Biodegradation of 1,4-dioxane by aerobic bacteria: Experimental studies and modeling of oxidation kinetics, co-contaminant effects, and biochemical pathways

1,4-Dioxane, a probable human carcinogen, is typically found as an environmental contaminant in conjunction with groundwater solvent plumes. Natural and enhanced bioremediation may provide effective, inexpensive, in situ treatment for dioxane in water supplies. The objective of this research was to develop a mechanistic understanding of the enzymes, pathways and kinetics of dioxane degradation by bacteria, and to evaluate the effects of relevant contaminant mixtures on dioxane degradation rates.;In this study, the hypothesis that bacteria expressing monooxygenase enzymes are capable of degrading dioxane was tested. Pseudonocardia dioxanivorans CB1190 was characterized as a new species in this research. Dioxane served as a growth substrate for P. dioxanivorans CB1190 and P. benzenivorans B5. Cometabolic transformation of dioxane was confirmed for monooxygenase-expressing strains that were induced with methane, propane, tetrahydrofuran, or toluene. Measured dioxane oxidation rates ranged from 0.01-0.6 mg hr-1 (mg protein)-1.;A biochemical pathway was proposed for monooxygenase-catalyzed dioxane biodegradation, in which the major intermediates were identified as 2-hydroxyethoxyacetic acid (HEAA), ethylene glycol, glycolate, and oxalate. Dioxane was ultimately mineralized to CO2. Bioremediation of dioxane via this pathway is not expected to cause accumulation of toxic compounds in the environment.;Dioxane is used as a solvent stabilizer for 1,1,1-trichloroethane (TCA). Consequently, TCA, and its chemical breakdown product, 1,1-dichloroethene (DCE), are common co-contaminants of dioxane in groundwater. Degradation rates of dioxane were inhibited in the presence of these chlorinated compounds. The inhibition was non-competitive and reversible for strains metabolizing dioxane. In contrast, both TCA and DCE were transformed by, and competitively inhibited dioxane degradation by the strains cometabolizing dioxane. These data suggest that the strategies for biostimulation or bioaugmentation of dioxane will need to consider the presence of chlorinated solvents during site remediation.;This is the first study to confirm the role of monooxygenases in dioxane degradation using multiple independent lines of evidence, to describe the complete biodegradation pathway, and to quantify the kinetics of metabolic and cometabolic degradation of dioxane alone, and in mixtures with chlorinated solvents. The results of this study will facilitate the improved strategies and the development of monitoring tools for dioxane bioremediation in the environment.