Degradation Pathways and Complete Defluorination of Chlorinated Polyfluoroalkyl Substances (Clx−PFAS)

Chlorinated polyfluoroalkyl substances (Clx−PFAS) have been developed and applied for decades, but they have just been recognized as an emerging class of pollutants. This study systematically investigated the degradation of three types of Clx−PFAS structures, including omega-chloroperfluorocarboxylates (ω-ClPFCAs, n=1,2,4,8 Cl−CnF2nCOO−), 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate (F-53B, Cl−(CF2)6−O−(CF2)2SO3−) and polychlorotrifluoroethylene oligomer acids (CTFEOAs, n=1,2,3 Cl−(CF2CFCl)nCF2COO−) under UV/sulfite treatment. The results lead to a series of transformative insights. After initial reductive dechlorination by hydrated electron (eaq–), multiple pathways occur, including hydrogenation, sulfonation, and dimerization. In particular, this study identified the unexpected hydroxylation pathway that convert the terminal ClCF2− into −OOC−, which is critical for the rapid and deep defluorination of F-53B. The hydroxylation of the middle carbons in CTFEOAs also triggers the cleavage of C−C bonds, yielding multiple −COO− groups to promote defluorination. Hence, the Cl atoms in Clx−PFAS enhance defluorination in comparison with the perfluorinated analogs. After UV/sulfite treatment, the HO• oxidation of the residue leads to ~100% defluorination of all ω-ClPFCAs and CTFEOAs, without generating toxic ClO3− from Cl−. 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.