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 H-2 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 (epsilon(e)). Under ZVI-limited conditions (initial Fe-0/TCE = 1.6 mol/mol), S-mZVI(bm) gave surface-area normalized reduction rate constants (k'(sA)) and epsilon(e) that were similar to 2- and 10-fold greater than the unsulfidated ball-milled control (mZVI(bm)). Under TCE-limited conditions (initial Fe-0/TCE = 2000 mol/mol), sulfidation increased ksA and epsilon(e) approximate to 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 beta-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.