Adsorbed Polymer and NOM Limits Adhesion and Toxicity of Nano Scale Zerovalent Iron to E. coli

Nanoscale zerovalent iron (NZVI) is used for groundwater remediation. Freshly synthesized bare, i.e. uncoated NZVI is bactericidal at low mg/L concentration, but the impact of surface modifiers and aging (partial oxidation) on its bactericidal properties have not been determined. Here we assess the effect that adsorbed synthetic polymers and natural organic matter (NOM) and aging (partial oxidation) have on the bactericidal properties of NZVI to the gram-negative bacterium, Escherichia coli. Exposure to 100 mg/L of bare NZVI with 28% Fe0 content resulted in a 2.2-log inactivation after 10 min and a 5.2-log inactivation after 60 min. Adsorbed poly(styrene sulfonate) (PSS), poly(aspartate) (PAP), or NOM on NZVI with the same Fe0 content significantly decreased its toxicity, causing less than 0.2-log inactivation after 60 min. TEM images and heteroaggregation studies indicate that bare NZVI adheres significantly to cells and that the adsorbed polyelectrolyte or NOM prevents adhesion, thereby decreasing NZVI toxicity. The 1.8-log inactivation observed for bare NZVI with 7% Fe0 content was lower than the 5.2-log inactivation using NZVI with 28% Fe0 after 1 h; however, the minimum inhibitory concentration (MIC) after 24 h was 5 mg/L regardless of Fe0 content. The MIC of PSS, PAP, and NOM coated NZVI were much higher: 500 mg/L, 100 mg/L, and 100 mg/L, respectively. But the MIC was much lower than the typical injection concentration used in remediation (10 g/L). Complete oxidation of Fe0 in NZVI under aerobic conditions eliminated its bactericidal effects. This study indicates that polyelectrolyte coatings and NOM will mitigate the toxicity of NZVI for exposure concentrations below 0.1 to 0.5 g/L depending on the coating and that aged NZVI without Fe0 is relatively benign to bacteria.