Influence of Water Hardness on Silver Ion and Silver Nanoparticle Fate and Toxicity Toward Nitrosomonas europaea

Abstract

Abstract

This study investigated the influence of water hardness (Mg²⁺ and Ca²⁺) on the fate and toxicity of 20 nm citrate silver nanoparticles (AgNPs) and Ag⁺ toward Nitrosomonas europaea, a model ammonia-oxidizing bacterium. Nitrification inhibition of N. europaea by 1 ppm AgNPs and 0.5 ppm Ag⁺ was reduced from 80% and 83%, respectively, in the absence of Mg²⁺ to 2% and 33%, respectively, in the presence of 730 μM Mg²⁺. Introduction of Mg²⁺ resulted in the rapid aggregation of the AgNP suspensions and reduced the 3 h Ag⁺ dissolution rates from 30%, in the absence of Mg²⁺, to 9%, in the presence of 730 μM Mg²⁺. Reduced AgNP dissolution rates resulted in decreased concentrations of silver that were found adsorbed to N. europaea cells. Increasing AgNP concentrations in the presence of Mg²⁺ increased the observed inhibition of nitrification, but was always less than what was observed in the absence of Mg²⁺. The presence of Mg²⁺ also reduced the adsorption of Ag⁺ to cells, possibly due to multiple mechanisms, including a reduction in the negative surface charge of the N. europaea membrane and a competition between Mg²⁺ and Ag⁺ for membrane binding sites and transport into the cells. Ca²⁺ demonstrated similar protection mechanisms, as Ag⁺ toxicity was reduced and AgNP suspensions aggregated and decreased their dissolution rates. These results indicate that the toxicity of Ag⁺ and AgNPs to nitrifying bacteria in wastewater treatment would be less pronounced in systems with hard water.