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.

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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.