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Theoretical and experimental approaches to elucidating the redox-driven degradation mechanisms of perfluoroalkyl substances
Van Hoomissen, Daniel
Van Hoomissen, Daniel
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2019
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2020-09-30
Abstract
Chemical scientists catalyze technological innovations across numerous disciplines in modern society. To some environmentalists, however, it’s evident that our inventions can do more harm than good. A lack of hindsight and foresight continually plague human progress as ongoing chemical pollution accompanies our technological discoveries and destroys the biosphere. Our deficiency in these aspects is consequentially related to our poor understanding of the fundamental physiochemical properties of recalcitrant compounds. This continues to stymie our engineering efforts to mitigate the harm of environmental pollution. To fill these knowledge gaps, my research focused on elucidating and contextualizing the chemical mechanisms associated with the remediation and destruction of persistent organic pollutants. Perfluoroalkyl substances (PFASs), anthropogenic contaminants of emerging concern, will be the subject of a large fraction of this work and are extensively described from a physical-organic perspective. A fundamental premise of this work the application of complementary experimental, computational and spectroscopic techniques can facilitate a bottom-up molecular approach to tackle these macroscopic existential problems. PFASs are a class of compounds which have one or more perfluorinated alkyl chain(s) attached to a diverse and extensive set of organic functional groups. Because of the unique properties of C—F bonds, these compounds are utilized in numerous commercial, industrial and pharmaceutical applications, and their continued use in these aspects constitutes billions of dollars in economic value. Their economic and technological usefulness is offset by their inability to biodegrade in the environment, and subsequently, biosphere wide contamination of fluorinated compounds has emerged as a worldwide dilemma. Much has been done to quantify the risk of PFASs and understand the degree to which they are released into the environment. Recently, bench-top investigations utilizing photochemical techniques have shown promise to degrade PFASs; however, to date, degradation-based methods are not implemented on larger scales. To this end, this thesis is focused on a synergistic computational-experimental approach to understand the degradation mechanisms of fluorinated organic contaminants, especially those associated with UV-mediated redox chemistry. First, many fundamental properties of PFASs are described, from their synthesis, to their use and destruction. Next, I will describe our approach to predict the ability for Co(I)-catalysts to reductively cleave C—F bonds in PFASs. This is followed by developing an understanding about their redox behavior and the subsequent isomerization reactions of activated degradation intermediates. Inspired by our work on isomerization events in fluorinated radicals, parallel methods were also used to describe the relative stability of other environmentally relevant radical species, allyl and benzyl radicals. Density functional theory (DFT) was imperative to understand the vital attributes for each component of this thesis and was essential in the development of structure activity relationships for a broad suite of PFASs. In short, the polar functional group and the chain length of the perfluoroalkyl acids was found to govern which C—F bond would cleave under reductive conditions. In conjunction with DFT, ongoing development of laser flash photolysis techniques will support the measurement of the reduction rate constants between PFAS and UV-generated hydrated electrons and complements ongoing work to compute the rate of electron transfer with Marcus theory. These methods will help us improve our understanding of existing practices for the remediation of PFASs and could facilitate the development of novel technologies to remediate a wide-range of PFASs-impacted natural waters.
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