THERMODYNAMICS OF CIS-1,2-DICHLOROETHENE DECHLORINATION UNDER METHANOGENIC AND SULFATE-REDUCING CONDITIONS IN WETLAND PEAT

摘要

In the present study, the potential for biodegradation of cis-1,2- dichloroethene (cis-1,2-DCE) under H2/CO2-dependent methanogenesis and sulfatereducing conditions in laboratory microcosms were assessed using a thermodynamic approach. A peat/compost/sand mixture was selected for study because, based on earlier biodegradation, sorption and geotechnical tests, the soil mixture was found to be a most promising material for construction of a treatment wetland for groundwater contaminated with chlorinated aliphatic organic compounds. The Gibbs free energies (?Gr) under the actual incubation conditions were calculated for the reduction of cis-1,2-DCE, sulfate, and production of methane using the concentrations of the reactants and products. The calculated ?Gr values were used to determine the thermodynamic feasibility of various terminal electron accepting processes (TEAPs) in the peat mixture. Thermodynamic calculations showed that concomitant dechlorination of cis-1,2-DCE and reduction of sulfate is thermodynamically feasible as also attested by reactants/products monitoring data. The calculated energy yields for methanogenesis (-1 to 60 Kj/mol H2) during active dechlorination and sulfate-reduction indicated that methane production was thermodynamically not possible as evidenced by nearly constant concentrations of methane. Except for the reduction of chlorinated solvents, methanogenesis (-20 Kj/mol) and sulfate reduction (-11 to -5 Kj/mol) were observed to occur close to their thermodynamic thresholds. Degradation kinetics of cis-1,2-DCE was observed to be faster by a factor of about two under methanogenic conditions than under sulfatereducing conditions. The peat mixture was able to sustain dechlorination reactions for over 18 months without exogenous electron donor indicating its good potential for construction of treatment wetlands. Results and the approach used in the present work may be used to predict the potential for dechlorination of cis-1,2-DCE and other chlorinated organic compounds in sedimentary environments under various reducing conditions.