Silver Nanoparticles Entering Soils via the Wastewater–Sludge–Soil Pathway Pose Low Risk to Plants but Elevated Cl Concentrations Increase Ag Bioavailability

Peng Wang, Neal W. Menzies, Paul G. Dennis, Jianhua Guo, Christian Forstner, Ryo Sekine, Enzo Lombi, Peter Kappen, Paul M. Bertsch, Peter M. Kopittke

The widespread use of silver nanoparticles (Ag-NPs) results in their movement into wastewater treatment facilities and subsequently to agricultural soils via application of contaminated sludge. On-route, the chemical properties of Ag may change and further alterations are possible upon entry to soil. In the present study, we examined the long-term stability and (bio)availability of Ag along the 'wastewater-sludge-soil' pathway. Synchrotron-based X-ray absorption spectroscopy (XAS) revealed that ca. 99% of Ag added to the sludge reactors as either Ag-NPs or AgNO3 was retained in sludge, with ≥ 79% of this being transformed to Ag2S, with the majority ( ≥ 87%) remaining in this form even after introduction to soils at various pH values and Cl concentrations for up to 400 d. Diffusive gradients in thin films (DGT), chemical extraction, and plant uptake experiments indicated that the potential (bio)availability of Ag in soil was low but increased markedly in soils with elevated Cl, likely due to the formation of soluble AgClx complexes in the soil solution. Although high Cl concentrations increased the bioavailability of Ag markedly, plant growth was not reduced in any treatment. Our results indicate that Ag-NPs entering soils through the 'wastewater-sludge-soil' pathway pose low risk to plants due to their conversion to Ag2S in the wastewater treatment process, although bioavailability may increase in saline soils or when irrigated with high-Cl water.