Groundwater geochemical constituents controlling the reductive dechlorination of TCE by nZVI: Evidence from diverse anaerobic corrosion mechanisms of nZVI

Abstract

The corrosion mechanisms of nanoscale zero-valent iron (nZVI) vary with different geochemical constituents, which affect the reductive dechlorination process of trichloroethylene (TCE). In this study, the effect of nZVI anaerobic corrosion on the reductive dechlorination of TCE with different groundwater geochemical constituents (Ca2+-SO42-, Ca2+-HCO3-, Na+-NO3-) was investigated. Microscopic characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM) combined with pH, oxidation-reduction potential (ORP) and dissolved Fe2+ in solutions to illustrate the corrosion mechanism of nZVI. In the four systems including ultrapure water (UPW), the reduction of TCE conformed to pseudo-first-order kinetics, the generation of Cl− accorded with zero-order kinetics, and multi-step reaction kinetics was used to fit the generation and degradation of chlorinated byproducts (Dichloroethylene, DCEs). Compared with UPW system, the dissolution corrosion of Ca2+-HCO3- and Ca2+-SO42- promoted the reductive dechlorination of TCE (kobs, TCE = 0.658 ± 0.010 & 0.245 ± 0.028 d−1 and kobs, Cl- = 41.682 ± 1.016 & 20.623 ± 1.923 μM⋅d−1 for Ca2+-HCO3- & Ca2+-SO42-, respectively) and the degradation of DCEs (0.444 ± 0.036 & 0.244 ± 0.040 μM⋅d−1 for Ca2+-HCO3- & Ca2+-SO42-, respectively); redox-active NO3− competed for electrons and passivated the surface of nZVI, which limited the reductive dechlorination of TCE (kobs, TCE = 0.111 ± 0.025 d−1 & kobs, Cl- = 14.943 ± 0.664 μM⋅d−1) and the degradation of DCEs (0.078 ± 0.018 μM⋅d−1), and the passivation layer promoted the adsorption of TCE. This study from the perspective of nZVI corrosion provides a theoretical basis for the long-term application of nZVI technology in the remediation of TCE-contaminated sites with different groundwater geochemical types.