Pathways and Kinetics of Chlorinated Ethylene and Chlorinated Acetylene Reaction with Fe(0) Particles

Abstract

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).

Understanding Trichloroethylene Chemisorption to Iron Surfaces Using Density Functional Theory

“…These hypotheses that chemical adsorption may be involved in choroalkene reduction are consistent with data showing that reduction rates of TCE are not controlled by an outer-sphere electron transfer reaction (8). Other indirect evidence for a catalytic role of the iron surface in chloroalkene reduction is the observation that alkyl halides, such as 1,2-dichloroethane, are much less reactive than vinyl halides, such as 1,2-dichloroethene, despite having a more positive reduction potential for one and two electron reduction reactions (10). Although there is some indirect evidence, actual chemisorption of chloroalkenes may be difficult to confirm using spectroscopic methods due to the fact that reactions with the iron metal occur beneath the oxide layer coating the iron surfaces (11).…”

Kinetics of TNT Degradation in The Presence of Zero Valent Iron Nanocatalyst

“…7 and the correlation coefficient (R 2 ) of this line is > 0.99, which indicates the observed rate constant (kobs) is proportional to the concentration of nanoscale zero-valent iron and kobs can be improved by increasing the concentration of nano-Fe0 powder. It is in accordance with the results observed by other workers (32,33) , where conventional zero iron powder was applied to reduce chlorinated ethylene and chlorinated acetylene. …”

Carbon and Chlorine Isotope Analysis to Identify Abiotic Degradation Pathways of 1,1,1-Trichloroethane

“…The occurrence of distinct mechanisms is supported by previous studies indicating that production of ethene/ethane from TCE mainly proceeds via β-dichloroelimination to chloroacetylene. 46,60 For the Fe(0) pathway, an ε bulk C of −13.6 ± 0.5‰ from a previous study is available for comparison. 36 This value is larger than the one determined in our study (−7.8 ± 0.4‰), however, significant variation ranging from −10.3 to −14.0‰ has also been recently observed during reduction of 1,1,1-TCA by biotically mediated FeS formation in microcosm experiments.…”