Date of Award

12-2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Science

Advisor

Freedman, David L

Committee Member

Lee , Cindy M

Committee Member

Carraway , Elizabeth R

Abstract

Halogenated methanes, including carbon tetrachloride (CT) and chloroform (CF), are significant groundwater contaminants. Options for bioremediation of high concentrations are limited. Previous studies have shown that an enrichment culture (designated DHM-1) that grows on corn syrup has the potential for use in bioaugmentation. DHM-1 cometabolically biotransforms high concentrations of CT and CF to nontoxic end products (mainly CO, CO2, and organic acids) in the presence of vitamin B12 (approximately 3% of chlorinated methanes on a molar basis). Sulfide is also required by this culture to function properly. However, insufficient data are available on its performance under field conditions. Also, it is not yet known how this culture would perform at low pH levels that may exist in situ, and how the rate of CF biodegradation is influenced by the concentration of B12.
The objectives of this study were 1) to evaluate biostimulation (using corn syrup with and without B12) and bioaugmentation as approaches to treat CT and CF; 2) to evaluate the effectiveness of an enrichment culture (developed during this study) that anaerobically grows on DCM as its sole carbon and energy source, for biodegradation of DCM produced during reaction of CT and CF with zero valent iron (ZVI); 3) to determine the effect of B12 concentration on the rate of CF dechlorination by the DHM-1 enrichment culture; and 4) to determine the effect of pH on the rate of CF transformation by the DHM-1 enrichment culture.
To address the first two objectives, microcosms were prepared with subsurface material and groundwater from two industrial sites (Sites A and B) contaminated with high concentrations of CT and CF. To address the other objectives, the DHM-1 culture was subjected to various pH levels and B12 concentrations.
The results obtained from the microcosm experiment conducted for Site A indicated that addition of ZVI followed by bioaugmentation was the most promising approach evaluated for removal of CT and CF from the high concentration part of the plume (i.e., in the source zone). This strategy achieved complete removal of CT, nearly complete removal of CF and partial removal of the DCM that accumulated during reaction of the CT and CF with ZVI. There is still a need to determine if the other volatile compounds formed during reaction with ZVI pose a concern for downgradient remediation. Biostimulation with corn syrup plus B12 was effective for removal of CT and almost all of the CF from the medium concentration plume. Bioaugmentation did not improve the rate of transformation; however, addition of DHM-1 ensured complete removal of CF. SDC-9 (a commercially available bioaugmentation culture) and a sulfate-reducing enrichment culture (developed for use in this study) were less effective for removal of CT and CF in the medium concentration microcosms.
An anaerobic enrichment culture that grows on DCM as its sole carbon and energy source was successfully developed. It was acclimated to consume up to 500 mg/L of DCM. Use of the culture for bioaugmentation to remove DCM, in Site A microcosms treated with ZVI, was partially effective; additional work is needed to optimize the use of the culture. Although ZVI plus bioaugmentation removed CT and CF at a faster rate, the accumulation of DCM was a concern that was only partially addressed by bioaugmentation with the DCM enrichment culture.
For the Site A low concentration plume, biostimulation with corn syrup plus B12 was the most promising approach evaluated for removal of CT and CF. Adjusting the pH of the groundwater improved the rate of transformation, although this must be weighed against the cost of in situ pH adjustment. No attempt was made to remove the low level of DCM present in the low concentration plume.
The results of the microcosm experiment performed for Site B indicated biostimulation with corn syrup was effective in removing CT; however, addition of B12 and bioaugmentation did not appreciably enhance the rate or extent of CT biodegradation. Biostimulation and bioaugmentation did not improve the rate of CF removal over the unamended treatment. The rate of CF removal was slow and consistent in the biotic treatments. None of the amendments were effective in removing DCM, although it was present at a lower concentration than CT and CF. Attempts to stimulate the effectiveness of B12 and bioaugmentation by generating sulfide via sulfate reduction were not effective. Although 12 mM of sulfate was consumed in the treatment bioaugmented with DHM-1, there was no enhancement in the rate or extent of CT or CF biodegradation. This suggests that the subsurface material has a considerable sulfide demand that needs to be satisfied before the effectiveness of B12 and DHM-1 can be realized.
The results from the experiments conducted to further characterize the DHM-1 enrichment culture indicated that the maximum CF biodegradation rates for DHM-1 increased with increasing pH from 5.0 to 7.7. Its activity is severely inhibited by pH levels below 6.0. At pH 5.0, it lost its ability to biotransform CF. Between pH 6.4 and 7.3, the rate of CF biodegradation by DHM-1 is somewhat stable; however, it increases rapidly between pH 7.3 and 7.7. Given the inhibitory effect of pH levels below 6.0, the pH of the groundwater should be adjusted within the neutral range for successful bioaugmentation to occur. The sensitivity of DHM-1 to low pH values must be considered for application in aquifers that have pH levels below 6. With respect to the dose of B12, CF biodegradation rates for DHM-1 increased with increasing vitamin B12 concentrations. The relationship between the B12 to CF ratio and the rate of CF biodegradation can be described by a modification of the Michaelis-Menten kinetics model. A Vmax of 664.6 mg CF/L∙d and a B12/Km ratio of 0.00500.0010 mol B12 per mol CF were obtained from fitting experimental results to this model. This information can help in the selection of a cost-effective dose for B12 when it is used as an amendment to facilitate bioremediation of chlorinated methanes.

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