Oxidation of Per- and Polyfluoroalkyl Ether Acids and Other Per- and Polyfluoroalkyl Substances by Sulfate and Hydroxyl Radicals: Kinetic Insights from Experiments and Models
Chuhui Zhang, Tiffany Tang, Detlef R.U. Knappe
Abstract
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 (SO 4 •– ) and hydroxyl ( • OH) radicals can oxidize some PFAS, but the behavior of per- and polyfluoroalkyl ether acids (PFEAs) in processes involving SO 4 •– 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 SO 4 •– and • OH. Among the studied PFAS, 6:2 fluorotelomer sulfonate reacted most readily with • OH [ k •OH = (1.1–1.2) × 10 7 M –1 s –1 ], while polyfluoroalkyl ether acids containing an -O-CFH- moiety reacted more slowly [ k •OH = (0.5–1.0) × 10 6 M –1 s –1 ]. In the presence of SO 4 •–, polyfluoroalkyl ether acids with an -O-CFH- moiety reacted more rapidly [ k SO4 •– = (0.89–4.6) × 10 6 M –1 s –1 ] than perfluoroalkyl ether carboxylic acids (PFECAs) and a chloro-perfluoro-polyether carboxylic acid (ClPFPECA) [ k SO4 •– = (0.85–9.5) × 10 4 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. SO 4 •– 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 SO 4 •– attack was the -O-CFH- moiety. Perfluoroalkyl ether sulfonic acids were not oxidized by SO 4 •– and • OH under the conditions evaluated in this study.