Abstract
Sulfide-modified nanoscale zero-valent iron (S/nZVI) has been widely studied for groundwater remediation, but the potential environmental risks are poorly understood. This study examined the toxicity of S/nZVI to Escherichia coli in aqueous solutions. The sulfidation could reduce toxicity of nZVI, and S/nZVI exhibited a weaker toxicity at lower Fe/S molar ratio, resulting from the lower Fe0 content and higher sulfate and iron oxide. The toxicity of S/nZVI was significantly alleviated in the presence of N-Acetyl-L-cysteine (a scavenger for reactive oxygen species (ROS)), revealing that the ROS-induced oxidative stress was the principal mechanism. Moreover, Transmission Electron Microscopy images elucidated that the membranes of S/nZVI-treated cells were disrupted and S/nZVI existed on E. coli surface and in the cytoplasm. S/nZVI might have interacted with the amine, carboxyl, and ester groups on E. coli cell surface, as demonstrated by Fourier Transform Infrared Spectroscopy analysis. However, the presence of individual groundwater component (e.g., Ca2+, SO42−, HCO3− and humic acid) could more or less alleviate the toxicity of S/nZVI. Furthermore, S/nZVI only exhibited slight toxic effect (<0.15-log after 1 h) in the presence of the mixed components. The same faint toxicity was observed for the aged S/nZVI, indicating that S/nZVI could lose its toxicity over time.
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•S/nZVI exhibited a weaker toxicity than nZVI toward E. coli.•The toxicity of S/nZVI was weaker at lower Fe/S molar ratio.•ROS-induced oxidative stress was the primary mechanism for the toxicity of S/nZVI.•The presence of groundwater components could alleviate the toxicity of S/nZVI.•S/nZVI could gradually loss its toxicity with aging time.