Influence of ammonia on silver nanoparticle dissolution and toxicity to Nitrosomonas europaea

Nitrosomonas europaea, a model ammonia oxidizing bacterium, was sensitive to both ionic silver (Ag+) and 20 nm citrate capped silver nanoparticles (AgNPs). AgNP toxicity has been previously shown to be primarily due to the dissolution of Ag+. The rate of AgNP dissolution dramatically increased in test medium containing ammonium sulfate ((NH4)2SO4) and HEPES buffer compared to test medium containing either deionized water or HEPES buffer alone. The AgNP dissolution rates accelerated with increases in ammonia (NH3) concentrations either through increases in pH or through higher (NH4)2SO4 concentrations. Ammonia likely participated in the oxidation of the AgNP to form Ag(NH3)2+ in solution leading to the observed increase in AgNP dissolution rates. AgNP toxicity was enhanced as NH3 concentrations increased. However, Ag+ toxicity was constant at all NH3 concentrations tested. Therefore, it can be concluded that the increased AgNP toxicity was due to increased Ag+ release and not due to a synergistic effect between NH3 and Ag+. The results of this study may provide insights in the fate and toxicity of AgNPs in high NH3 environments including wastewater treatment plants, eutrophic waterways and alkaline environments.