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Kinetics and mechanisms of hydrated electron reactions during advanced reduction processes
Amador, Camille K.
Amador, Camille K.
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2023
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2024-10-18
Abstract
Advanced reduction processes (ARPs) are a class of water treatment technologies that have great potential in remediating recalcitrant contaminants that are resistant towards traditional oxidation methods such as chlorinated solvents, toxic oxyanions, and more recently, per- and polyfluoroalkyl substances (PFASs). Although ARPs have been shown to completely mineralize persistent organic pollutants, their operation efficiency is highly dependent on sourcewater composition since the reactive species, hydrated electron (???−), can be rapidly quenched by non-target constituents present in natural water. Current means of optimizing ARP reaction conditions involve UV photolysis experiments that require performing multiple batch experiments which can be time and resource intensive. Moreover, means of measuring fundamental kinetics values of ???− reactions (i.e., k2; M-1 s-1) such as laser flash photolysis (LFP) and pulse radiolysis contradict results in photolysis experiments and values vary widely in the literature.
Kinetic models are an alternative approach for gaining quantitative information regarding the photochemistry dictating ARP treatment results. This work investigates the kinetics and mechanisms of ???− reactions during ARPs for the development of a photochemical model to quantitatively predict PFAS degradation in diverse environments. The first objective of this thesis focused on measuring kinetics and elucidating mechanisms of PFAS degradation by ???− using laser flash photolysis (LFP) and density functional theory (DFT). Reactivity of the compounds in the dataset varied widely despite their structural similarities and it was proposed that polyfluorinated carboxylates can undergo non-degradative reaction pathways posing additional challenges for treating diverse PFAS mixtures. The second objective involved quantitatively describing the effect of solution pH and acid-base speciation of common water constituents on ???− availability during ARPs. Carbonate species, ubiquitous in natural waters, were found to significantly inhibit PFAS degradation, and species-specific k2 values were determined using LFP and nonlinear regression analysis. The third and final objective was to develop a photochemical model for predicting PFAS degradation during UV-sulfite treatment using the kinetics values and mechanistic insights obtained in the first two objectives. This model facilitates the (1) quantitative interpretation of the effect of system and solution parameters on treatment efficacy, (2) reconciliation of UV photolysis and LFP/pulse radiolysis methodologies, and (3) prediction of PFAS degradation during UV-sulfite treatment in different environments based on geochemical solution conditions and UV light source.
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