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 (AgS) are more likely to be the form that Ag enters the environment. The retention of silver sulfide nanoparticles (AgS-NPs) in natural soils is critical for bioavailability and toxicity but remains unclear. Here, we examined the retention of AgS-NPs in 11 natural soils with varying properties using batch assays. More than 99% of AgS-NPs were retained in soil solids, irrespective of soil properties. Such high retention of AgS-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 AgS-NPs ranging from 47.1% to 61.7% in porewater. In contrast to AgS-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 AgS-NPs in natural soils.