Photo-oxidative/reductive decomposition of perfluorooctanoic acid (PFOA) and its common alternatives: Mechanism and kinetic modeling

Per- and poly-fluoroalkyl substances (PFAS) are a group of diverse anthropogenic chemicals that are of global concern. This study explored degradation of perfluorooctanoic acid (PFOA), and its common alternatives, perfluorobutanoic acid (PFBA) and hexafluoropropylene oxide dimer acid (HFPO-DA or GenX) through photochemical processes employing sulfite and persulfate mediators. Specific objectives were to i) assess key operational parameters and their interactions, ii) determine the roles of direct photolysis, generated active species, and UV wavelengths, iii) explore the impacts of copresent organic and inorganic constituents, and iv) develop a kinetic model for sulfite-mediated processes to determine key optical parameters and intrinsic rates of the studied PFAS with hydrated electron (eaq−). Photochemical experiments were conducted in well-controlled collimated beam setups, facilitating the fluence-based study. In both sulfite- and persulfate-mediated systems, solution pH is the most significant factor, followed by mediator concentration. The Φ254 nm and Φ185 nm of sulfite are 0.104 ± 0.007 and 1.38 ± 0.11 mol/einstein, highlighting the synergistic impact of introducing 185 nm to the sulfite-mediated process. Scavengers and water matrices influence active species, with eaq− and SO4•– dominating in sulfite- and persulfate-mediated systems, respectively. The study highlights the potential role of O2•– in GenX decomposition and elucidates intermediates evolution and decomposition mechanisms, where eaq− and SO4•– lead to distinct CO bond breakage in GenX. The persulfate-mediated process exhibits slower kinetics, resulting in higher intermediates and lower defluorination efficiency. Kinetic modeling reveals intrinsic rate constants for PFOA, PFBA, and GenX with eaq− as (8.34 ± 0.08) × 107, (5.85 ± 0.07) × 107, and (1.70 ± 0.10) × 107 M−1 s−1, respectively.