Pathways and kinetics through which chlorinated ethylenes and their daughter products react with Fe(0) 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 tetrachloroethylene (PCE), 97% of trichloroethylene (TCE), 94% of cis-dichloroethylene (cis-DCE), and 99% of trans-dichloroethylene (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 dichloroacetylene 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(0).

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HOCl is often assumed to represent the active oxidant in solutions of free available chlorine (FAC). We present evidence that Cl(2)O and Cl(2) can play a greater role than HOCl during chlorination of the herbicide dimethenamid. Reaction orders in [FAC] were determined at various solution conditions and ranged from 1.10 +/- 0.13 to 1.78 +/- 0.22, consistent with the concurrent existence of reactions that appear first-order and second-order in [FAC]. Solution pH, [Cl(-)], [FAC], and temperature were systematically varied so that the reactivity and activation parameters of each FAC species could be delineated. Modeling of kinetic data afforded calculation of second-order rate constants (units: M(-1) s(-1)) at 25 degrees C: k(Cl2O) = (1.37 +/- 0.17) x 10(6), k(Cl2) = (1.21 +/- 0.06) x 10(6), and k(HOCl) = 0.18 +/- 0.10. Under conditions typical of drinking water chlorination, Cl(2)O is the predominant chlorinating agent of dimethenamid. To the extent that Cl(2)O represents the active species in reactions with other disinfection byproduct (DBP) precursors, the influence of [FAC] and pH on DBP precursor reaction rates is different than if HOCl were the principal oxidant. Moreover, these findings call into question the validity of apparent rate constants (k(app)) commonly reported in the chlorination literature.

The reduction of tetrachloroethylene (PCE) and trichloroethylene (TCE) catalyzed by vitamin B12 was examined in homogeneous and heterogeneous (B12 bound to agarose) batch systems using titanium(III) citrate as the bulk reductant. The solution and surface-mediated reaction rates at similar B12 loadings were comparable, indicating that binding vitamin B12 to a surface did not lower catalytic activity. No loss in PCE reducing activity was observed with repeated usage of surface-bound vitamin B12. Carbon mass recoveries were 81−84% for PCE reduction and 89% for TCE reduction, relative to controls. In addition to sequential hydrogenolysis, a second competing reaction mechanism for the reduction of PCE and TCE by B12, reductive β-elimination, is proposed to account for the observation of acetylene as a significant reaction intermediate. Reductive β-elimination should be considered as a potential pathway in other reactive systems involving the reduction of vicinal polyhaloethenes. Surface-bound catalysts such as vitamin B12 may have utility in the engineered degradation of aqueous phase chlorinated ethenes.

Information concerning the pathways and products of reaction of 1,1,1-trichloroethane (1,1,1-TCA) with zero-valent metals may be critical to the success of in situ treatment techniques. Many researchers assume that alkyl polyhalides undergo reduction via stepwise hydrogenolysis (replacement of halogen by hydrogen). Accordingly, 1,1,1-TCA should react to 1,1-dichloroethane (1,1-DCA), to chloroethane, and finally to ethane. Experiments conducted in laboratory-scale batch reactors indicate, however, that with zinc, iron, and two bimetallic reductants (nickel-plated iron and copper-plated iron) this simplistic stepwise scheme cannot explain observed results. 1,1,1-TCA was found to react rapidly with zinc to form ethane and 1,1-DCA. Independent experiments confirmed that 1,1-DCA reacts too slowly to represent an intermediate in the formation of ethane. In reactions with iron, nickel/ iron, and copper/iron, cis-2-butene, ethylene, and 2-butyne were also observed as minor products. Product ratios were dependent on the identity of the metal or bimetallic reductant, with zinc resulting in the lowest yield of chlorinated product. For reactions with iron and bimetallic reductants, a scheme involving successive one-electron reduction steps to form radicals and carbenoids can be invoked to explain the absence of observable intermediates, as well as the formation of products originating from radical or possibly from carbenoid coupling.