Kinetics and Mechanism of Ultrasonic Defluorination of Fluorotelomer Sulfonates

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Ultrasound degrades “legacy” per- and polyfluoroalkyl substances (PFAS) via thermolysis at the interface of cavitation bubbles. However, compared to “legacy” PFAS, polyfluoroalkyl substances have a lesser affinity to the interface and may react with ^•OH. To understand the effect of size on degradation kinetics and mechanism of polyfluoroalkyl substances, this work compared ultrasonic treatment (f = 354 kHz) of n:2 fluorotelomer sulfonates (FTSAs) of varying chain lengths (n = 4, 6, 8). Of the congeners tested, 4:2 fluorotelomer sulfonate (FtS) degraded the fastest in individual solutions and in mixtures. Sonolytic rate constants correlated to diffusion coefficients of FTSAs, indicating that diffuse short-chain FTSAs outcompete long-chain FTSAs to adsorb and react at the bubble interface. Interestingly, 4:2 and 8:2 FtS had different evolutions of fluoride-to-sulfate ratios, [F^–]/[SO₄^2–], over time. Initially, [F^–]/[SO₄^2–]_4:2 FtS and [F^–]/[SO₄^2–]_8:2 FtS were respectively higher and lower than theoretical ratios. This difference was attributed to the lower maximum surface excess of 8:2 FtS, hindering its ability to pack and, consequently, defluorinate at the interface. In the presence of an ^•OH scavenger, FTSAs had similar %F^– release compared to no scavenger, whereas %SO₄^2– release was drastically diminished. Therefore, thermolysis is the primary degradation pathway of FTSAs; ^•OH supplements SO₄^2– formation. These results indicate that ultrasound directly cleaves C–F bonds within the fluoroalkyl chain. This work shows that ultrasound efficiently degrades FTSAs of various sizes and may potentially treat other classes of polyfluoroalkyl substances.