Rethinking Stability of Silver Sulfide Nanoparticles (Ag₂S‑NPs) in the Aquatic Environment: Photoinduced Transformation of Ag₂S‑NPs in the Presence of Fe(III)

The stability of engineered nanomaterials in a natural aquatic environment has drawn much attention over the past few years. Silver sulfide nanoparticles (Ag₂S-NPs) are generally assumed to be stable in a natural environment as a result of their physicochemical property; however, it may vary depending upon environmental conditions. Here, we investigated whether and how the environmentally relevant factors including light irradiation, solution pH, inorganic salts, dissolved organic matter (DOM), and dissolved oxygen (DO) individually and in combination influenced the stability of Ag₂S-NPs in an aquatic environment. We presented for the first time that transformation of Ag₂S-NPs can indeed occur in the aqueous system with an environmentally relevant concentration of Fe³⁺ under simulated solar irradiation and natural sunlight within a short time (96 h), along with significant changes in morphology and dissolution. The photoinduced transformation of Ag₂S-NPs in the presence of Fe³⁺ can be dramatically influenced by solution pH, Ca²⁺/Na⁺, Cl^–/SO₄^2–, DOM, and DO. Moreover, Ag₂S-NP dissolution increased within 28 h, followed rapid decline in the next 68 h, which may be a result of the reconstitution of small Ag₂S-NPs. Taken together, this work is of importance to comprehensively evaluate the stability of Ag₂S-NPs in an aquatic environment, improving our understanding of their potential risks to human and environmental health