Per- and polyfluoroalkyl substances (PFAS) are widely used anthropogenic chemicals that are associated with environmental persistence, bioaccumulation, toxicity, and global mobility. The production and use of long-chain perfluoroalkyl acids (PFAAs) has been phased out in many countries; in their place, novel PFAS, such as per- and polyfluoroalkyl ether acids (PFEAs) are frequently employed. PFAAs are not destroyed in typical drinking and wastewater treatment processes. Advanced oxidation processes (AOPs) can transform some PFAS, but the fate of PFEAs is poorly understood. Furthermore, second order rate constants that describe the oxidation of PFAS by sulfate and hydroxyl radicals are sparse. In this study, we investigated the oxidation of 18 PFAS including 15 novel per- and polyfluoroalkyl ether acids (PFEAs) by sulfate and hydroxyl radicals. 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 [k_SO4^●– = (0.89–4.6) × 10⁶ M^–1 s^–1] than perfluoroalkyl ether carboxylic acids (PFECAs) and a chloro-perfluoro-polyether carboxylic acid (ClPFPECA) [k_SO4^●– = (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. Oxidation products of PFEAs included ultra-short-chain PFAS, such as trifluoroacetic acid, but PFAS mineralization was low at the studied experimental conditions.

Chuhui Zhang

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