Elucidating the impact of sulfur precursors on the reactivity, toxicity, and colloidal stability of post-sulfidized nanoscale zerovalent iron

Abstract

The sulfur precursor-induced sulfidation effectively enhances the nanoscale zerovalent iron (nZVI) reactivity. However, due to the batch-to-batch variation in synthesis and reaction conditions, the sulfur precursor that confers superior reactivity to sulfidized nZVI (S-nZVI) and its application potential are undetermined. This study evaluated the impact of six sulfur precursors (Na2S, Na2S2O4, S0, Na2S2O3, K2S6, and sulfate reduction effluent [a sulfide-containing effluent]) on the reactivity, toxicity, and colloidal stability of S-nZVI. The reactivity enhancement induced by sulfidation of the different sulfur precursors was inconsistent and dependent on the contaminant. In the removal of trichloroethylene (TCE) and hexavalent chromium Cr(VI) in aqueous solution, S-nZVI, with a high Fe0 content, hydrophobicity, and electron transfer properties (i.e., S-nZVINa2S and S-nZVINa2S2O3), yielded relatively high capacities. In the removal of tetracycline (TC) by S-nZVI, the performance was relatively unaffected by the sulfur precursors. The toxicity evaluation demonstrated that S-nZVIs showed a limited toxicity (less than 0.35-log) to Escherichia coli (E. coli) in simulated groundwater. Despite sulfidation marginally improving the colloidal stability of nZVI—by reducing the interparticle forces—, the S-nZVI particle settling was observed, within 60 min, at the bottom of the vial containing the suspension. Overall, none of the sulfur precursors was manifestly preferable to the others. Na2S, Na2S2O3, and SR-effluent were recommended for use in low sulfur content post-sulfidation processes considering the material performances and costs. These findings contribute to the understanding of the effect of the sulfur precursors on the reactivity and application potential of S-nZVI, thereby providing guidance for tuning S-nZVI properties and selecting suitable sulfur precursors for specific in situ remediation scenarios.