Degradation and Defluorination of Per- and Polyfluoroalkyl Substances by Direct Photolysis at 222 nm
Xiaoyue Xin, Juhee Kim, Daniel C. Ashley, Ching‐Hua Huang
Abstract
High Resolution Image Download MS PowerPoint Slide The susceptibility of 19 representative per- and polyfluoroalkyl substances (PFAS) to direct photolysis and defluorination under far-UVC 222 nm irradiation was investigated. Enhanced photolysis occurred for perfluorocarboxylic acids (PFCAs), fluorotelomer unsaturated carboxylic acids (FTUCAs), and GenX, compared to that at conventional 254 nm irradiation on a similar fluence basis, while other PFAS showed minimal decay. For degradable PFAS, up to 81% of parent compound decay (photolysis rate constant ( k 222 nm ) = 8.19–34.76 L·Einstein –1; quantum yield (Φ 222 nm ) = 0.031–0.158) and up to 31% of defluorination were achieved within 4 h, and the major transformation products were shorter-chain PFCAs. Solution pH, dissolved oxygen, carbonate, phosphate, chloride, and humic acids had mild impacts, while nitrate significantly affected PFAS photolysis/defluorination at 222 nm. Decarboxylation is a crucial step of photolytic decay. The slower degradation of short-chain PFCAs than long-chain ones is related to molar absorptivity and may also be influenced by chain-length dependent structural factors, such as differences in p K a, conformation, and perfluoroalkyl radical stability. Meanwhile, theoretical calculations indicated that the widely proposed HF elimination from the alcohol intermediate (C n F 2 n +1 OH) of PFCA is an unlikely degradation pathway due to high activation barriers. These new findings are useful for further development of far-UVC technology for PFAS in water treatment.