Date of Award

1-2011

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

Falta , Ronald W

Abstract

1,2-Dichloroethane (DCA) and ethylene dibromide (EDB) are among the top 15 chlorinated aliphatic compounds on the Agency for Toxic Substances and Disease Registry's Priority List of Hazardous Substances. Co-contamination of groundwater with EDB and 1,2-DCA resulted mainly from environmental releases of leaded gasoline; these compounds were added as lead scavengers. A microcosm study by Henderson et al. with soil and groundwater from a site contaminated with leaded gasoline demonstrated anaerobic biodegradation of EDB at a higher rate, and to a greater extent, than 1,2-DCA. Both compounds were transformed mainly by dihaloelimination to ethene. The objectives of this study were to measure maximum specific growth rates, half saturation coefficients, and lag times in enrichment cultures that use 1,2-DCA and EDB as terminal electron acceptors; and to evaluate if the presence of EDB has an effect on the kinetics of 1,2-DCA dehalogenation, and vice versa. The effect of each compound on biodegradation rates of the other was evaluated at the high concentrations that may be found at industrial sites (e.g., >10 mg/L) and at the lower concentrations that have been reported at leaded gasoline sites.
Two enrichment cultures were developed; one was grown with EDB as its terminal electron acceptor, the other with 1,2-DCA. The cultures dehalogenated weekly additions of approximately 24 mg/L of EDB and 1,2-DCA. Lactate was provided as the electron donor and ethene was the predominant product from both compounds. The enrichment cultures were used as a source of inoculum in experiments designed to measure the maximum specific growth rate (μ ̂) and half saturation coefficient (KS) under different conditions: with EDB and 1,2-DCA alone, using their respective enrichment cultures as inoculum; with EDB alone, using the 1,2-DCA enrichment culture as inoculum; with 1,2-DCA alone, using the EDB enrichment culture as inoculum; and with both compounds present together, in one case with the EDB enrichment culture as inoculum and another with the 1,2-DCA enrichment culture as inoculum. Both enrichment cultures grew on either compound, even though the EDB enrichment had never previously been exposed to 1,2-DCA and vice versa.
Based on batch depletion experiments performed at high concentrations of 1,2-DCA and EDB, the maximum specific growth rate (μ ̂) ranged from 0.19 to 0.58 d-1 for 1,2-DCA and from 0.30 to 0.45 d-1 for EDB, with somewhat lower rates for EDB when the 1,2-DCA culture was used as the inoculum source. Maximum transformation rates were 130 µM/d for 1,2-DCA and 74 µM/d for EDB. The half saturation coefficient for 1,2-DCA (5.7-15.7 mg/L, or 58-158 µM) was notably higher than what has been reported for polychlorinated ethenes (e.g., 1.6-3.9 µM for tetrachloroethene, 1.8-2.8 µM for trichloroethene, 1.8-1.9 µM for cis-dichloroethene) but similar in magnitude to vinyl chloride (63-602 µM); the higher KS values occurred when EDB was present along with 1,2-DCA. The KS values for EDB were considerably lower than for 1,2-DCA, with three of the four treatments at or below 15 µg/L (0.082 µM). Nevertheless, the KS for EDB is two orders of magnitude higher than its maximum contaminant level (MCL; 0.00027 µM). In nearly all of the bottles used to measure μ ̂ and KS, the rate of consumption slowed down faster than what was predicted by Monod kinetics. At this transition point, EDB and 1,2-DCA levels reached a plateau or decreased at a considerably slower rate. To account for this behavior, the Monod model was modified to include a transition concentration (St), which was subtracted from the substrate (S) concentration. Without St in the model, the error associated with KS was significantly higher for 1,2-DCA and EDB; the KS value for 1,2-DCA did not change substantially or increased somewhat; and the KS value for EDB was either unchanged or decreased significantly. St levels ranged from 12.9 to 127.5 µg/L for 1,2-DCA and 0.5 to 9.2 µg/L for EDB.
Most importantly, in treatments when EDB and 1,2-DCA were both added, the EDB was always consumed first and adversely impacted the kinetics of 1,2-DCA utilization. In separate experiments with 1,2-DCA provided alone, dechlorination of 1,2-DCA was interrupted by adding EDB at a concentration more than 100 times lower than the remaining 1,2-DCA; use of 1,2-DCA did not resume until the EDB decreased close to its MCL.
Lag times prior to the onset of 1,2-DCA dechlorination ranged from 10-75 days, versus only 2-15 days for EDB. The longest lag periods for 1,2-DCA occurred in treatments that were inoculated with the EDB enrichment culture. Also, the presence of EDB with 1,2-DCA significantly increased the lag times prior to the onset of 1,2-DCA utilization. However, the lag time for EDB was not impacted by the presence of 1,2-DCA (regardless of the inoculum source).
Batch experiments were also performed at lower 1,2-DCA and EDB concentrations, similar to those found near the source zone of leaded gasoline spills. When the 1,2-DCA enrichment culture was provided as the inoculum, EDB was consumed first, reaching its MCL level in 11-22 days. In the treatment with only 1,2-DCA added, its MCL was reached in 28-45 days. With EDB present, biodegradation of 1,2-DCA started shortly after the EDB reached its MCL and the 1,2-DCA was consumed at an equivalent or slightly faster rate than in the treatment with 1,2-DCA alone, suggesting that prior exposure to EDB had a slightly beneficial impact. With the EDB enrichment culture as inoculum, EDB was consumed first, in the presence or absence of 1,2-DCA. The treatment with EDB alone was faster and reached the MCL first; however, the time required to reach the MCL was notably longer (34-38 days) than with the 1,2-DCA enrichment culture. The lag time prior to the onset of 1,2-DCA biodegradation was 49-61 days, considerably longer than for the 1,2-DCA enrichment culture. In the treatment with both compounds present, biodegradation of 1,2-DCA started around the time when EDB reached its MCL. Overall, these experiments confirmed the preferential consumption of EDB over 1,2-DCA and that both contaminants can reach their respective MCL levels, regardless of the type of enrichment culture used as inoculum.
Evidence continues to accumulate for the need to monitor 1,2-DCA and EDB contamination of groundwater, especially at former leaded-gasoline site. Corresponding interest in remediation approaches is likely to increase. Bioaugmentation is a candidate approach for sites where monitored natural attenuation is infeasible. Although considerable information is available on cultures that can dechlorinate 1,2-DCA, most have not been tested for their ability to debrominate EDB. Of the two enrichment cultures evaluated in this study, the 1,2-DCA culture has the advantage of more rapid transition to 1,2-DCA after EDB is consumed. Additional information is needed on the ability of enrichment cultures to dehalogenate 1,2-DCA and EDB in the presence of persistent fuel hydrocarbons.

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