Sulfidation of Zero-Valent Iron by Direct Reaction with Elemental Sulfur in Water: Efficiencies, Mechanism, and Dechlorination of Trichloroethylene

Sulfidation can enhance both the reactivity and selectivity (i.e., electron efficiency, ε e ) of zero-valent iron (ZVI) in contaminant removal, which may make this technology cost-effective for a wider range of water treatment applications. However, current sulfidation methods involve either hazardous or unstable sulfidation agents (e.g., Na 2 S, Na 2 S 2 O 3, and Na 2 S 2 O 4 ) or energy-intensive preparations (e.g., mechanochemical sulfidation with elemental sulfur). In this study, we demonstrate that very efficient sulfidation of microscale ZVI (mZVI) can be achieved at all S/Fe molar ratios (∼100% sulfidation efficiency, ε s ) simply by direct reaction between elemental sulfur (S 0 ) and ZVI in an aqueous suspension at ambient temperature. In comparison, the ε s values obtained using Na 2 S, Na 2 S 2 O 3, or Na 2 S 2 O 4 as the sulfidation agents were only ∼23, ∼75, and ∼38%, respectively. The sulfidated mZVI produced using the new method reacts with trichloroethylene (TCE) with very high rates and electron efficiencies: rate constants and electron efficiencies were 800- and 79-fold higher than those of the unsulfidated mZVI. The enhanced performance of this material, together with the operational advantages of S 0 for sulfidation (including safety, stability, and cost), may make it a desirable product for full-scale engineering applications.