Near-Quantitative Defluorination of Perfluorinated and Fluorotelomer Carboxylates and Sulfonates with Integrated Oxidation and Reduction

Abstract

The UV-sulfite reductive treatment using hydrated electrons (<i>e</i>_aq^-) is a promising technology for destroying perfluorocarboxylates (PFCAs, C_nF_2n+1COO^-) in any chain length. However, the C-H bonds formed in the transformation products strengthen the residual C-F bonds and thus prevent complete defluorination. Reductive treatments of fluorotelomer carboxylates (FTCAs, C_nF_2n+1-CH₂CH₂-COO^-) and sulfonates (FTSAs, C_nF_2n+1-CH₂CH₂-SO₃^-) are also sluggish because the ethylene linker separates the fluoroalkyl chain from the end functional group. In this work, we used oxidation (<i>Ox</i>) with hydroxyl radicals (HO•) to convert FTCAs and FTSAs to a mixture of PFCAs. This process also cleaved 35-95% of C-F bonds depending on the fluoroalkyl chain length. We probed the stoichiometry and mechanism for the oxidative defluorination of fluorotelomers. The subsequent reduction (<i>Red</i>) with UV-sulfite achieved deep defluorination of the PFCA mixture for up to 90%. The following use of HO• to oxidize the H-rich residues led to the cleavage of the remaining C-F bonds. We examined the efficacy of integrated oxidative and reductive treatment of n = 1-8 PFCAs, n = 4,6,8 perfluorosulfonates (PFSAs, C_nF_2n+1-SO₃^-), n = 1-8 FTCAs, and n = 4,6,8 FTSAs. A majority of structures yielded near-quantitative overall defluorination (97-103%), except for n = 7,8 fluorotelomers (85-89%), n = 4 PFSA (94%), and n = 4 FTSA (93%). The results show the feasibility of complete defluorination of legacy PFAS pollutants and will advance both remediation technology design and water sample analysis.