Per- and polyfluoroalkyl substances (PFASs) are widely used anthropogenic chemicals. Because of the strong electronegativity of fluorine and the strong carbon-fluorine bonds that exist within PFASs, they are not destroyed in typical drinking and wastewater treatment processes. Advanced oxidation processes (AOPs) can transform some PFASs, but the fate of per- and polyfluoroalkyl ether acids (PFEAs) is poorly understood. Furthermore, second order rate constants that describe the oxidation of PFASs by sulfate and hydroxyl radicals are sparse. In this study, we investigated the degradation of 13 PFASs including 10 novel per- and polyfluoroalkyl ether acids (PFEAs) using sulfate and hydroxyl radical based-AOPs. None of the studied PFASs showed measurable degradation by ultraviolet (UV) photolysis up to a UV dose of ~24,000 mJ/cm2. Among the studied PFASs, 6:2 fluorotelomer sulfonate (6:2 FtS) was the only compound that was oxidized by hydroxyl radicals generated by UV photolysis of hydrogen peroxide (UV/H2O2). Using UV photolysis of persulfate (UV/PS), polyfluoroalkyl ether carboxylic and sulfonic acids with a -O-CFH- moiety degraded more rapidly than perfluoroalkyl mono- and multi-ether carboxylic acids and a chloro-perfluoro-polyether carboxylic acid (ClPFPECA). Perfluoroalkyl ether sulfonic acids were not oxidized at any of the tested conditions. PFAS oxidation followed second-order kinetics, and and oxidation rates were impacted by solution pH and the presence of matrix constituents such as chloride and natural organic matter (NOM). We employed a kinetic model to simulate PFAS destruction and estimate second-order rate constants. For sulfate radicals, second-order rate constants ranged from 1.1–3.5 x 107 M-1s-1 for polyfluoroalkyl ether carboxylic and sulfonic acids with a -O-CFH- moiety to 1.5–5.4 x 105 M-1s-1 for perfluoroalkyl mono- and multi-ether carboxylic acids and ClPFPECA. The model was able to simulate the PS decay, pH changes, radical concentrations, chloride and NOM degradation, as well as chlorate formation. This study provided insight into remediation strategies that include hydroxyl and sulfate radicals as oxidizing agents for the degradation of PFASs.

Degradation of per- and polyfluoroalkyl ether acids by sulfate and hydroxyl radical-based advanced oxidation processes

ACS Spring 2022

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