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.

Automated summary

Ultrasound degrades legacy per- and polyfluoroalkyl substances (PFAS) via thermolysis at the interface of cavitation bubbles. This study compared the ultrasonic treatment of n:2 fluorotelomer sulfonates (FTSAs) with varying chain lengths (n = 4, 6, 8) to understand the effect of size on degradation kinetics and mechanism. The results showed that ultrasound efficiently degrades FTSA of different sizes and may have the potential to treat other classes of polyfluoroalkyl substances.