Description
Due primarily to their remarkably effective antimicrobial properties, silver nanoparticles (AgNPs) are commonly incorporated into a myriad of consumer products. The rapid and unregulated introduction of these products to the consumer market has raised concerns over the potential for AgNPs to impair essential biological processes within wastewater treatment plants. Nitrosomonas europaea, a model ammonia oxidizing bacteria, plays an important role in the removal of organic nitrogen from wastewater. In the studies presented herein, the toxicity and adsorption of 20 nm citrate capped AgNPs to N. europaea was investigated in the presence and absence of surrogate wastewater and natural water constituents, including bovine serum albumin (BSA), alginate, and Suwannee River Humic Acid (SRHA). The AgNPs were found to be highly susceptible to rapid dissolution to Ag⁺ in the presence of NH₃, resulting in their increased toxicity to N. europaea. The presence of 100 ppm BSA yielded relatively uniform AgNP dissolution and subsequently enhanced toxicity to N. europaea independently of NH₃ concentrations, suggesting that the BSA concentration was a better predictor of AgNP dissolution than NH₃. Unlike BSA, the high concentration of alginate (600 ppm) used in these studies did not result in uniform dissolution of the AgNPs. However, the protective coating afforded by the adherence of alginate to the surface of the AgNPs was compromised by NH₃ and is reflected by the enhanced dissolution of the AgNPs and the subsequent increase in their toxicity to N. europaea. In addition, NH₃ compromised the protective coating endowed by 15 ppm SRHA, resulting in increased dissolution and toxicity of the AgNPs to N. europaea. Furthermore, results from these studies indicate a strong affinity for the AgNPs to adsorb to cells. At lower concentrations (40 ppm), BSA appeared to coat the surface of the AgNPs and reduced their adsorption to N. europaea cells through electrosteric hindrances. When compared with BSA, a significantly higher concentration of alginate (800 ppm) was required to saturate the surface of the AgNPs and reduce their adsorption potential to cells through electrosteric hindrances, indicating that polysaccharides may, in general, interact weakly with the AgNPs surface. Similar to BSA, a relatively low concentration of 10 ppm SRHA was required to saturate the surface of the AgNPs and significantly reduce their adsorption to cells. However, the electrostatic stabilization afforded by SRHA was quickly compromised in waters with a high electrolyte concentration.
