Effect of competitive terminal electron acceptor processes on dechlorination of cis-1,2-dichloroethene and 1,2-dichloroethane in constructed wetland soils

Abstract

Thermodynamic calculations were coupled with time-series measurements of chemical species (parent and daughter chlorinated solvents, H2, sulfite, sulfate and methane) to predict the anaerobic transformation of cis-1,2-dichloroethene (cis-1,2-DCE) and 1,2-dichloroethane (1,2-DCA) in constructed wetland soil microcosms inoculated with a dehalorespiring culture. For cis-1,2-DCE, dechlorination occurred simultaneously with sulfite and sulfate reduction but competitive exclusion of methanogenesis was observed due to the rapid H2 drawdown by the dehalorespiring bacteria. Rates of cis-1,2-DCE dechlorination decreased proportionally to the free energy yield of the competing electron acceptor and proportionally to the rate of H2 drawdown, suggesting that H2 competition between dehalorespirers and other populations was occurring, affecting the dechlorination rate. For 1,2-DCA, dechlorination occurred simultaneously with methanogenesis and sulfate reduction but occurred only after sulfite was completely depleted. Rates of 1,2-DCA dechlorination were unaffected by the presence of competing electron-accepting processes. The absence of a low H2 threshold suggests that 1,2-DCA dechlorination is a cometabolic transformation, occurring at a higher H2 threshold, despite the high free energy yields available for dehalorespiration of 1,2-DCA. We demonstrate the utility of kinetic and thermodynamic calculations to understand the complex, H 2-utilizing reactions occurring in the wetland bed and their effect on rates of dechlorination of priority pollutants. © 2006 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.