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-mZVIbm) 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 (εe). Under ZVI-limited conditions (initial Fe0/TCE = 1.6 mol/mol), S-mZVIbm gave surface-area normalized reduction rate constants (k′SA) and εe that were ∼2- and 10-fold greater than the unsulfidated ball-milled control (mZVIbm). Under TCE-limited conditions (initial Fe0/TCE = 2000 mol/mol), sulfidation increased k SA and εe ≈ 5- and 50-fold, respectively. The major products from TCE degradation by S-mZVIbm 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.