Date of Award
Master of Science (MS)
Environmental Engineering and Science
Freedman, David L
Finneran , Kevin T
Lee , Cindy M
Tetrachloroethene (PCE) and trichloroethene (TCE) are the predominant contaminants at hazardous waste sites in the United States. Although less prevalent, dichloromethane (DCM) is also found at a number of sites. EPA classifies PCE and DCM as likely to be carcinogenic in humans by all routes of exposure, while TCE is classified as carcinogenic to humans by all routes. At some sites, releases of PCE, TCE and DCM comingle in the groundwater. Field evidence from one such site in California suggests that DCM is used as the electron donor for reductive dechlorination of TCE. Nevertheless, definitive evidence that DCM can serve as an electron donor for complete reduction of chlorinated ethenes to ethene is lacking. The primary objective of this thesis was to evaluate the use of DCM as an electron donor for reductive dechlorination of PCE to ethene. Two anaerobic enrichment cultures were used. One grows by organohalide respiration of PCE and TCE to ethene, with lactate as the electron donor. The other uses DCM as its sole source of carbon and energy and releases formate and acetate as fermentation products. The experimental design included treatments with a combination of the two cultures and addition of only DCM and PCE. A secondary objective was to perform a preliminary assessment of the microbe responsible for biodegrading DCM.
In the treatment inoculated with both cultures and provided with only PCE (2.4 mg/L) and DCM (9.7 mg/L), biodegradation of DCM and reductive dechlorination of PCE started at the same time. Repeated additions of DCM were consumed in 4-7 days, with only minor accumulation of chloromethane. Repeated additions of PCE were also consumed, with increases and then decreases of chlorinated ethene daughter products. Ethene started to accumulate after approximately three months and by the end of the incubation period (80-130 days), ethene was the only daughter product detected. Formate, acetate, and propionate were detected as products from biodegradation of DCM. Other treatments confirmed that the chlorinated ethene culture can use formate and hydrogen as electron donors, but not acetate; which is consistent with the observed use of DCM as a sole electron donor. A treatment inoculated with the chlorinated ethene culture that received no electron donor failed to reduce PCE. Furthermore, the chloroethene culture was unable to biodegrade DCM and the DCM culture was unable to reduce PCE (with lactate provided as the electron donor).
The only isolate obtained in previous research that is able to grow anaerobically on DCM as a sole carbon and energy source via fermentation is Dehalobacterium formicoaceticum strain DMC. Preliminary attempts were made to evaluate if the DCM enrichment culture developed during this research also contains Dehalobacterium spp. PCR analysis of the enrichment culture tested positively for the presence of members of the phylum Firmicutes, which includes Dehalobacterium. Microscopic evaluation of the enrichment revealed an abundance of short rods, which were gram positive, which is also consistent with Dehalobacterium. Nevertheless, additional research is needed to determine a more specific identification of the microbe responsible.
The results of this study provide definitive evidence that it is possible for DCM to serve as an electron donor for reductive dechlorination of PCE to ethene. While it is inconceivable that DCM would ever be intentionally added to serve as an electron donor, the results are relevant to those sites where these contaminants are comingled.
Kanitkar, Yogendra, "EVALUATION OF DICHLOROMETHANE AS AN ELECTRON DONOR FOR REDUCTIVE DECHLORINATION OF TETRACHLOROETHENE TO ETHENE" (2012). All Theses. 1456.