In this study, we comprehensively evaluate chloride- and ionic-strength-mediated changes in the physical morphology, dissolution, and bacterial toxicity of silver nanoparticles (AgNPs), which are one of the most-used nanomaterials. The findings isolate the impact of ionic strength from that of chloride concentration. As ionic strength increases, AgNP aggregation likewise increases (such that the hydrodynamic radius [HR] increases), fractal dimension (Df) strongly decreases (providing increased available surface relative to suspensions with higher Df), and the release of Ag(aq) increases. With increased Ag(+) in solution, Escherichia coli demonstrates reduced tolerance to AgNP exposure (i.e., toxicity increases) under higher ionic strength conditions. As chloride concentration increases, aggregates are formed (HR increases) but are dominated by AgCl(0)(s) bridging of AgNPs; relatedly, Df increases. Furthermore, AgNP dissolution strongly increases under increased chloride conditions, but the dominant, theoretical, equilibrium aqueous silver species shift to negatively charged AgClx((x-1)-) species, which appear to be less toxic to E. coli. Thus, E. coli demonstrates increased tolerance to AgNP exposure under higher chloride conditions (i.e., toxicity decreases). Expression measurements of katE, a gene involved in catalase production to alleviate oxidative stress, support oxidative stress in E. coli as a result of Ag(+) exposure. Overall, our work indicates that the environmental impacts of AgNPs must be evaluated under relevant water chemistry conditions.

Metal and metal-oxide nanoparticles (NPs) are used in numerous applications and have high likelihood of entering engineered and natural environmental systems. Careful assessment of the interaction of these NPs with bacteria, particularly biofilm bacteria, is necessary. This perspective discusses mechanisms of NP interaction with bacteria and identifies challenges in understanding NP–biofilm interaction, considering fundamental material attributes and inherent complexities of biofilm structure. The current literature is reviewed, both for planktonic bacteria and biofilms; future challenges and complexities are identified, both in light of the literature and a dataset on the toxicity of silver NPs toward planktonic and biofilm bacteria. This perspective aims to highlight the complexities in such studies and emphasizes the need for systematic evaluation of NP–biofilm interaction.

Hydraulic performance issues in drinking-water biofilters have sometimes been associated with phosphorus limitation and increased production of extracellular polymeric substances in previous bench-scale studies. However, field studies utilizing phosphorus supplementation to improve biofilter hydraulic performance have produced mixed results. Here, we determined the ratio of activities for phosphatase to glycosidase (PHO:GLY), which are enzymes involved in acquiring orthophosphate and biodegradable organic carbon from complex organic substrates, to assess phosphorus limitation in 21 pilot-and full-scale biofilters. Supplementation of the pilot-scale biofilter influents with 37 μg/L orthophosphate-P reduced the PHO:GLY from 1.8-40.3 (mean 14.8) to 0.3-15.9 (mean 5.3), demonstrating that increased orthophosphate availability decreases PHO:GLY. In the absence of phosphorus supplementation, the PHO:GLY of the pilot-and full-scale biofilters ranged from 0.3 to 40.3 (mean 10.1), and no hydraulic performance issues were noted. Thus, severe phosphorus limitation appears uncommon in the field, suggesting that phosphorus supplementation is unlikely to improve hydraulic performance in typical drinking water biofilters.

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