Mathematisch-Naturwissenschaftliche Fakult ät

1. Mathematisch-NaturwissenschaftlicheFakultät FachbereichGeowissenschaften Modeling Reductive Biodegradation of Chlorinated Ethenes ShenghuaYue, Dr. Fernando MazoD’Affonseca, Prof. Dr.PeterGrathwohl shenghua.yue@student.uni-tuebingen.de Center for Applied Geosciences (ZAG), EberhardKarls university Tuebingen Introduction Results 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 H2 and provides insight into the relationship between rate-limited mass transfer from DNAPLs and aqueous phase dechlorination kinetics. Fig. 4: Data from the study of Fennell and Gossett (1998) Fig. 3: Simulation from PHREEQC. 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. Background Theory • 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. 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. For dechlorination to occur, dechlorinators must outperform other hydrogen utilizers. Iron and sulfate are apparently the most important competitors in the environment. Fig. 5: Simulation for hydrogen competition.. Coupling with biological dechlorination , DNAPL dissolution was significantly enhanced. Fig.1: Sequential dechlorination of chlorinated ethenes(Smidt et al., 2000). Fig. 6: Simulation of DNAPL dissolution... Conclusions The conceptual model is diagrammed in Fig.2. It depicts the reactions between six microbial populations in a batch system: fermenters, dechlorinators, methanogens, acetotropicmethanogens, iron reducers and sulfate reducers.Moreover, DNAPL dissolution and mineral-aqueous interactions are incorporated. Model Development • The PHREEQC model is verified by the comparison with the study of 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 H2 for ferric iron. • H2 level was an indicator of the dominant electron acceptor process. • The presence of biodegradation significantly enhances the DNAPL dissolution. Acknowledgements Many thanks to Prof. Dr. Peter Grathwohl, Dr. Fernando MazoD’Affonseca, Prof. Dr. Lewis Semprini and Diplo. DominikHöyng for their support. Fig.2: Main processes involved in the model simulations. References Fennell, D. E. and J. M. Gossett (1998). "Modeling the Production of and Competition for Hydrogen in a Dechlorinating Culture." Environmental Science & Technology32(16): 2450-2460. Smidt, H., A. D. L. Akkermans, J. van derOost and W. M. de Vos (2000). "Halorespiring bacteria-molecular characterization and detection." Enzyme and microbial technology27(10): 812-820