Mathematisch-Naturwissenschaftliche Fakultät Fachbereich Geowissenschaften Modeling Reductive Biodegradation of Chlorinated Ethenes Shenghua Yue, Dr. Fernando Mazo D’Affonseca, Prof. Dr.Peter Grathwohl Center for Applied Geosciences (ZAG), Eberhard Karls university Tuebingen Background Theory Chlorinated ethenes are the most common organic pollutants in the groundwater. As chlorinated ethenes form DNAPL zones in the subsurface, they can serve as persistent contaminant sources. Reductive dechlorination is considered as a crucial process in natural attenuation of chloroethenes. Through the development of batch reactor models in PHREEQC-2, this study seeks to better understand the competition between the different electron acceptors for the limited H 2 and provides insight into the relationship between rate-limited mass transfer from DNAPLs and aqueous phase dechlorination kinetics. Introduction In the presence of suitable microorganisms, hydrogen acting as electron donor can replace a chlorine on a chloroethene molecule, which results in sequential dechlorination from PCE to TCE to DCE to VC to ethene (Fig.1). Meanwhile the mediating organism obtains energy for growth. For dechlorination to occur, dechlorinators must outperform other hydrogen utilizers. Iron and sulfate are apparently the most important competitors in the environment. Fig.1: Sequential dechlorination of chlorinated ethenes(Smidt et al., 2000).Smidt et al., 2000 Model Development The conceptual model is diagrammed in Fig.2. It depicts the reactions between six microbial populations in a batch system: fermenters, dechlorinators, methanogens, acetotropic methanogens, iron reducers and sulfate reducers.Moreover, DNAPL dissolution and mineral- aqueous interactions are incorporated. Fig.2: Main processes involved in the model simulations. Results A good agreement between PHREEQC simulation and lab data from the study of Fennell and Gossett (1998) was found, which demonstrated the ability of the PHREEQC model to simulate the complex dechlorination process. Fig. 3: Simulation from PHREEQC. Fig. 4: Data from the study of Fennell and Gossett (1998)Fennell and Gossett (1998 The results suggested the superiority of iron reduction over dechlorination on electron donor consumption and showed the competitive advantage of dechlorination over sulfate reduction and methanogenesis. Fig. 5: Simulation for hydrogen competition.. Coupling with biological dechlorination, DNAPL dissolution was significantly enhanced. Fig. 6: Simulation of DNAPL dissolution... References  The PHREEQC model is verified by the comparison with the study of Fennell and Gossett (1998).Fennell and Gossett (1998  Dechlorination is more efficient under methanogenic or sulfate reducing conditions than under iron reduction condition (scenario 2), which reflects a greater availability of H 2 for ferric iron.  H2 level was an indicator of the dominant electron acceptor process.  The presence of biodegradation significantly enhances the DNAPL dissolution. Conclusions Many thanks to Prof. Dr. Peter Grathwohl, Dr. Fernando Mazo D’Affonseca, Prof. Dr. Lewis Semprini and Diplo. Dominik Höyng for their support. Acknowledgements Fennell, D. E. and J. M. Gossett (1998). "Modeling the Production of and Competition for Hydrogen in a Dechlorinating Culture." Environmental Science & Technology32(16): Smidt, H., A. D. L. Akkermans, J. van der Oost and W. M. de Vos (2000). "Halorespiring bacteria-molecular characterization and detection." Enzyme and microbial technology27(10):