Deciphering the nature of secondary AgS species in soils polluted with Ag nanoparticles

Ag₂S nanoparticles (Ag₂S-NPs) are the main silver species in sewage sludge and soils polluted with silver nanoparticles (AgNPs). Uncertainties about Ag⁺ release and nano-Ag₂S toxicity prompted us to further investigate Ag speciation and distribution under realistic conditions. In a previous study we found by bulk X-ray absorption spectroscopy (XAS) that besides Ag₂S, a secondary AgS species was present in sewage sludge and soils. This species accounted for 24–36 % of total silver. The objective of the present work was to investigate the nature of this AgS species. A combination of X-ray absorption near edge structure (XANES) spectroscopy at Ag K and L_III edges at micro and nano resolutions and of S K-edge micro XANES were used. Nano X-ray Fluorescence (nXRF) was used to investigate the nanoparticle size distribution and to evaluate the presence of mixed metal sulfides. We identified the secondary AgS species as Ag bound to thiol groups of organic matter (Ag-thiolate). It was present around the Ag₂S-NPs, and likely resulted from the complexation of Ag⁺ by thiol groups of soil organic matter. Ag₂S-NPs displayed a variety of sizes, with 10 % to 40 % of particles bellow 100 nm. Mixed metal sulfides were also present. This study improves our understanding of the fate of Ag-NPs in terrestrial environments, highlights the potential reactivity of Ag₂S-NPs in soils, and shows the primary role of soil organic matter as a sink for released Ag⁺. This multi-scale, high-resolution approach represents a novel and powerful methodology for studying nanoparticle behavior in complex environmental matrices.