Degradation and Defluorination of Per- and Polyfluoroalkyl Substances by Direct Photolysis at 222 nm

Far-UVC 222 nm irradiation is capableof degrading groupsof PFAS including PFCAs, FTUCAs, and GenX, and the photolysis rateand extent are influenced by molar absorptivity and possibly severalchain-length-dependent structural factors. The susceptibility of 19 representative per- and polyfluoroalkylsubstances (PFAS) to direct photolysis and defluorination under far-UVC222 nm irradiation was investigated. Enhanced photolysis occurredfor perfluorocarboxylic acids (PFCAs), fluorotelomer unsaturated carboxylicacids (FTUCAs), and GenX, compared to that at conventional 254 nmirradiation on a similar fluence basis, while other PFAS showed minimaldecay. For degradable PFAS, up to 81% of parent compound decay (photolysisrate constant (k (222 nm)) = 8.19-34.76L & BULL;Einstein(-1); quantum yield (& phi;(222 nm)) = 0.031-0.158) and up to 31% of defluorination were achievedwithin 4 h, and the major transformation products were shorter-chainPFCAs. Solution pH, dissolved oxygen, carbonate, phosphate, chloride,and humic acids had mild impacts, while nitrate significantly affectedPFAS photolysis/defluorination at 222 nm. Decarboxylation is a crucialstep of photolytic decay. The slower degradation of short-chain PFCAsthan long-chain ones is related to molar absorptivity and may alsobe influenced by chain-length dependent structural factors, such asdifferences in pK (a), conformation, andperfluoroalkyl radical stability. Meanwhile, theoretical calculationsindicated that the widely proposed HF elimination from the alcoholintermediate (C n F2n+1OH) of PFCA is an unlikely degradation pathway due to highactivation barriers. These new findings are useful for further developmentof far-UVC technology for PFAS in water treatment.