Mechanochemically Sulfidated Microscale Zero Valent Iron: Pathways, Kinetics, Mechanism, and Efficiency of Trichloroethylene Dechlorination

In water treatment processes that involve contaminant reduction by zerovalent iron (ZVI), reduction of water to dihydrogen is a competing reaction that must be minimized to maximize the efficiency of electron utilization from the ZVI. Sulfidation has recently been shown to decrease H₂ formation significantly, such that the overall electron efficiency of (or selectivity for) contaminant reduction can be greatly increased. To date, this work has focused on nanoscale ZVI (nZVI) and solution-phase sulfidation agents (e.g., bisulfide, dithionite or thiosulfate), both of which pose challenges for up-scaling the production of sulfidated ZVI for field applications. To overcome these challenges, we developed a process for sulfidation of microscale ZVI by ball milling ZVI with elemental sulfur. The resulting material (S-mZVI^bm) exhibits reduced aggregation, relatively homogeneous distribution of Fe and S throughout the particle (not core-shell structure), enhanced reactivity with trichloroethylene (TCE), less H₂ formation, and therefore greatly improved electron efficiency of TCE dechlorination (ϵ_e). Under ZVI-limited conditions (initial Fe⁰/TCE = 1.6 mol/mol), S-mZVI^bm gave surface-area normalized reduction rate constants (k′_SA) and ϵ_e that were ∼2- and 10-fold greater than the unsulfidated ball-milled control (mZVI^bm). Under TCE-limited conditions (initial Fe⁰/TCE = 2000 mol/mol), sulfidation increased k_SA and ϵ_e ≈ 5- and 50-fold, respectively. The major products from TCE degradation by S-mZVI^bm were acetylene, ethene, and ethane, which is consistent with dechlorination by β-elimination, as is typical of ZVI, iron oxides, and/or sulfides. However, electrochemical characterization shows that the sulfidated material has redox properties intermediate between ZVI and Fe₃O₄, mostly likely significant coverage of the surface with FeS.