Several bacterial groups capture energy for growth from the reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) to predominantly cis-1,2-dichloroethene (cis-DCE). Field observations indicate that cis-DCE and vinyl chloride (VC) often accumulate, even after biostimulation with nutrient amendments, due to the lack of appropriate organisms or insufficient environmental conditions to promote complete detoxification to ethene, an environmentally benign product.
This project explored the fate of cis-DCE and VC under different redox conditions. Specifically, the following objectives were addressed: (1) identification of the processes and bacterial groups involved in detoxification, (2) determination of mechanisms for enhancing degradation, (3) isolization and characterization of degrading organisms, (4) investigation of the role of site-specific characteristics, and (5) development of a protocol for evaluating a site and identifying the most promising remedial option.
In the past, research focused on the anaerobic transformation of PCE and TCE. Relatively little was known about the types of microorganisms and environmental conditions associated with the dechlorination of cis-DCE and VC. Four microbial processes are involved in their fate in groundwater, including anaerobic chlororespiration, anaerobic energy-yielding oxidation, aerobic co-oxidation, and aerobic energy-yielding oxidation. The microbiology of each process was characterized, and each was evaluated for its potential applicability to groundwater remediation.
This research identified metabolic reductive dechlorination ([de]chlororespiration) and aerobic energy-yielding oxidation as the dominant microbial processes involved in cis-DCE and VC detoxification. Dehalococcoides populations were present in all cultures capable of complete reductive dechlorination to ethene. Dehalococcoides sp. strain BAV1 was obtained in pure culture and is the first isolate capable of growth with all dichloroethene isomers and VC. Characterization of strain BAV1 provided insight into the requirements of Dehalococcoides populations for enhanced dechlorination activity and efficient detoxification. The aerobic work suggested that organisms utilizing cis-DCE as the source of carbon and energy are rare in the environment, but VC-oxidizers were found at most aerobic sites. VC-assimilating organisms, including Ralstonia sp., were obtained in pure culture. The Ralstonia isolate utilized cis-DCE, VC, and ethene as sources of carbon and energy, while trans-DCE was co-metabolized with VC as the primary substrate. Co-oxidation of cis-DCE was observed with all cultures growing with VC or ethene as primary substrates. A site screening protocol was developed to guide practitioners in evaluating site assessment data and in determining the most promising bioremediation approach.
This effort has provided a solid scientific understanding of the critical factors controlling the anaerobic and aerobic transformation of cis-DCE and VC. Qualitative and quantitative molecular tools targeting key dechlorinators were designed for site assessment and bioremediation monitoring. In collaboration with Regenesis, a PCE-to-ethene dechlorinating consortium was commercialized for bioaugmentation applications. The site screening protocol will facilitate implementation of enhanced bioremediation at chloroethene-contaminated sites. (Project Completed - 2004)