Degradation of novel PFOA alternatives in fluoropolymer production by UV activated persulfate: Efficiency, mechanism and structural effects

Ultraviolet-activated persulfate (UV/PS) represents a promising advanced oxidation process (AOP) for the elimination of toxic and bio-refractory organic pollutants in wastewater due to the advantages of SO₄•^-. C7 HFPO-TA, one of the latest alternatives to PFOA in fluoropolymers production, with a unique structural fragment (CF₃O-CF(CF₃)-) that has been frequently identified in effluents, surface waters, and sediments in proximity to fluorochemical industrial zone. In this study, we evaluated the degradation of C7 HFPO-TA and co-existing PFHxA in AOPs, mainly focusing on the molecular structure effect on the chemical affinity with different reactive species (RSs). The results showed that > 99.9 % C7 HFPO-TA and PFHxA could be decomposed after 480 min in UV/PS, significantly outperforming UV/H₂O₂ (<10 %), as the result of the synergistic effect of radical/non-radical (SO₄•^-/HSO₄•, O₂•^-, HO•, ¹O₂⁾ and chain reactions. The degradation mechanism of C7 HFPO-TA was primarily governed by the DHEH mechanism, which facilitated radical chain reactions and resulted in the formation of a new perfluoroalkyl ester intermediate (C₆F₁₂O₃), as optimized by Transition State theory. The degradation of five HFPO and three PFCA was highly dependent on their molecular structures. Degradation kinetics and DFT calculations demonstrated the longer branched fluorocarbon chain could diminish the steric hindrance of α-CF₃ in HFPO and made the electron distribution more flexible for the reaction with the RSs. The comprehensive analysis of degradation kinetics, theoretical calculations and intermediates analysis elucidated the transformation mechanisms of C7 HFPO-TA and PFHxA in UV/PS.