Reductive dechlorination of chlorinated ethenes by ball milled and mechanochemically sulfidated microscale zero valent iron: A comparative study.

Ball milling is an effective technique to not only activate and reduce the size of commercial microscale zero valent iron (mZVI) but also to mechanochemically sulfidate mZVI. Yet, little is known about the difference between how chlorinated ethenes (CEs) interact with ball milled mZVI (mZVI) and mechanochemically sulfidated mZVI (S-mZVI). We show that simple ball milling exposed the active Fe sites, while mechanochemical sulfidation diminished Fe sites and meanwhile increased S sites. Mechanochemical sulfidation with [S/Fe] increased from 0 to 0.20 promoted the particle reactivity most for TCE dechlorination (∼14-fold), followed by PCE and 1,1-DCE while it diminished the reactivity for trans-DCE (∼0.4-fold), cis-DCE (∼0.02-fold) and VC (∼0.002-fold) compared to simple ball milling. Sulfidation also improved the electron efficiency of CE dechlorination, except for cis-DCE and VC. The k of cis-DCE, VC and trans-DCE dechlorination positively correlated with surface Fe content, suggesting their dechlorination was mainly mediated by Fe site or reactive atomic hydrogen. The k of TCE dechlorination positively correlated with surface S content and the dechlorination mainly occurred on S sites via direct electron transfer. Increased sulfidation favored direct electron transfer mechanism. The k of PCE and 1,1-DCE was not dependent on either parameter and their dechlorination was equally achieved through either mechanism.