A New Electron Donor for Reductive Dechlorination of Perchloroethylene

One of the main problems associated with remediation of chlorinated solvent groundwater contamination is the presence of NAPLs in the source zone. NAPLs are often better treated through solvent extraction, surfactant washing, or vapor extraction than through bioremediation. Removal of NAPL through solvent extraction would likely leave a substantial residual of both the extraction solvent and the chlorinated solvent. We hypothesized that utilization of the remaining extraction solvent as an electron donor could be exploited to clean up the remaining chlorinated solvent following source zone remediation. The main question is whether a suitable extraction solvent could work as an electron donor to drive reductive dechlorination. Many different compounds have been used as electron donors to stimulate reductive dechlorination, including volatile fatty acids (VFAs), sugars, hydrogen, and polymeric compounds. Generally, it is regarded that fermentation of compounds yields hydrogen, the true electron donor for reductive dechlorination. Compounds with high hydrogen yields generally are thought to be the best electron donors for bioremediation. After looking at several potential candidate solvents that might serve as electron donors for reductive dechlorination, we decided to investigate the “green technology” solvent ethyl lactate. Ethyl lactate (EL) is reported to be easily broken down in the environment (although data is notably sparse on the exact metabolites). Possible metabolites would likely be ethanol and lactate. Since the latter is an excellent source of hydrogen through fermentative processes, we investigated whether a mixed culture known to degrade PCE could grow on EL and whether it could stimulate PCE dechlorination. An anaerobic flow through reactor with decanting was developed to harvest culture cellmass. The reactor was fed EL and PCE separately and inoculated with a mixed culture previously shown to dechlorinate PCE with lactate as the electron donor. Following an initial start-up period, cellmass production commenced (as shown by protein production) and PCE dechlorination was observed (by chloride release). Analysis of PCE metabolites was performed in batch incubations and the theoretical chloride release was compared to that observed in the culture. The results demonstrate the EL can serve as an electron donor to drive reductive dechlorination of PCE, and may hold promise for in situ bioremediation applications following source zone remediation. INTRODUCTION Chlorinated solvents are hazardous compounds that are common contaminates of soil, sediments, and groundwater (GW) aquifers. The most prevalent chlorinated solvents are the chloroethenes (CEs), which frequently occur as dense non-aqueous phase liquids (DNAPLs) in GW. Some commonly found CEs are perchloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE) and vinyl chloride (VC). Because of their known adverse health effects, treatment is required when human exposure is possible. Potential treatments for in situ GW remediation in current use include: pump-and-treat, solvent flushing/cosolvent extraction, air sparging and bioremediation. Due to strong partitioning to the solid phase, the pump-and-treat method is very slow for hydrophobic compounds and can be hindered by DNAPL formation. Air sparging can be effective for VOCs but typically has high installation costs. The cosolvent extraction method has been shown to be effective but ultimately leaves a residue of the extraction solvent and contaminants that may require further treatment. Bioremediation of CEs can occur both aerobically or anaerobically. Many studies have been made on aerobic dechlorination of CEs (Vogel et al., 1987; Wackett et al., 1992; Nielson et al., 1990). PCE is totally recalcitrant to biodegradation under aerobic conditions. VC is the only CE that can serve as a primary substrate, whereas TCE and DCE can be degraded only cometabolically (compounds are transformed fortuitously by the organisms using some other compound as substrate) under aerobic conditions. VC can also be degraded by cometabolism. Under anaerobic conditions, CEs can be degraded by reductive dechlorination in which CEs are utilized as electron acceptors and alternate carbon sources are used as electron donors. Reductive dechlorination often supports growth of the cultures resulting in either partially or completely dechlorinated products (Ferguson and Pietari, 2000). Many obligately anaerobic microbes are found to degrade CEs. PCE and TCE can act as electron acceptors by dechlorinating cultures like Dehalospirillum multivorans (DHM) and Desulfitobacterium which derive energy from the oxidation of electron donors like lactate and H2 in a process known as halorespiration (Neumann et al., 1995; Gerritse et al., 1996). Pure cultures like DHM (Neumann et al., 1995) and methanogens like Methanosarcina sp. and Methanobacterium thermoautotrophicum (Fathepure et al., 1987) showed partial dechlorination of PCE. The former dechlorinates PCE to cisDCE while the latter dechlorinates PCE to TCE. Complete transformation of PCE to ethene (ETH) was first observed in methanogenic cultures (Freedman and Gossett, 1989). PCE to ethene (ETH) transformation was also VC ETH C C