185-nm UV Direct Photolysis of PFAS Compounds: Kinetics and Degradation Mechanisms

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; C_nF_2n+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., C_nF_2n+1H and C_nF_2n+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.