducted to summarize the findings. Risk of bias assessment was not performed, as this is a narrative systematic review without quantitative synthesis. The review protocol was registered in the OSF (registration DOI: 10.17605/OSF.IO/FTQHN). First, aging pathways and the alterations in the physicochemical properties of MNPs caused by aging are summarized, mainly including changes in surface morphology, crystallinity, and chemical composition, etc. Second, the aging mechanism of MNPs and the factors affecting the aging were discussed. Third, the biotoxicity of aged MNPs on both microorganisms and humans was reviewed, which is mainly due to three sources: plastic particles themselves, released chemicals, and the combination of plastics with coexisting pollutants. Furthermore, this review also criticized the limitations in current studies, the lack of comprehensive evaluation of multiple environmental factors and the identification of specific toxicity; it also provides suggestions for future research. This overview is meaningful for better understanding the environmental fate and risks of MNPs.