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Catalytic strategies for enhancing electrochemical oxidation of 1,4-dioxane: TiO2 dark activation and microbial stimulation

Date

2016

Authors

Jasmann, Jeramy R., author
Borch, Thomas, advisor
Blotevogel, Jens, advisor
Farmer, Delphine, committee member
Neilson, James, committee member
Sanford, William, committee member
Elliot, Michael, committee member

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Abstract

1,4-dioxane, a probable human carcinogen, is an emerging contaminant currently being reviewed by the U.S. Environmental Protection Agency for possible health-based maximum contaminant level regulations. As both stabilizer in commonly used chlorinated solvents and as a widely used solvent in the production of many pharmaceuticals, personal care products, (PPCPs), 1,4-dioxane has been detected in surface water, groundwater and wastewater around the U.S. It is resistant to many of the traditional water treatment technologies such as sorption to activated carbon, air stripping, filtering and anaerobic biodegradation making 1,4-dioxane removal difficult and/or expensive. State-of-the art technologies for the removal of 1,4-dioxane usually apply advanced oxidation processes (AOPs) using strong oxidants in combination with UV-light and sometimes titanium dioxide (TiO2) catalyzed photolysis. These approaches require the use of expensive chemical reagents and are limited to ex situ (i.e. pump and treat) applications. Here, at Colorado State University’s Center for Contaminant Hydrology, innovative flow-through electrolytic reactors have been developed for treating groundwater contaminated with organic pollutants. The research presented in this dissertation has investigated catalytic strategies for enhancing electrochemical oxidation of 1,4-dioxane in flow-through reactors. Two types (abiotic and biotic) of catalysis were also explored: (1) dark, electrolytic activation of insulated, inter-electrode TiO2 pellets to catalyze the degradation of organic pollutants in the bulk solution by reactive oxygen species (ROS), and (2) adding permeable electrodes upstream of dioxane-degrading microbes, Pseudonocardia dioxanivorans CB1190, to pre-treat mixed contaminant water and provide O2 stimulation to these aerobic bacteria. For the abiotic form of catalysis, we characterized the properties of novel TiO2 inter-electrode material, and elucidated the properties most important to its catalytic activity, using 1,4-dioxane as the model contaminant. The TiO2 was novel in its use as an “inter-electrode” catalyst (not coated on the electrode and not used as a TiO2 slurry) and in the mechanism of its catalytic activation occurring in dark (not photocatalysis) and insulated (not typical electrocatalysis) conditions. Further studies were performed using electrochemical batch reactors and probe molecules in order to gain mechanistic insights into dark catalysis provided by detached TiO2 pellets in an electrochemical system. The results of our investigations show that electrolytic treatment, when used in combination with this catalytically active inter-electrode material, can successfully and efficiently degrade 1,4-dioxane. Benefits of catalyzed electrolysis as a green remediation technology are that (1) it does not require addition of chemicals during treatment, (2) it has low energy requirements that can be met through the use of solar photovoltaic modules, and (3) it is very versatile in that it could be applied in situ for contaminated groundwater sites or installed in-line on above-ground reactors to remediate contaminated groundwater. Although, 1,4-dioxane appears to be resistant to natural attenuation via anaerobic biodegradation, some aerobic bacteria have been shown to metabolize and co-metabolize 1,4-dioxane. For example, growth-supporting aerobic metabolism/degradation of 1,4-dioxane by Pseudonocardia dioxanivorans CB1190, has been demonstrated in laboratory studies. However, previous studies showed that this biodegradation process is inhibited by the presence of chlorinated solvents such as 1,1,1-trichlorethane (1,1,1-TCA) and trichloroethene (TCE). This could dramatically impact the success for in situ 1,4-dioxane biodegradation with P. dioxanivorans since chlorinated solvents are common co-contaminants of 1,4-dioxane. Our previous investigations into electrolytic treatment of organic pollutants both ex and in situ showed that effective degradation of chlorinated solvents like TCE was achievable. In addition, the electrolysis of water generates molecular O2 required by the CB1190 bacteria as well. This led us to hypothesize that the generation of O2 could enhance aerobic biodegradation processes, and the concurrent degradation of co-solvents could reduce their inhibitory impact on 1,4-dioxane biodegradation. In flow-through sand column studies presented here, we investigate the electrolytic stimulation of Pseudonocardia dioxanivorans CB1190, with the expectation that anodic O2 generation would enhance aerobic biodegradation processes, and concurrent degradation of TCE would reduce the expected inhibitory impact on 1,4-dioxane biodegradation. Results show that when both electrolytic and biotic processes are combined, oxidation rates of 1,4-dioxane substantially increased suggesting that aerobic biodegradation processes had been successfully stimulated. In summary, the results of this dissertation provide evidence of (1) efficient removal of recalcitrant 1,4-dioxane, especially with the addition of inter-electrode TiO2 catalysts, (2) elucidate possible mechanistic pathways for electro-activated dark TiO2 catalysis, and (3) provide evidence for successful synergistic performance for electro-bioremediation treatment during simulated mixed, contaminant plume conditions.

Description

Includes bibliographical references.
2016 Summer.
Zip file contains supplementary materials.

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Subject

1,4-dioxane
electrochemical oxidation
titanium dioxide
contaminant degradation

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