Effects of pH, Electrolyte, Humic Acid, and Light Exposure on the Long-Term Fate of Silver Nanoparticles

We investigated the evolution in silver nano particle (AgNP) properties during a series of 10-50 day experiments on suspensions with different pH (5-9), electrolyte type (NaNO3 and NaCl) and concentration (2 and 6 mM), Suwannee River humic acid (SRHA) concentration (0-13.2 mg C/L), and light exposure (artificial sun light exposure for 8 h per day or dark). Of these factors, pH most influenced the AgNPs' properties as it modifies surface charge as well as AgNP dissolution and oxidation and Ag reduction reactions. As a result, particle behavior differed in basic and acidic conditions. Trends with pH varied, however, based on the electrolyte and SRHA concentration. In the presence of chloride which forms AgCl(s), for example, we observed the particle size decreased with increasing pH. The opposite was observed in identical systems in NaNO3. This behavior was modified by SRHA, with increasing SRHA reducing dissolution and enhancing stability. Light exposure enhanced processes resulting in AgNP dissolution, resulting in higher dissolved Ag concentrations than under similar conditions in the dark. Overall, our results highlight how AgNP properties evolve over time and provide insight needed to confidently extend model system behavior to predict the environmental fate of AgNPs.