Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(O) particles

Pathways and kinetics through which chlorinated ethylenes and their daughte r products react with Fe(0) particles were investigated through batch exper iments. Substantial intra- and interspecies inhibitory effects were observe d, requiring the use of a modified Langmuir-Hinshelwood-Hougen-Watson (LHHW ) kinetic model in which species compete for a limited number of reactive s ites at-the particle-water interface. Results indicate that reductive beta- elimination accounts for 87% of tetrachloroethylene (PCE), 97% of trichloro ethylene (TCE), 94% of cis-dichloroethylene (cis-DCE), and 99% of trans-dic hloroethylene (trans-DCE) reaction. Reaction of 1,1-DCE gives rise to ethyl ene, consistent with a reductive alpha-elimination pathway. For the highly reactive chloro- and dichloroacetylene intermediates produced from the redu ctive elimination of TCE and PCE, 100% and 76% of the reaction, respectivel y, 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 wi th increasing halogenation, counter to the trend that might be anticipated if the rate-limiting step were to involve dissociative electron transfer. W e propose that the reaction of vinyl halides proceeds via a di-sigma-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 rea ction with Fe(0).