Mechanistic Insights into the Transient Reactions of Environmentally Persistent Free Radicals on Common Microplastics: An Important Role of Air Humidity

The air relative humidity (RH) was reported to significantly influence the photoaging processes of microplastics (MPs), while few research studies have been conducted to obtain deep insights into the underlying mechanisms. In the present study, an in situ electron paramagnetic resonance (EPR) apparatus equipped with a humidity control system was developed to investigate the dynamics of environmentally persistent free radicals (EPFRs) during MP aging processes. The experimental results indicate that the lower RH tends to promote EPFR accumulation on MPs and inhibit their decay, particularly for polyvinyl chloride microplastics (PVC-MPs). Through theoretical computations, the mechanism involving water molecules in the alteration of EPFRs is interpreted. In addition, low RH conditions favor the generation of superoxide radical anions on both polystyrene MPs (PS-MPs) and PVC-MPs. As the RH level rises, the cytotoxicity of aged PS-MPs and aged PVC-MPs underwent decline and enhancement, respectively, in which superoxide radical anions and hydroxyls played the dominant role. In essence, this study presents new results for the formation of EPFRs on MP surfaces, which would provide a foundation for future research into the intricate aging mechanisms of MPs, e.g., discerning how various environmental parameters shape the aging process of MPs and, consequently, their environmental behaviors.