| A number of dissolved halogenated volatile organic compounds (VOCs) undergo abiotic transformation reactions under environmental conditions. These compounds also sorb to natural sediments despite their relative hydrophilicity. To date, research has not been performed that considers how sorption and transformation processes combine to influence the fate of these contaminants.;1,1,2,2 tetrachloroethane (TeCA) has been shown by others to undergo base-mediated dehydrohalogenation, producing trichloroethene (TCE) in solution. In this research, reaction rates published by Cooper et al. (1987) were confirmed and were not affected by the presence of sulfide in buffered aqueous solution.;In this work, adsorption of TeCA was observed to be linear and consistent with the Freundlich isotherm as well as predictive models based on the octanol-water partition coefficient. Desorption was shown to be reversible for a small number of desorption steps then became irreversible. The results compared favorably with a model previously published by Kan et al. (1998) using semivolatile organic compounds.;In a column study with TeCA sorbed to activated carbon, the sorbed reaction occurred with a rate constant approximately half the dissolved reaction rate constant. TeCA in the irreversible compartment of Dickinson Bayou sediment did not react to form TCE. The result is consistent with pH effects, the existence of an energetically stable irreversible compartment, and/or size-related phenomena such as incorporation of the TeCA molecule into the hydrophobic interior of humic acid. TeCA sorbed in the irreversible compartment is apparently not accessible to the hydrophilic hydroxide ion. In a batch reactor, the reaction rate was evaluated by comparing experimental TeCA concentrations with a model based on both the expected reaction rate in solution and the expected impact of sorption on reaction rate as derived from the activated carbon study. The observed reaction rate was approximately two times greater than expected. Although the activated carbon experiment indicated that sorption hinders reactivity, this observation is consistent with heterogeneous catalysis, possibly due to metals incorporated into the humic/fulvic acid structure and natural organic matter. |
