Date of Award

12-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Engineering and Earth Science

Committee Member

Dr. David L. Freedman, Committee Chair

Committee Member

Dr. Ronald W. Falta

Committee Member

Dr. Kevin T. Finneran

Committee Member

Dr. Harry D. Kurtz, Jr.

Committee Member

Dr. Lawrence C. Murdoch

Abstract

Used mainly as a solvent stabilizer, 1,4-dioxane is present at many sites contaminated along with chlorinated solvents and other chemical compounds. Considered a probable human carcinogen by the U.S. Environmental Protection Agency, this contaminant has raised considerable concerns because of its potential adverse effects on health. Therefore, remediation of 1,4-dioxane has gained importance, and although there are several approaches for its treatment, such as ex situ physicochemical processes, bioremediation is a key alternative because it is a low energy demanding process. Anaerobic conditions are present at most contaminated sites, however, there is insufficient scientific evidence for anaerobic biodegradation of 1,4-dioxane. On the other hand, aerobic biodegradation of 1,4-dioxane has been widely studied under metabolic and cometabolic conditions. Nevertheless, limited information is known about the rate of 1,4-dioxane cometabolism with substrates, such as propane, that can be used for in situ bioremediation. Bacteria that grow on 1,4-dioxane have a low affinity for the contaminant since their half saturation coefficient (Ks) values are often high, but the contaminant half saturation coefficients (Kc) associated with cometabolism are usually lower. However, kinetic parameters for cometabolic biodegradation of 1,4-dioxane with a non-toxic and convenient substrate such as propane have not been evaluated. Based on the gaps in the scientific literature, and in order to expand the understanding of 1,4-dioxane biodegradation and its potential in situ bioremediation applications, the objectives of this study included: 1) Estimate the kinetic parameters for 1,4-dioxane metabolism and for cometabolism by propane-oxidizing bacteria that are relevant to field applications in bioremediation; and 2) Evaluate the potential for in situ bioremediation of a 1,4-dioxane plume using metabolic and cometabolic biosparging and bioaugmentation, based on simulations using a subsurface transport model; and 3) Evaluate the potential for anaerobic biodegradation of 1,4-dioxane. To achieve the first objective, kinetic parameters for aerobic cometabolic biodegradation of 1,4-dioxane by propane-oxidizing bacteria were evaluated for a pure culture, Rhodococcus ruber ENV425, and a mixed culture, ENV487. The 1,4-dioxane metabolizer Pseudonocardia dioxanivorans CB1190 was also tested for its kinetic parameters. Kinetics for metabolic and cometabolic biodegradation of 1,4-dioxane were successfully modeled using modified Monod equations. Results indicate that the propanotrophic bacteria have lower half saturation constants (KC = 6.05 ± 0.26 and 3.25 ± 0.05 mg COD L-1) for 1,4-dioxane than CB1190 (KS = 11.5 ± 0.4 mg COD L-1). Other parameters measured included the biomass yield (Y) for propane and 1,4-dioxane, transformation capacity (TC), half saturation coefficients for oxygen (KSO and KCO), biomass decay coefficient (b), and substrate utilization rates (kS and kC). Coinhibition parameters (KiS and KiC) between propane and 1,4-dioxane were also estimated. Batch simulations showed that cometabolism is more advantageous than metabolism when the initial concentration of 1,4-dioxane is low (~1 mg L-1) and that both processes are heavily impacted by dissolved oxygen concentrations less than 2 mg L-1. The second objective was achieved by simulating the effect of biodegradation reactions on a 1,4-dioxane subsurface plume treated with biosparging and bioaugmentation. The effect of the injection rates of propane, biomass and oxygen as well as the initial 1,4-dioxane concentrations were evaluated in terms of the time to reach an average 1,4-dioxane level of 1 µg L-1, as well as the percentage of 1,4-dioxane that underwent biodegradation. Data from a biosparging pilot study at Vandenburg Air Force base was used to calibrate the model as it applied to propanotrophic cometabolism. The simulation results indicated that propanotrophic cometabolism achieves remediation at a faster rate when the initial 1,4-dioxane concentration is less than 7.5 mg L-1; lower concentrations do not support enough growth of microbes that grow of 1,4-dioxane to adequately offset the effect of cell decay. A continuous supply of propane to support cometabolism negates the effect of cell decay. The model provides a framework for comparing metabolic and cometabolic approaches to in situ bioremediation at other sites. To achieve the third objective, microcosms were prepared with groundwater and sediment from two contaminated sites at which the field data suggest that 1,4-dioxane is undergoing anaerobic biodegradation. The groundwater contains high levels of acetone and isopropanol, which ensure anaerobic conditions. High levels of halogenated solvents are also present. The microcosms were amended with uniformly labeled [14C]-1,4-dioxane to characterize degradation products. Amendments included Fe(III) oxide, Fe(III)-ethylene-diaminetetraacetic acid (Fe(III)-EDTA), anthraquinone disulfonate (AQDS), sulfate and oxygen. Following four years of incubation, biodegradation of many of the halogenated solvents was observed, as was iron reduction, sulfate reduction, and methanogenesis. However, there was no significant evidence to support biodegradation of 1,4-dioxane under anaerobic conditions, although partial mineralization in aerobic microcosms was observed. Further laboratory studies are needed to determine the feasibility of anaerobic biodegradation of 1,4-dioxane. Until then, aerobic treatment remains the only viable bioremediation alternative.

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