Pathways and kinetics through which chlorinated ethylenes and their daughter products react with Fe(O) particles were investigated through batch experiments. Substantial intra- and interspecies inhibitory effects were observed, requiring the use of a modified Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model in which species compete for a limited number of reactive sites at the particle-water interface. Results indicate that reductive β-elimination accounts for 87 of tetrachloro-ethylene (PCE), 97 of trichloroethylene (TCE), 94 of cisdichloroethylene (cis-DCE), and 99 of trans-dichloro-ethylene (trans-DCE) reaction. Reaction of 1,1-DCE gives rise to ethylene, consistent with a reductive α-elimination pathway. For the highly reactive chloro- and dichloro-acetylene intermediates produced from the reductive elimination of TCE and PCE, 100 and 76 of the reaction, respectively, occur via hydrogenolysis to lesser chlorinated acetylenes. The branching ratios for reactions of PCE or TCE (and their daughter products) with iron particles are therefore such that production of vinyl chloride is largely circumvented. Reactivity of the chlorinated ethylenes decreases markedly with increasing halogenation, counter to the trend that might be anticipated if the rate-limiting step were to involve dissociative electron transfer. We propose that the reaction of vinyl halides proceeds via a di-σ-bonded surface-bound intermediate. The reactivity trends and pathways observed in this work explain why lesser-chlorinated ethylenes have only been reported as minor products in prior laboratory and field studies of PCE and TCE reaction with Fe(O).
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