Controlled evaluation of silver nanoparticle sulfidation in a full-scale wastewater treatment plant

Sulfidation of silver nanoparticles (AgNPs), which is known to alter AgNP toxicity, occurs during transport through wastewater treatment plants. In this study, arrays of immobilized AgNPs fabricated by nanosphere lithography (NSL) were used to study AgNP sulfidation in a full-scale wastewater treatment plant (WWTP). A detailed laboratory study preceded field deployment. The characteristic NSL pattern remained discernible by atomic force microscopy and transmission electron microscopy after both lab and field exposures. Growth of AgNPs due to an increase in density upon sulfidation permitted the study of sulfidation kinetics in the WWTP. Sulfidation occurred almost exclusively in anaerobic zones of the WWTP, where the initial sulfidation rate was 11-14 nm of Ag converted to Ag2S per day. Measurements of the chemical composition and crystallinity of AgNPs exposed to primary influent for ∼ 10 d confirmed that they had been converted almost entirely to Ag2S. Laboratory experiments revealed that the sulfidation process is not uniform and that partially sulfidized AgNPs retain the potential to release toxic Ag(+) ions. The results indicate that primary AgNPs are sulfidized directly without dissolving and forming secondary precipitates. This study demonstrates the utility of immobilized AgNPs for detailed, in situ investigations of nanomaterial tranformations.