UC Riverside

Per- and polyfluoroalkyl substances (PFAS) comprise a large class of chemically stable compounds causing ubiquitous pollution. Their detrimental effects to humans and the environment are exacerbated by their mobility in aquatic systems. Effective and efficient methods are necessary to chemically destroy these aqueous contaminants. This thesis study focuses on understanding structure-reactivity relationships of aqueous PFAS within the UV/sulfite system, developing and optimizing this system for effective and efficient PFAS destruction, and identifying next-generation PFAS design for rapid and complete defluorination. A photochemical system equipped with a 254 nm Hg lamp is used to irradiate an aqueous solution amended with a photosensitizer (i.e., sulfite, SO32–), spontaneously generating reactive hydrated electrons (eaq–) and sulfite radicals (SO3⦁–), in order to probe the reactivity with PFAS. A systematic investigation using the UV/sulfite system reveal critical structure–reactivity relationships for legacy (e.g., carboxylates, sulfonates, and telomer carboxylates) and emerging (e.g., ether carboxylates) aqueous PFAS. Decay kinetics, transformation products, and defluorination (i.e., percent C–F bond cleavage) results highlight distinct reaction pathways. Quantum chemical calculations on bond dissociation energies and reaction simulations provide mechanistic interpretation of experimental results in order to elucidate destruction pathways. System parameters, including solution pH and photosensitizer concentration, are optimized to achieve the deepest and most efficient defluorination. Kinetic studies reveal competition between reductive and oxidative defluorination mechanisms directly influencing overall system performance. Increased reactivity and deeper defluorination is achieved under increasingly basic conditions, where the reaction pathways with SO32– are thermodynamically mediated. Effective and efficient PFAS treatment within the UV/sulfite system is highly dependent on structure. Perfluorocaboxamides (PFCAms), containing the distinct amide functional group, can be rapidly destroyed and deeply defluorinated within the UV/sulfite system. By taking advantage of a novel defluorination mechanism upon reactivity with SO3⦁–, PFCAms exhibit higher reactivity resulting in deeper defluorination in modest basic solution in the presence of UV-irradiated SO32–. Furthermore, N-substituted PFCAms observe even faster reactivity and resistance to hydrolysis, demonstrating the possibility of designing future PFAS for rapid and complete defluorination.