The transformation and fate of silver nanoparticles in paddy soil: effects of soil organic matter and redox conditions

The transformation and fate of silver nanoparticles (AgNPs) in soils could provide insight into their environmental risks. Here Ag transformation and partitioning in paddy soil, spiked with polyvinylpyrrolidone-coated AgNPs or AgNO3, were investigated over 69 days, with particular consideration given to soil redox potential (Eh) and organic matter (OM). Synchrotron-based X-ray absorption spectroscopy demonstrated that after 28 days of incubation, most Ag in the flooded soil was present as Ag2S (normally >94%). Other species (e.g., metallic AgNPs and AgCl) were also identified within the first 2 days of incubation, with the fractions varying according to soil Eh and the initial form of added Ag. The majority of Ag was retained in soil solids as opposed to the liquid phase. However, changing soil OM and Eh may induce the release of dissolved Ag on longer time scales: the presence of soil OM increased Ag retention in the soil solids and decreased dissolved Ag levels whereas high Eh tended to decrease Ag sulfidation and increase the release of dissolved Ag. These results suggest that soil OM and Eh influence the fate of AgNPs in natural soils and should be considered when addressing the risks associated with AgNP amendments to paddy (and other) soils.