Deeper Defluorination and Mineralization of a Novel PFECA (C7 HFPO-TA) in Vacuum UV/Sulfite: Unique Mechanism of H/OCF 3 Exchange.

C7 HFPO-TA is a newly identified alternative to PFOA, which possesses a unique structure fragment (CF 3 O-CF(CF 3 )-). 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 -CF 3 at the α-position carbon posed as the spatial hindrance, shielding the attack of SO 4 •- 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/OCF 3 exchange that converted the CF 3 O-CF(CF 3 )- into H-CF(CF 3 )- directly, and the SO 3 •- involved mechanism of C7 HFPO-TA for the first time. We revealed the branched -CF 3 connected to the same carbon next to the CF 3 O- group affected the C-O bond cleavage site, preferring the H/OCF 3 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.