Wastewater Treatment Processing of Silver Nanoparticles Strongly Influences Their Effects on Soil Microbial Diversity

Silver nanoparticles (NPs) are entering soils with biosolids via wastewater treatment, and on-route, undergo changes (e.g., sulfidation) that alter silver bioavailability and toxicity. While additions of fresh Ag-NPs to soil have been show to influence bacterial diversity, it is unclear whether these effects are representative of realistic exposure pathways. To investigate the effects of wastewater treatment processing on the ecotoxicology of Ag-NPs, we ran sequencing batch reactors for 28 d to produce three batches of sludge: (1) a control, (2) Ag-NP sludge, and (3) AgNO₃ sludge. The effects of processed silver on the diversity and composition of soil bacterial and fungal communities were compared to those of fresh Ag-NPs, Ag₂S-NPs, and AgNO₃, which were added with the control sludge, at two concentrations (1 and 10 mg Ag kg^-1 dry soil) over time (3, 7, 30, and 90 d). The effects of processed Ag-NPs on the composition of soil bacterial communities were larger and more persistent than those of fresh Ag-NPs, Ag₂S-NPs, and AgNO₃. Treatment effects on fungi were relatively minor. These findings suggest that the potential ecological impacts of Ag-NPs entering soils via more realistic exposure pathways (e.g., sludge) are underestimated when extrapolated from studies that focus on applications of fresh Ag-NPs.