Sulfophilic metal ions in groundwater induce particle structure and dechlorination efficiency change of sulfidated zero-valent iron

Sulfidated microscale zero-valent iron (S-mZVI) is a promising ZVI material for remediation of chlorinated hydrocarbons (CHCs). However, the structure and dechlorination behavior change of S-mZVI induced by sulfophilic metal (Me) ions in groundwater are barely studied. Here we show that Me ion-amended S-mZVI (S-mZVI^Me) have a rate sequence of S-mZVI^Co>S-mZVI^Ni>S-mZVI>S-mZVI^Cu>S-mZVI^Cd$\approx$S-mZVI^Zn and S-mZVI^Ni>S-mZVI^Cd>S-mZVI$\approx$S-mZVI^Zn$\approx$S-mZVI^Cu$\approx$S-mZVI^Co for trichlorethylene (TCE) dechlorination and hydrogen evolution reaction (HER), respectively. This results in the highest ever reported electron efficiency (98.6 %) for TCE dechlorination by S-mZVI^Co. Cross-section SEM-EDS, XRD, and XPS analyses confirm the formation of MeS_x on the surface of all S-mZVI^Me. Additionally, Ni⁰, Cu⁰, and possibly Cd° formed on the S-mZVI^Ni, S-mZVI^Cu, and S-mZVI^Cd, respectively. Theoretical calculations indicate that the nascent metal sulfides are more hydrophobic than FeS, indicating the faster HER with Ni and Cd amendment is likely due to formation of bimetallic structures. Correlation analyses suggest that both low band gap and high work function of the semi-conductive Co sulfide contribute to the high reactivity of S-mZVI^Co. Column studies further show that implementing Co²⁺ enables the dechlorination of TCE from 2000 µg/L to <70 µg/L up to 1000 pore volumes by S-mZVI, compared to >1.2 mg/L without Co²⁺. These findings have important implications for remediation of CHC-contaminated sites using S-mZVI.