A Critical Review of an Environmental Risk Substance Induced by Aging Microplastics: Insights into Environmentally Persistent Free Radicals

Microplastics (MPs), as an emerging contaminants category, can undergo complex aging in a variety of environmental matrices in which the chemical bonds of polymer molecules can be broken to form free radicals. While the existence of free radicals in aged plastics has been known for over half a century, only recently has significant research on a new type of environmentally risky substance, namely environmentally persistent free radicals (EPFRs), present in aged MPs and their environmental effects, been started, but it is still in its infancy. To address these issues, this work examines EPFR generation on MPs and their environmental effect by reviewing publications from 2012 to 2023. The aging processes and mechanisms of MPs in the environment are first summarized. Then, the occurrence and formation mechanisms of EPFRs on aged MPs are specifically discussed. Additionally, the reactivity of EPFRs on aging MPs and their influencing factors are comprehensively considered, such as their physicochemical properties, oxygen content, and coexisting substances. Due to their reactivity, EPFRs can interact directly with some substances (e.g., p-nitrophenol and proteins, etc.) or induce the generation of reactive oxygen species, leading to diverse environmental effects, including pollutant transformation, biotoxicity, and health risks. Finally, research challenges and perspectives for EPFRs formation on aging MPs and related environmental implications are presented. Given the environmental fate and risk of MPs-EPFRs, our urgent call for a better understanding of the potential hazards of aged MPs is to help develop a sustainable path for plastics management.