Dynamic exchange between Ag⁺ and Ag⁰: Implications for the environmental fate of Ag nanoparticles

The environmental transformation and fate of nanomaterials are critical for their consequences. Although Ag nanoparticles (Ag NPs) are frequently detected in the environment, their transformation and fate, particularly in relation to sediment or mineral surfaces, remain poorly understood. In this study, we investigated the behaviors of Ag NPs on γ-Al₂O₃, a model mineral, under environmentally relevant aqueous conditions. We discovered that Ag NPs gradually diminished during light irradiation, ultimately transforming into ultrasmall NPs, clusters, and single atoms. The newly formed secondary Ag⁰ on the surface of γ-Al₂O₃ exhibited low sunlight absorption but high dissolution activity. A chemical equilibration was established quickly between Ag⁺ in the solution bulk and Ag⁰ on the γ-Al₂O₃ surfaces, but dynamic exchange between Ag⁺ and Ag⁰ continued to evolve Ag⁰ speciation afterwards. Light drove these processes, with O₂ involved. The dynamic exchange persisted continuously, although its rate could be influenced by environmental parameters. SYNOPSIS: In a simulated environmental photochemical process, Ag⁰ and Ag⁺ undergo rapid dynamic exchange, leading to the downsizing of Ag nanoparticles on γ-Al₂O₃ surfaces into ultrasmall nanoparticles, clusters, and single atoms.