Deeper Defluorination and Mineralization of a Novel PFECA (C7 HFPO-TA) in Vacuum UV/Sulfite: Unique Mechanism of H/OCF3 Exchange

C7 HFPO-TA is a newly identified alternative to PFOA, which possesses a unique structure fragment (CF₃O-CF(CF₃)-). In this study, we evaluated the chemical reactivity of C7 HFPO-TA in advanced oxidation and reduction processes for the first time, which revealed a series of unexpected transformation mechanisms. The results showed that reductive degradation based on hydrated electrons (e_aq^-) was more feasible for the degradation of C7 HFPO-TA. For oxidative degradation, the branched -CF₃ at the α-position carbon posed as the spatial hindrance, shielding the attack of SO₄^•- to -COO^-. The synergistic effects of HO^•/e_aq^- and direct photolysis led to deeper defluorination and mineralization of C7 HFPO-TA in the vacuum UV/sulfite (VUV/SF) process. We identified a unique H/OCF₃ exchange that converted the CF₃O-CF(CF₃)- into H-CF(CF₃)- directly, and the SO₃^•- involved mechanism of C7 HFPO-TA for the first time. We revealed the branched -CF₃ connected to the same carbon next to the CF₃O- group affected the C-O bond cleavage site, preferring the H/OCF₃ exchange pathway. The defluorination of C7 HFPO-TA was compared with PFOA and three PFECAs in the VUV/SF process, which was highly dependent on structures. Degradation kinetics, theoretical calculations, and products' analysis provided an in-depth perspective on the degradation mechanisms and pathways of C7 HFPO-TA.