Oxidation of Per- and Polyfluoroalkyl Ether Acids and Other Per- and Polyfluoroalkyl Substances by Sulfate and Hydroxyl Radicals: Kinetic Insights from Experiments and Models
Zhang, C., Tang, T., & Knappe, D. R. U. (2023, May 24). ENVIRONMENTAL SCIENCE & TECHNOLOGY.
Per- and polyfluoroalkyl substances (PFAS) are widely used anthropogenic chemicals. Because of the strength of the carbon–fluorine bond, PFAS are not destroyed in typical water treatment processes. Sulfate (SO4•–) and hydroxyl (•OH) radicals can oxidize some PFAS, but the behavior of per- and polyfluoroalkyl ether acids (PFEAs) in processes involving SO4•– and •OH is poorly understood. In this study, we determined second-order rate constants (k) describing the oxidation of 18 PFAS, including 15 novel PFEAs, by SO4•– and •OH. Among the studied PFAS, 6:2 fluorotelomer sulfonate reacted most readily with •OH [k•OH = (1.1–1.2) × 107 M–1 s–1], while polyfluoroalkyl ether acids containing an -O-CFH- moiety reacted more slowly [k•OH = (0.5–1.0) × 106 M–1 s–1]. In the presence of SO4•–, polyfluoroalkyl ether acids with an -O-CFH- moiety reacted more rapidly [kSO4•– = (0.89–4.6) × 106 M–1 s–1] than perfluoroalkyl ether carboxylic acids (PFECAs) and a chloro-perfluoro-polyether carboxylic acid (ClPFPECA) [kSO4•– = (0.85–9.5) × 104 M–1 s–1]. For homologous series of perfluoroalkyl carboxylic acids, linear and branched monoether PFECAs, and multiether PFECAs, PFAS chain length had little impact on second-order rate constants. SO4•– reacted with the carboxylic acid 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•– attack was the -O-CFH- moiety. Perfluoroalkyl ether sulfonic acids were not oxidized by SO4•– and •OH under the conditions evaluated in this study.