The 185-nm ultraviolet (UV) direct photolysis of PFAS is unknown due to interference from radical-driven degradation and photon competition with coexisting aqueous species. This study established an N2O-saturated condition to evaluate the kinetics and mechanisms of 185-nm UV direct photolysis and defluorination of three PFAS subclasses: perfluorocarboxylates (PFCAs; CnF2n+1-COOH, n = 1-9), perfluorosulfonic acids (PFSAs), and per- and polyfluoroalkyl ether carboxylic acids (PFECAs). PFCAs and PFECAs underwent significant direct photolysis, whereas PFSAs remain resistant. First-order direct photolysis rate constants strongly correlated with molar absorption coefficients (68-3396 M-1cm-1), exhibiting enhanced kinetics for longer-chain PFCAs (n > 7), PFECAs (n > 6), and multiether PFECAs. 185-nm quantum yields ranged from 0.12 to 0.28 mol/Einstein, resulting in 30-84% defluorination. Products analysis identified neutral gaseous transformation products (e.g., CnF2n+1H and CnF2n+2), indicating dominant C-C cleavage for PFCAs and combined C-O and C-C cleavage for PFECAs. Kinetic modeling revealed that the contribution of direct photolysis to overall PFAS degradation in 185-nm water photolysis increased with decreasing pH, accounting for 87% at pH 7 and 10% at pH 11, respectively. This study discovered the existence and extent of direct PFAS photolysis at 185 nm and provided mechanistic insights for future UV PFAS water treatment.
Liu et al. (Tue,) studied this question.