Copper(I) Promotes Silver Sulfide Dissolution and Increases Silver Phytoavailability.

Metal sulfides, including acanthite (AgS), are persistent in the environment. In colloidal form, however, they can serve as a "Trojan horse", facilitating the mobility of trace metal contaminants. The natural processes that lead to the in situ dissolution of colloidal metal sulfides in soil are largely unknown. In this study, the dissolution of colloidal AgS in topsoil and Ag phytoavailability to wheat were examined in AgS-Cu(II)-thiosulfate systems. Cu(II) and thiosulfate strongly increased silver release (up to 83% of total Ag) from AgS in the dark. Electron paramagnetic resonance, X-ray photoelectron spectroscopy, and Cu K-edge X-ray absorption spectroscopy identified Cu(I) as the driving force of AgS dissolution. Density functional theory calculations further demonstrated the ability of Cu(I) to substitute for surface Ag on AgS in an energetically favorable manner. However, excess Cu(II) could enhance the formation of precipitates containing Cu(I), Ag, and S. Our results indicate that at ambient temperature and in the dark, Cu(I) can promote the dissolution of AgS and act as a precipitating agent. These findings reveal previously unrecognized biogeochemical processes of colloidal AgS and their importance in determining the fate of metal sulfides in the environment and probably also in vivo.