There is a large body of evidence concerning the aerobic and anaerobic metabolism of tetrachloroethene (PCE) and trichloroethene (TCE); however, there is uncertainty about the environmental fates of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC). The microorganisms at some sites seem unable to transform PCE and TCE beyond cDCE, and large amounts of cDCE often accumulate. The microbial communities at other sites can effect complete dechlorination to ethene, although VC reductive dechlorination is often rate-limiting. Whether incomplete dechlorination at some sites is due to a lack of appropriate organisms or inappropriate environmental conditions is not known.
The objective of this project was to establish a better understanding of aerobic and anaerobic transformation of cDCE and VC.
Anaerobic biological processes have, in some cases, resulted in the natural destruction (intrinsic bioremediation) of chlorinated solvents, thus precluding expensive ex situ treatment. cDCE and VC, intermediates in the biological reductive dehalogenation of PCE and TCE, appear to require molecular hydrogen as a key electron donor for these reductive processes; however, the transformation of cDCE to VC and of VC to ethene is very slow. A major question is why the process does not go to completion to ethene.
In the initial phase of this project, microorganisms, enzymes, and mechanisms involved in cDCE and VC anaerobic reduction were studied. In Phase II, cDCE and VC reductive dehalogenation kinetics were evaluated, and in Phase III, the potential chemical factors that may affect these processes were also evaluated. Phase IV involved development of a field procedure for estimating the presence and rate of release of electron donors for reductive dehalogenation. In the final phase, anaerobic cDCE and VC oxidizing microorganisms were isolated and anaerobic oxidation kinetics determined.
Major findings include: (1) identification of a gene for evaluating, predicting, and monitoring in situ reductive VC dehalogenation; (2) evidence confirming the inhibitory effect of cDCE on VC dechlorination; (3) evidence confirming that microbial growth and dechlorination can be achieved under lower hydrogen concentrations, although, based on kinetic limitations, it may be unlikely that chlorinated ethenes can be reduced to acceptable EPA drinking water levels; and (4) the first report of an organism obtaining energy for growth through every step of the reduction of TCE to ethene.
This project provided new basic information on the mechanism of biological reductive dehalogenation of cDCE and VC. Other benefits include molecular probes (patent obtained) for reductive VC dehalogenation, a refined mathematical model for growth limitation at low substrate concentrations, data on the inhibition of cDCE and VC reductive dehalogenation by other co-contaminants, and a field procedure to determine the availability of hydrogen for reductive dehalogenation.