Stimulating Bacteria to Degrade Industrial Chlorinated Solvents

Abstract

The widespread use and improper storage of chlorinated solvents in industries like agriculture, dry cleaning, and manufacturing has resulted in these toxic chemicals being released into the environment. In Minnesota, perchloroethylene (PCE) has been identified in 50 to 100 remediation sites. Once in the environment, remediation of these solvents is difficult due to the stability of its carbon-halogen bond and its volatility. Biological dechlorination of chlorinated solvents such as PCE is a well-known method of remediation. Although these methods can be successful, additional work is needed to limit the formation of toxic intermediates that are present due to incomplete dechlorination. Current remediation methods also rely on the addition of an external carbon source, such as methanol or lactate, that acts as an electron donor, and presents an additional cost for these remediation techniques.Biological dechlorination is performed by a variety of microorganisms. For this study, these microorganisms are referred to as obligate organohalide respiring bacteria (OHRB), facultative OHRB, and hydrolytic dechlorinators. Obligate OHRB perform reductive dechlorination and use chlorinated solvents as their sole terminal electron acceptors and an external carbon source as their electron donor. These bacteria contain reductive dehalogenase (rdh) genes that help facilitate dehalogenation and generate cellular energy. Facultative OHRB can use a variety of electron acceptors and contain rdh genes as well as dehalogenase (dh) genes to facilitate the dechlorination process. These organisms also use chlorinated solvents as an electron acceptor, but can use other electron acceptors, such as Fe(III). Hydrolytic dechlorinators have been found in both anaerobic and aerobic environments and can use a variety of electron donors and acceptors to perform a substitutive dehalogenation catalyzed by hydrolytic dhs. The work presented in this thesis describes the effect of a substance known as chlorinated natural organic matter (Cl-NOM) on these groups of bacterial dechlorinators. Cl-NOM is derived from natural organic matter that reacts in the environment with free chloride and reactive oxygen species. Batch reactors with known obligate OHRB were operated with varying levels of Cl-NOM and PCE to determine if Cl-NOM amendment would affect PCE dechlorination. Experiments showed that this amendment did in fact accelerate the dechlorination of PCE; it was unclear whether obligate OHRB grew on Cl-NOM itself or grew on PCE in the presence of Cl-NOM. A continuous flow reactor was also operated to better understand if Cl-NOM addition could enrich facultative OHRB or hydrolytic dechlorinators present in uncontaminated soil. Results showed that two facultative OHRB were slightly enriched during reactor operation. Rdh genes were found at higher gene copies than dh genes, suggesting that both facultative OHRB and hydrolytic dechlorinators were enriched. No known obligate OHRB were detected in the reactor.