Elucidating the role of overlooked environmentally persistent free radicals and reactive chlorine species on photoaged chlorine-containing microplastics: New insights into formation mechanisms and health risks

Microplastics (MPs) are capable of inducing the generation of free radicals during aging. However, the formation mechanisms and potential health risks of environmentally persistent free radicals (EPFRs) and reactive chlorine species (RCS) on photoaged chlorine-containing microplastics (Cl-MPs) remain unknown. In this study, the formation of multiple EPFRs on photoaged polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC) was observed by the electron paramagnetic resonance technique. Further multiple characterization analysis and density functional theory (DFT) calculations revealed that the unpaired electrons generated from the homolysis of chemical bonds and the oxidation of PVC and PVDC under light irradiation may be the precursors for the formation of a variety of EPFRs (secondary alkyl radicals, tertiary alkyl radicals, tertiary alkyl peroxy radicals and tertiary alkoxy radicals). Meanwhile, besides the formation of various reactive oxygen species (ROS), a minor amount of RCS formation on photoaged PVC and PVDC was firstly observed. In addition, photoaged PVDC exhibited higher oxidative potential (OP) levels and cytotoxicity compared to PVC, likely due to its stronger EPFRs and reactive species (including RCS and ROS) generation capacities. This study is the first to identify and reveal the formation mechanisms and pathways of EPFRs and RCS on photoaged Cl-MPs, with implications on health risks. The results provide new insights and scientific basis for the health risk assessment of Cl-MPs in the atmosphere.