Oxidation of Per- and Polyfluoroalkyl Ether Acids and Other Per- and Polyfluoroalkyl Substances by Sulfate and Hydroxyl Radicals: Kinetic Insights from Experiments and Models

Per- and polyfluoroalkyl substances(PFAS) are widely used anthropogenicchemicals. Because of the strength of the carbon-fluorine bond,PFAS are not destroyed in typical water treatment processes. Sulfate(SO4 (& BULL;-)) and hydroxyl ((OH)-O-& BULL;) radicals can oxidize some PFAS, but the behavior of per- andpolyfluoroalkyl ether acids (PFEAs) in processes involving SO4 (& BULL;-) and (OH)-O-& BULL; is poorlyunderstood. In this study, we determined second-order rate constants(k) describing the oxidation of 18 PFAS, including15 novel PFEAs, by SO4 (& BULL;-) and (OH)-O-& BULL;. Among the studied PFAS, 6:2 fluorotelomer sulfonatereacted most readily with (OH)-O-& BULL; [k (& BULL;OH) = (1.1-1.2) x 10(7) M-1 s(-1)], while polyfluoroalkyl etheracids containing an -O-CFH- moiety reacted more slowly [k (& BULL;OH) = (0.5-1.0) x 10(6) M-1 s(-1)]. In the presence of SO4 (& BULL;-), polyfluoroalkyl ether acids withan -O-CFH- moiety reacted more rapidly [k (SO4) (& BULL;-) = (0.89-4.6) x 10(6) M-1 s(-1)] than perfluoroalkylether carboxylic acids (PFECAs) and a chloro-perfluoro-polyether carboxylicacid (ClPFPECA) [k (SO4) (& BULL;-) = (0.85-9.5) x 10(4) M-1 s(-1)]. For homologous series of perfluoroalkyl carboxylicacids, linear and branched monoether PFECAs, and multiether PFECAs,PFAS chain length had little impact on second-order rate constants.SO4 (& BULL;-) reacted with the carboxylicacid headgroup of perfluoroalkyl carboxylic acids and PFECAs. In contrast,for polyfluoroalkyl ether carboxylic and sulfonic acids with an -O-CFH-moiety, the site of SO4 (& BULL;-) attackwas the -O-CFH- moiety. Perfluoroalkyl ether sulfonic acids were notoxidized by SO4 (& BULL;-) and (OH)-O-& BULL; under the conditions evaluated in this study. Second-order rate constants describing the oxidation ofper- and polyfluoroalkyl substances, including novel per- and polyfluoralkylether acids, by hydroxyl and sulfate radicals were determined fromexperiments and kinetic models.