Heading - Research Brief from the Western Region Hazardous Substance Research Center.
Brief #4
August 2004

Summary of the Problem

Dense Non-Aqueous Phase Liquids (DNAPLs) are chemicals that do not mix well with water. When they spill, they can sink deep within aquifers and form highly concentrated "source zones" that are extremely difficult to clean up. One possible clean-up approach is in situ bioremediation -- a method where native or introduced microbes feed on the chemicals and convert them into harmless end products. In situ bioremediation is an attractive solution because it potentially has lower costs than other cleanup technologies, particularly ones that require pumping contaminants to the surface.

WRHSRC researchers Seungho Yu and Lewis Semprini are investigating one biochemical process that could be exploited for in situ bioremediation of contaminant source zones. The process, called anaerobic reductive dechlorination, degrades one common type of DNAPL, the chlorinated ethylenes. The toxic solvents Tetrachloroethylene (PCE) and trichloroethylene (TCE) are both members of this class of chemicals.


About the WRHSRC

The Western Region Hazardous Substance Research Center (WRHSRC) is one of five university-based hazardous substance research centers in the United States. The Centers are funded by grants from the US EPA Office of Research and Development and Office of Solid Waste and Emergency Response. Our Research Briefs are designed to enhance our communication with environmental professionals and others interested in emerging technologies for hazardous substance cleanup. For more information about the WRHSRC visit: http://wrhsrc.orst.edu or call 541-737-2751.

 
   

Anaerobic Reductive Dechlorination

Groundwater contaminated with high concentrations of chlorinated solvents such as TCE and PCE is extremely difficult to clean up. One process that shows promise for in situ remediation is anaerobic reductive dechlorination. In this process, anaerobic microbes trigger a sequence of reactions that transform PCE and TCE to the harmless substance ethene.

Click on highlighted words and definitions and illustrations will pop up.

A first step toward developing reductive dechlorination for bioremediation is to define the kinetics and inhibition of the transformation reactions. WRHSRC researchers Lewis Semprini and Seungho Yu are working on this problem. They have defined kinetic parameters for each step in the reaction sequence and developed kinetic models that successfully predict dechlorination rates for a wide range of PCE and TCE concentrations. Their work is described in a forthcoming paper in Biotechnology and Bioengineering.

Defining Kinetic Parameters

Anaerobic reductive dechlorination of TCE and PCE occurs as a sequence of reactions (Figure 1). Microbes replace chloride molecules in the chemical's structure with hydrogen molecules. Replacement of one chloride molecule transforms PCE to TCE. Replacement of another transforms TCE to cis-dichloroethylene (cis-DCE), then cis-DCE to vinyl chloride, and finally vinyl chloride to the harmless substances ethene and chloride.

Yu and Semprini carried out a series of stepwise experiments to determine kinetic parameters for each reaction in the sequence (Yu, 2003). The experiments used batch kinetic reactors -- small glass bottles that maintained anaerobic conditions and contained enriched dechlorinating cultures. For each experiment, they introduced a known concentration of a compound, for example c-DCE, and measured its disappearance and the production of its daughter compound. They then purged the reactor and added a higher concentration of the parent compound and repeated the measurements (Figure 2). They repeated this procedure in steps and determined kmax and Ks values from regression of the transformation rate vs. concentration results.

Modeling Transformation and Inhibition

The team used the kinetic parameters to develop models of the complete dechlorination reaction sequence. The models mathematically predicted the transformation rate for each intermediate product and the production rate of the end product ethene. They compared the models predictions with results from experiments with a wide range of starting concentrations of PCE and TCE. They were particularly interested in whether they could successfully model dechlorination for high concentrations, up to the solubility limit of PCE and half the solubility limit of TCE.

The team’s initial model used competitive Michaelis-Menten kinetics to predict dechlorination rates. This model took into account the ability of some products in the reaction sequence to inhibit other reaction steps. For example, in earlier experiments they found that the more chlorinated compounds inhibited dechlorination of the less chlorinated compounds. When they compared their model predictions with laboratory experiments, they found good agreement for dechlorination of low concentrations of PCE and TCE. However, the model did not predict the transformation rates they observed for high contaminant concentrations -- model transformation rates were much higher than those observed in the experiments. This suggested that an additional inhibition mechanism affects reaction rates when PCE and TCE concentrations are very high. Yu and Semprini then developed a second model which included both competitive and Haldane inhibition kinetics. In Haldane kinetics, the compound itself inhibits its own transformation. This model was successful -- it predicted the transformation rates they observed for experiments with very high concentrations of PCE and TCE (Figure 3).

Next Steps

The researchers are now experimenting with the cultures in continuous-flow columns (Figure 4). These studies will help evaluate the potential of anaerobic reductive dechlorination under flow conditions that are more representative of in-situ remediation. They will also use molecular techniques to identify and monitor the microbial cultures. Each research step brings the scientists closer to a practical cleanup solution for groundwater contaminated with high concentrations of chlorinated solvents.

For More Information

Contact Dr. Lewis Semprini or read more in the following references:

Yu, S., 2003, Kinetic and modeling investigations of the anaerobic reductive dechlorination of chlorinated ethylenes using single and binary mixed cultures and silicon-based organic compounds as slow-release substrates, Ph.D. Dissertation, Department of Civil, Construction, and Environmental Engineering, Oregon State University.

Yu, S. and Semprini, L., 2004, Kinetics and modeling of reductive dechlorination at high PCE and TCE concentrations, Biotechnology and Bioengineering, in press.

Yu, S. and Semprini, L., 2002a, Comparison of trichloroethylene reductive dehalogenation by microbial communities stimulated on silicon-based organic compounds as slow-release anaerobic substrates, Water Research, vol. 36, p.4985-4996.

Yu, S. and Semprini, L., 2002b, Dechlorination of PCE DNAPL with TBOS Using a Binary Mixed Culture. In Remediation of Chlorinated and Recalcitrant Compounds, Battelle Press, Columbus, OH, Paper 2B-49.

Additional References

ATSDR, 1997, Public Health Statement for Tetrachloroethylene, CAS# 127-18-4, http://www.atsdr.cdc.gov/toxprofiles/phs18.html .

Squillace, P.J., Scott, J.C., Moran, M.J., Nolan, B.T., and Kolpin, D.W., 2002, VOCs, pesticides, nitrate, and their mixtures in groundwater used for drinking water in the United States: Environmental Science & Technology, v. 36, no. 9, p. 1923-1930.(report --pdf version). Supplemental material (MS Excel format).


Copyright © 2004 Oregon State University
WRHSRC News
Western Region Hazardous Substance Research Center

Oregon State University - Web Disclaimer
Please send comments and questions to: wrhsrc@engr.orst.edu
Link to the WRHSRC home page. Link to Oregon State University's College of Engineering. Link to Stanford University's College of Engineering.