Chlorinated ethenes are among the most prevalent legacy contaminants affecting groundwater quality. A common treatment for chlorinated ethenes in the subsurface is in situ anaerobic bioremediation where the organohalide-respiring bacteria, Dehalococcoides mccartyi, convert the contaminants to non-toxic ethene via…
Chlorinated ethenes are among the most prevalent legacy contaminants affecting groundwater quality. A common treatment for chlorinated ethenes in the subsurface is in situ anaerobic bioremediation where the organohalide-respiring bacteria, Dehalococcoides mccartyi, convert the contaminants to non-toxic ethene via hydrogen (H2) dependent reductive dehalogenation. Typically, D. mccartyi obtain H2 through the fermentation of organic substrates by fermentative bacteria. However, stimulation of H2 competing processes causing production of methane (a potent greenhouse gas), rapid substrate consumption of simple substrates, and well/pore clogging by viscous complex substrates often challenge bioremediation, leading to slow rates of dehalogenation or stalls at chlorinated intermediates.This dissertation details the potential of microbial chain elongation as a technology for bioremediation of chlorinated ethenes. In chain elongation, bacteria reliably produce H2 and carboxylates (e.g., butyrate (C4)) using simple compounds (e.g., ethanol (C2) and acetate (C2)) as substrates. Under certain conditions, production of alcohols (e.g., butanol (C4)) can also occur. Here, chain elongation was demonstrated to drive reductive dehalogenation of trichloroethene via direct rapid-release H2 and slow-release H2 during fermentation of elongated products. Results showed chain elongation suppressed methanogenesis, supporting chain elongation as a potential solution for bioremediation when typical fermentable substrates do not meet treatment goals. Next, the potential for chain elongation was evaluated using groundwater and soil from a Superfund site experiencing challenges with bioremediation. Soils from the site were found to contain chain elongating bacteria, while groundwater not previously stimulated with ethanol and acetate was steered to chain elongate with bioaugmentation. Additional chain elongation substrate combinations relevant to bioremediation were identified. Results are being used to inform the design of a pilot study at the site. Lastly, this research identified and demonstrated higher ethanol concentrations, higher total pressures, and higher H2 partial pressure improves chain elongation activity and production of butanol, an important biofuel. These results aid in efforts to make chain elongation relevant as a bioprocess in a circular economy and bioremediation. Cumulatively, this dissertation research demonstrated the potential of chain elongation in bioremediation of chlorinated ethenes, indicating it should be considered when evaluating solutions for contaminated sites.
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Microbial chain elongation (CE) has been shown at laboratory scale to drive reductive dehalogenation (RD) of chlorinated ethenes through both primary (oxidation of ethanol) and secondary (fermentation of medium chain carboxylates) hydrogen (H2) production. This process can offer engineers a…
Microbial chain elongation (CE) has been shown at laboratory scale to drive reductive dehalogenation (RD) of chlorinated ethenes through both primary (oxidation of ethanol) and secondary (fermentation of medium chain carboxylates) hydrogen (H2) production. This process can offer engineers a sustainable in situ bioremediation alternative to address the challenges of conventional treatment technologies and processes. To aid in moving this process into field scale applications, a greater understanding of the specific microbiomes involved in both primary and secondary processes is needed. In this study, microbial community analysis was conducted on groundwater microcosms under various CE substrate combinations to quantify the extent of CE and the effect on RD of cis-1,2-dichloroethene (cis-DCE). Taxonomic classification of amplicon sequence variants obtained from DNA extracted from groundwater microcosms were used to characterize microbiomes using QIIME 2. Pielou’s eveness and beta diversity (via unweighted UniFrac distances) analyses were performed to assess the diversity of microbiomes. Overall, low concentration microcosms (excluding L-7:1 EtOH:Butyrate and L-9:1 EtOH:Acetate + Soil) underwent complete RD, as evidenced by significant ethene production. Alpha and beta diversity analyses confirm the findings of chemical data that the overall substrate concentrations played a major role in determining the extent of CE and RD.
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Chlorinated ethene contamination is present at hundreds of sites around the U.S. and threatens the health and quality of living in many communities. Complete reductive dechlorination of chlorinated ethenes to ethene is possible by the anaerobic bacteria Dehalococcoides mccartyi which…
Chlorinated ethene contamination is present at hundreds of sites around the U.S. and threatens the health and quality of living in many communities. Complete reductive dechlorination of chlorinated ethenes to ethene is possible by the anaerobic bacteria Dehalococcoides mccartyi which uses H2 as an electron donor for the process. Microbial chain elongation (MCE) has recently shown viability as an H2 producing process for reductive dechlorination. This study examined the presence of native chain-elongating organisms in soil and groundwater samples from a Superfund site contaminated with chlorinated ethenes using batch microcosms experiments. The study’s findings have implications for the use of MCE to promote detoxification of chlorinated ethenes at contaminated sites.
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Trichloroethene (TCE) is a ubiquitous soil and groundwater contaminant. The most common bioremediation approach for TCE relies on the process of reductive dechlorination by Dehalococcoides mccartyi. D. mccartyi use TCE, dichloroethene, and vinyl chloride as electron acceptors and hydrogen as…
Trichloroethene (TCE) is a ubiquitous soil and groundwater contaminant. The most common bioremediation approach for TCE relies on the process of reductive dechlorination by Dehalococcoides mccartyi. D. mccartyi use TCE, dichloroethene, and vinyl chloride as electron acceptors and hydrogen as an electron donor. At contaminated sites, reductive dechlorination is typically promoted by adding a fermentable substrate, which is broken down to short chain fatty acids, simple alcohols, and hydrogen. This study explored microbial chain elongation (MCE), instead of fermentation, to promote TCE reductive dechlorination. In MCE, microbes use simple substrates (e.g., acetate, ethanol) to build medium chain fatty acids and also produce hydrogen during this process. Soil microcosm using TCE and acetate and ethanol as MCE substrates were established under anaerobic conditions. In soil microcosms with synthetic groundwater and natural groundwater, ethene was the main product from TCE reductive dechlorination and butyrate and hydrogen were the main products from MCE. Transfer microcosms using TCE and either acetate and ethanol, ethanol, or acetate were also established. The transfers with TCE and ethanol showed the faster rates of reductive dechlorination and produced more elongated products (i.e., hexanoate). The microbial groups enriched in the soil microcosms likely responsible for chain elongation were most similar to Clostridium genus. These investigations showed the potential for synergistic microbial chain elongation and reductive dechlorination of chlorinated ethenes.
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