Kinetics and pathways of chlorinated ethylene and chlorinated ethane reaction with zero-valent metals (Groundwater contamination)

Chlorinated ethylenes and chlorinated ethanes are commonly detected ground water contaminants. Recent interest has focused on using zero-valent metals, specifically iron, to remove or contain these compounds in situ. Despite initial success, little information is available concerning the detailed kinetics or pathways of these reactions. The pathways by which compounds are reduced, as well as the rates of reaction of the parent species and intermediates, will dictate the design parameters of in situ permeable reactive barriers.; In this work, batch and column reactors were used to study the reactions of chlorinated ethylenes and chlorinated ethanes with zero-valent iron and zinc. Parent species disappearance and the formation of reaction intermediates and daughter products were monitored. Using these data, kinetic models were developed, allowing determination of the relative importance of reductive elimination and hydrogenolysis pathways.; Results indicated that reductive β-elimination is an important pathway for reaction of chloroethylenes with iron and zinc. With iron, the chlorinated acetylenes formed via the reductive β-elimination of tetrachloroethylene and trichloroethylene undergo limited hydrogenation to form dichloroethylenes or vinyl chloride. Thus, production of vinyl chloride, a known human carcinogen, is largely circumvented. Polychlorinated ethanes containing α,β chlorine atoms reacted exclusively via reductive β-elimination with zinc. The exception was pentachloroethane, which also reacted via hydrolysis.; A pseudo-first order model fit the data for the reactions of chlorinated ethylenes with Zn(0) in batch reactors. The rates of reaction of some of the polychlorinated ethanes with Zn(0), however, were found to be mass transfer limited, even though the pseudo-first order reaction rate constant was independent of mixing speed. Intra- and interspecies competitive effects were observed during the reaction of chlorinated ethylenes by Fe(0) in batch and column reactors. This required the use of a Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model. The LHHW kinetics accurately modeled the network of 19 parallel and sequential reactions of the chlorinated ethylenes with Fe(0).; The results obtained in this work regarding reaction pathways and reaction kinetics will aid in the design of zero-valent metal treatment systems. This study has revealed that both product branching ratios and intraspecies competition may have important design ramifications.