Unanticipated Thio-Oxidation of Organophosphite Chemical Additives in PVC Microplastics Following <i>In Situ</i> Weathering

Microplastics have emerged as contaminants of concern due to their worldwide distribution and persistence. Following environmental weathering, the chemical composition of microplastics may be altered by physicochemical processes. In this study, nontargeted analysis was employed to examine changes in the chemical compositions of five types of microplastics that had been subjected to 16 weeks of in situ exposure to river water. The highest number of observed peak features was associated with polyvinyl chloride (PVC) (12,043), among which 2,086 were newly formed during exposure. It was unanticipated that three organothiophosphates, including triphenyl thiophosphate (TPTP), were identified with the highest abundance. The abiotic formation of organothiophosphates was confirmed via in-lab simulation trials following 6 weeks of artificial weathering. To further investigate potential reaction mechanisms, isotope-labeling trials were used to confirm the thio-oxidation of TPPi by polysulfides via the phosphonium intermediate. This is the first report of thio-oxidation of organophosphites, highlighting the importance of considering chemical transformations when conducting microplastic risk assessments.