Date of Award

8-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Science

Advisor

Freedman, David L

Committee Member

Brigmon , Robin L

Committee Member

Finneran , Kevin T

Abstract

Perchloroethene (PCE) is a pollutant of major environmental concern at hazardous waste sites worldwide. PCE and trichloroethene (TCE) are suspected carcinogens and are ranked 16th and 31st, respectively, on the Environmental Protection Agency's priority list for hazardous substances, developed under the Comprehensive Environmental Response, Compensation, and Liability Act. As a consequence of the widespread use of chlorinated solvents (including PCE and TCE) for dry cleaning, chemical feedstocks, metal degreasing and other purposes, chloroethenes are widely distributed in the environment. Many soils and groundwater throughout the world are contaminated by chloroethenes. Therefore, further improvements are needed in clean-up methods. Bioaugmentation has been used extensively to treat aquifers contaminated with chlorinated ethenes at sites that lack the microbes needed to accomplish reductive dechlorination at a reasonable rate. However, a major limitation to bioaugmentation has been aquifer pH. Dehalococcoides are required for achieving complete dechlorination to ethene, yet their reported pH optimum is approximately 6.5 to 7.5. To account for this in aquifers with a lower pH level, buffers have been added prior to injection of culture. However, buffer addition can lead to clogging by precipitates, the chemical costs can be substantial, and achieving homogenous distribution is very challenging. One alternative is to use bioaugmentation cultures that are able to function at lower pH levels. The observation of complete dechlorination of PCE and TCE at some sites with pH levels below 6 suggest this should be achievable. However, very limited information is available on bioaugmentation cultures that are capable of complete dechlorination of PCE and TCE at low pH levels. The objectives of this thesis were 1) to further develop an enrichment culture capable of anaerobic reductive dechlorination of PCE to ethene at a pH level of 5.5 or lower, in a large enough quantity to be used in a field demonstration (e.g., in a 19.6 L canister); 2) to evaluate the effect of solid support materials (perlite and sand) on the rate of ethene accumulation at pH 5.5 or lower; 3) to evaluate bioaugmentation with a low pH enrichment culture in groundwater that is poorly buffered; and 4) to test the effect of pH levels below 5.5 (e.g., 5.35 and 5.30) on the rate of reductive dechlorination of PCE, including the rate of ethene accumulation. The starting point for this research was an enrichment culture that showed promise at pH levels below 6. The culture was further enriched over approximately two years of incubation and multiple transfers in mineral salts medium. The volume of culture was scaled up from serum bottles to 2.6 L bottles and then to 19.6 L canisters, creating enough culture to be used in a pilot test at a hazardous waste site in which the aquifer pH is below 6. Consistent reductive dechlorination of PCE to ethene was achieved with the culture at a pH level of approximately 5.5. The highest rate of ethene accumulation was 3.8 µM/d. Supporting material was unnecessary for growth of this low pH enrichment culture. Perlite slightly reduced the lag time needed for the onset of PCE dechlorination and ethene accumulation, but once dechlorination activity was established, perlite did not improve the process. Likewise, sand offered no advantages for growth of the low pH enrichment culture. This is fortuitous, since the presence of solids would hinder application of the culture in the field. Having established consistent operation of the culture at pH 5.5, an experiment was performed to evaluate the effect of lower pH levels. The lowest pH evaluated was approximately 5.3. The culture continued to dechlorinate PCE to ethene; however, the rates were noticeably slower. Improvements in rate may be achievable at the lower pH levels with further incubation of the culture. A microcosm experiment was performed with soil and groundwater from a site in which the pH is consistently below 6. Reductive dechlorination of PCE was observed in the treatment that was bioaugmented with the low pH enrichment culture developed during this research; no dechlorination occurred in the unamended treatment or the treatment that received only lactate or lactic acid. Thus far, the main dechlorination product in the bioaugmented treatment is cDCE; VC has started to accumulate. Although preliminary, these results indicate the low pH enrichment culture shows potential for use in bioaugmentation of low pH sites, without the need for chemical adjustment of the pH. The enrichment culture was inefficient in terms of its use of lactate or lactic acid for reductive dechlorination; only ~1-2% of the electron equivalents were used for this purpose. The majority of electron donor use was for methanogenesis. Decreases in methanogenesis may be achievable by increasing the concentration of PCE added to a level that is inhibitory to methanogens. The results of this study indicate that bioaugmentation of aquifers that have a pH below 6 may be a feasible remediation strategy for treating PCE and TCE. A field trial with the enrichment culture developed during this research is recommended.

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