UC Riverside

Due to their unique properties including hydrophobicity, lipophobicity, and thermostability, per- and polyfluoroalkyl substances (PFASs) have been extensively used since the 1940s in a wide range of applications. However, concerns about the fate of PFASs have been rising because of their persistence, bioaccumulation, and toxicity, leading to worldwide efforts on PFAS regulation. Current environmental remediation efforts primarily focus on the “legacy” perfluorinated CnF2n+1−X (X = COO−, SO3−, and (CH2)m−R, where R represents highly diverse organic moieties). However, beyond the previously elucidated hydrodefluorination and decarboxylation, the degradation pathways of the legacy PFASs remain largely unknown. Additionally, although “alternative” PFAS containing –H and –Cl in the fluorinated moiety have also been systematically developed and extensively applied for decades, only a few studies have explored their degradation. In this thesis study, we first investigate the degradation of omega-hydroperfluorocarboxylates (ω-HPFCAs, H−CF2(CF2)n−1−COO−) with UV/sulfite. To our surprise, the presence of the H atom on the remote carbon makes ω-HPFCAs more susceptible than perfluorocarboxylates (PFCAs, CF3(CF2)n−1−COO−) to decarboxylation and less susceptible to hydrodefluorination. This study further systematically investigated the degradation of Clx−PFAS, including omega-chloroperfluorocarboxylates (ω-ClPFCAs, Cl−CnF2n−COO−), 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate (F-53B, Cl− (CF2)6−O−(CF2)2−SO3−) and polychlorotrifluoroethylene oligomer acids (CTFEOAs, Cl− (CF2CFCl)nCF2−COO−) under UV/sulfite treatment. After initial reductive dechlorination by hydrated electron (eaq–), multiple pathways occur, including hydrogenation, sulfonation, and dimerization. This study also identified the unexpected hydroxylation pathway that converts the terminal ClCF2− into −OOC−. The hydroxylation of the middle carbons in CTFEOAs also triggers the cleavage of C−C bonds, yielding multiple −COO− groups to promote defluorination. Based on the critical mechanistic understanding obtained from the degradation of Clx−PFAS, this study further reveals novel degradation pathways of legacy PFAS under UV/sulfite treatment via transformation product analyses of a series of legacy PFAS with various head groups and chain lengths. Beyond eaq–, several other active species could also be involved in the reaction and result in transformation products with different recalcitrance. This study renovates and further advances the mechanistic understanding of PFAS degradation in “advanced reduction” systems. It also suggests the synergy between “more degradable” molecular design and cost-effective degradation technology to achieve the balanced sustainability of fluorochemicals.