Biodegradation of 1,4-Dioxane in Co-Contaminant Mixtures
Bioremediation is a promising technology to degrade or detoxify various organic and inorganic compounds in polluted environments by using microbiological activity, but it is sensitive to biogeochemical conditions as well as co-occuring compounds at impacted sites. This study focused on biodegradation of 1,4-dioxane, which is a carcinogen and an emerging water contaminant. 1,4-Dioxane was utilized as a stabilizer of chlorinated solvents, such as 1,1,1-trichloroethane (TCA); and it has been found widespread in groundwater. Many US states are implementing lower regulatory advisory levels based on the toxicity profile of 1,4-dioxane and the potential public health risks. However, the unique chemical properties of 1,4-dioxane, such as high water solubility, low Henry’s law constant, and importantly, the co-occurrence with chlorinated solvents and other contaminants, increase the challenges to efficiently cleanup 1,4-dioxane contaminations. The objectives of this research were to measure and model the effects of chlorinated solvents on 1,4-dioxane metabolic biodegradation by laboratory pure cultures, elucidate the mechanisms of the inhibition, and test the effects of mixtures of co-contaminants in samples collected from actual 1,4-dioxane contaminated sites. It was determined that individual solvents inhibited biodegradation of 1,4-dioxane in the following order: 1,1-dichloroethene (1,1-DCE) > cis-1,2-diochloroethene (cDCE) > trichloroethene (TCE) > 1,1,1-trichloroethane (TCA). The results confirmed that 1,1-DCE was the strongest inhibitor of 1,4-dioxane biodegradation, even in chlorinated ethene mixtures. The energy production was delayed, and the genes coding for catalytic enzymes, dioxane monooxygenase (dxmB) and alcohol dehydrogenase (aldH) were down regulated, in the presence of chlorinated solvents. These results will be useful to scientists in understanding the fundamentals of enzymatic processes that catalyze biological degradation of hazardous compounds, and to environmental engineers by providing quantitative data valuable for the development of in-situ bioremediation approaches for contaminant mixtures.