High retention of silver sulfide nanoparticles in natural soils

Silver, either in ionic or nanoparticulate form, is widely used in consumer products. However, silver sulfide (Ag₂S) are more likely to be the form that Ag enters the environment. The retention of silver sulfide nanoparticles (Ag₂S-NPs) in natural soils is critical for bioavailability and toxicity but remains unclear. Here, we examined the retention of Ag₂S-NPs in 11 natural soils with varying properties using batch assays. More than 99% of Ag₂S-NPs were retained in soil solids, irrespective of soil properties. Such high retention of Ag₂S-NPs, at least partially, explained the distinct differences in phytoavailability performed in soil vs. liquid media in the literature. Nanoparticles containing Ag and S were identified in representative soil solids by high resolution transmission electron microscopy equipped with an energy dispersive X-ray spectrometer. Iron-rich acidic soil had a high dissolution of Ag₂S-NPs ranging from 47.1% to 61.7% in porewater. In contrast to Ag₂S-NPs, silver nanoparticles (AgNPs) and Ag⁺ in these soils were less retained (as described by Freundlich model) and the retention was closely associated with soil properties. These findings highlight the unique behaviors of Ag₂S-NPs in natural soils.