Reactive species-mediated stepwise photoaging mechanisms of microplastics transferred from freshwater to seawater

Microplastics (MPs) inevitably undergo photoaging following transfer from freshwater to seawater, where reactive species (RS) potentially serve as key drivers influencing their physicochemical properties. However, the mechanisms underlying RS-mediated photoaging of MPs in this environmental transition remain limited. This study elucidated a novel stepwise photoaging mechanism for polystyrene microplastics (PS MPs), initially mediated by reactive oxygen species (ROS) in freshwater and subsequently by reactive chloride species (RCS) in seawater. Results demonstrated that PS MPs transferred from freshwater conditions (sampled from Xiangjiang River; salinity undetected) to seawater conditions (sampled from Bohai Sea; salinity of 30‰) exhibited more advanced photoaging, with a 43% increase in polymer chain scission compared to those photoaged exclusively in freshwater. Notably, MPs modified with oxygen-containing functional groups (OFGs) facilitated the generation of ROS and RCS in seawater, thereby promoting the photoaging process. The OFGs-enriched photoaged PS MPs displayed a larger population of triplet-state PS (³PS*), a 312% enhancement in oxidative potential, and a 37% higher yield of dichloride radical anions (Cl₂•^-) compared to pristine PS MPs. These findings indicated a positive feedback loop: OFGs promoted ³PS* formation, which in turn facilitated ROS generation and oxidised chloride ions to form RCS. This cascade of RS promoted advanced photoaging, resulting in the accumulation of additional OFGs. Furthermore, density functional theory and molecular dynamics simulation results indicated that OFGs weakened the intrinsic molecular stability of MPs and introduced electrophilic-nucleophilic dual sites, thereby increasing their susceptibility to RS attack (especially by RCS). Collectively, this study highlights that OFGs serve as critical intermediates in the RS-mediated stepwise photoaging process, providing theoretical insights for elucidating the environmental fate of MPs in aquatic systems and further assessing their environmental risks.