Date of Award
Summer 9-15-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Organic contaminants are present in the aquatic environment and pose risks to human health. To address this concern, organic contaminants are degraded by oxidants in engineered aquatic systems. Halogen oxidants, including hypohalous acids and halogen radicals, are commonly used for the degradation of organic contaminants. However, one environmental concern associated with halogen oxidants is the formation of toxic halogenated byproducts. Thus, it is essential to develop effective strategies to degrade organic contaminants while mitigating the formation of halogenated byproducts. Consequently, this dissertation focused on evaluating the degradation kinetics and transformation mechanisms of organic compounds in three different engineered aquatic systems involving halogen oxidants. The first objective examined how halogen oxidants (e.g., hypochlorous acid) led to the formation of halogenated byproducts in conventional drinking water treatment during harmful algal blooms. Specifically, multiple oxidation stages in drinking water treatment may impact the formation of halogenated byproducts from algal organic matter (AOM) that occurred at high concentrations during harmful algal blooms. I found that transformations of organic moieties in AOM through additional oxidation processes changed the potential for halogenated byproduct formation from the reactions between halogen oxidants and AOM. The potentially increased formation of halogenated byproducts raises challenges for water treatment processes due to the escalating issue of harmful algal blooms on a global scale. The second objective investigated the impacts of halogen oxidants on the mechanisms and kinetics of organic compound degradation in hydraulic fracturing fluids. Specifically, persulfate is commonly used to generate sulfate radicals to break down polymer-based gels during hydraulic fracturing processes. However, high concentrations of halides in hydraulic fracturing fluids may convert sulfate radicals to halogen oxidants (i.e., hypohalous acids, halogen radicals). I found that halogen radicals were the key intermediates that led to the halogenation and selective degradation of organic compounds in hydraulic fracturing fluids. In addition, while previous research primarily investigated the direct reactions between halogen radicals and organic compounds, I demonstrated that the recombination products of halogen radicals (i.e., hypohalous acids) are also important oxidants that lead to the formation of halogenated byproducts in hydraulic fracturing fluids. The unintentional formation of halogen oxidants must be considered in future hydraulic fracturing designs to minimize their adverse effect on target organic compound degradation and to mitigate halogenated byproduct formation. The final objective evaluated how halogen oxidants contributed to organic compound degradation during brine treatment by plasma, a recently developed water treatment technology. Specifically, since hydroxyl radicals are the primary reactive species generated during plasma-based water treatment, halides in brines may convert hydroxyl radicals to halogen oxidants and thereby lead to selective degradation of organic compounds. I provided the first evidence that in contrast to the negligible effect of chloride, bromide that occurs at orders of magnitude lower concentrations significantly increased the formation of halogen oxidants during plasma treatment of brines. I also for the first time demonstrated that, via a radical-mediated pathway, halides led to the formation of hypohalous acids as the dominant oxidants during plasma treatment. The formation of hypohalous acids altered the degradation kinetics of some organic compounds from pseudo-first order to zero order, distinguishing plasma reactors from conventional radical-based treatment processes. The dominant role of hypohalous acids suggests that halides may contribute to the formation of halogenated byproducts, which is important to consider when expanding the application of plasma-based water treatment to more complex brines. Overall, this dissertation sheds light on the role of halogen oxidants in both conventional water treatment and emerging oxidative processes based on radicals. In conventional drinking water treatment, careful consideration of the oxidation of organic moieties is essential for effective strategies to control halogenated byproduct formation during harmful algal blooms. Moreover, in emerging oxidative treatment, it is important to acknowledge the role of hypohalous acids in assessing the degradation efficiency and halogenated byproduct formation when halides are present. These findings are necessary for future engineering designs to effectively degrade organic compounds while reducing the hazard posed by halogenated byproducts.
Language
English (en)
Chair
Kimberly Parker