Kinetics and Mechanism of Ultrasonic Defluorination of Fluorotelomer Sulfonates

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.