Mechanistic Roles of Microplastics in the Phototransformation of Silver Ions in Aquatic Environments

The role of microplastics (MPs) in transforming coexisting contaminants in aquatic environments is poorly understood. Herein, the mediation of polystyrene (PS) MPs on the phototransformation of silver ions (Ag⁺) was investigated. Ag-based nanoparticles (21.0-177.0 nm) formed from Ag⁺ in the presence of PS MPs (8.3-41.9 μm) after 40-day natural light and 96-h UV light irradiation, as detected by single-particle inductively coupled plasma mass spectrometry (sp-ICP-MS). Using a successive dissolution method, the species were identified as Ag⁰, Ag₂O, and Ag₂S nanoparticles. After 96 h of UV irradiation, Ag⁰ was the main nanoparticle species (93.4%), accounting for 78.0% of the total Ag. Both the MPs surface and the liquid phase were confirmed as Ag⁰ nanoparticle formation sites, whereas Ag₂O and Ag₂S were mainly formed in the liquid phase. Free radicals (superoxide radicals, oxygen-centered persistent free radicals) and oxygen-containing groups (aldehyde groups) on the MPs surface and dissolved organic matter in the liquid phase played crucial roles in Ag⁰ nanoparticle formation. Furthermore, sulfur species (e.g., S^2- and S₂O₃^2-) in the liquid phase were responsible for the Ag₂S nanoparticle formation. These findings are crucial for better understanding the environmental fate, geochemical cycle, and risk of both MPs and Ag⁺.