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, H₂-utilizing reactions occurring in the wetland bed and their effect on rates of dechlorination of priority pollutants.