Photoinduced transformation of silver ion by molybdenum disulfide nanoflakes at environmentally relevant concentrations attenuates its toxicity to freshwater algae

Abstract

The transformation of Ag+ is strongly correlated with its risks in aquatic environment. Considering the wide application of molybdenum disulfide (MoS2) and the inevitable release into the environment, the effects of MoS2 on Ag+ transformation and toxicity are of great concerns. This study revealed the pH-dependent reduction of Ag+ (0.5 mM) to Ag nanoparticles (AgNPs) by MoS2 (50 mg/L) and solar irradiation obviously accelerates the AgNPs formation (2.638 mg/L per day, pH=7.0) compared with dark condition (0.637 mg/L per day), ascribing to the electrons capture from electron-hole pairs of MoS2 by Ag+. Ionic strengths and natural organic matter decreased the AgNPs yield. Metallic 1 T phase of MoS2 primarily participated in AgNPs formation and was oxidized to soluble ions (MoO42−) due to the oxygen generation in valance band. The above processes also occurred between Ag+ and MoS2 at environmentally relevant concentrations. Further, photoinduced transformation of Ag+ by MoS2 (10–100 μg/L) significantly lowered its toxicity to freshwater algae. The AgNPs formation on MoS2 reduced the bioavailability of Ag+ to algae, which was the mechanism for attenuated Ag+ toxicity. The provided data are helpful for better understanding the roles of MoS2 on the environmental fates and risks of metal ions under natural conditions.