Kinetics and Mechanism of Ultrasonic Defluorination of Fluorotelomer Sulfonates

Ultrasounddegrades "legacy" per- and polyfluoroalkylsubstances (PFAS) via thermolysis at the interface of cavitation bubbles.However, compared to "legacy" PFAS, polyfluoroalkylsubstances have a lesser affinity to the interface and may react with (OH)-O-& BULL;. To understand the effect of size on degradationkinetics and mechanism of polyfluoroalkyl substances, this work comparedultrasonic treatment (f = 354 kHz) of n:2 fluorotelomer sulfonates (FTSAs) of varying chain lengths (n = 4, 6, 8). Of the congeners tested, 4:2 fluorotelomersulfonate (FtS) degraded the fastest in individual solutions and inmixtures. Sonolytic rate constants correlated to diffusion coefficientsof FTSAs, indicating that diffuse short-chain FTSAs outcompete long-chainFTSAs 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-)], overtime. Initially, [F-]/[SO4 (2-)](4:2 FtS) and [F-]/[SO4 (2-)](8:2 FtS) were respectivelyhigher and lower than theoretical ratios. This difference was attributedto the lower maximum surface excess of 8:2 FtS, hindering its abilityto pack and, consequently, defluorinate at the interface. In the presenceof an (OH)-O-& BULL; scavenger, FTSAs had similar %F- release compared to no scavenger, whereas %SO4 (2-) release was drastically diminished. Therefore, thermolysis is theprimary degradation pathway of FTSAs; (OH)-O-& BULL; supplementsSO(4) (2-) formation. These results indicatethat ultrasound directly cleaves C-F bonds within the fluoroalkylchain. This work shows that ultrasound efficiently degrades FTSAsof various sizes and may potentially treat other classes of polyfluoroalkylsubstances.