Wa. Arnold et Al. Roberts, Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(O) particles, ENV SCI TEC, 34(9), 2000, pp. 1794-1805
Citations number
70
Categorie Soggetti
Environment/Ecology,"Environmental Engineering & Energy
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).