Arsenite oxidation by reactive oxygen species generated from polystyrene nanoplastics under solar irradiation

Nanoplastics (NPs) are increasingly recognized as reactive environmental contaminants and are known to influence surrounding environments through the generation of reactive oxygen species (ROS) upon photolysis. However, their contribution to redox transformations of coexisting pollutants remains largely unexplored. In this study, we demonstrate that polystyrene nanoplastics (PS-NPs) exposed to solar light promote the oxidative conversion of arsenite (As(III)) to arsenate (As(V)) via ROS generation. Mechanistic studies identify superoxide (O₂^•-) and hydroperoxyl (HO₂^•) radicals as the key oxidizing species, underscoring an intrinsic photoinduced reactivity of PS-NPs under air-equilibrated conditions. The As(III) oxidation reactivity is governed by PS-NP properties, including concentration, particle size, and surface functionalization, in addition to aqueous conditions, with alkaline environments markedly favoring the reaction. Notably, As(III) oxidation was also confirmed under natural sunlight and in real water matrices, demonstrating the environmental relevance of this photochemical process. These findings confirm that nanoplastics can actively mediate arsenic oxidation through ROS-driven pathways, providing new insight into how NPs may influence arsenic speciation and mobility in aquatic environments. This study advances mechanistic understanding of NP-contaminant interactions and underscores the potential role of light-exposed NPs in shaping the redox chemistry of toxic elements in natural waters.