Mineralization of Trifluoromethanesulfonate via Subcritical Hydrothermal Reaction: Reaction Mechanism and Transformation Pathways

Hydrothermal alkaline treatment (HALT) is an innovative approach that was developed for the destruction of per- and polyfluoroalkyl substances (PFAS). While HALT has been shown to effectively destroy a wide range of PFAS detected in various sample matrices, a comprehensive understanding of the controlling reaction mechanisms and transformation pathways remains limited. Herein, we selected trifluoromethanesulfonate (TFMS), the shortest-chain and likely one of the most recalcitrant PFAS reported to date, to probe degradation mechanisms and identify transformation products. The results indicate that HALT of TFMS proceeds via general nucleophilic substitution and base-promoted pathways, leading to 65.7% mineralization of the parent compound after a 180 min reaction of 0.1 M TFMS at 350 °C in 1 M NaOH. For the degraded TFMS, approximately 100% of fluorine and sulfur were converted to fluoride and sulfate, respectively, while carbon was distributed mainly as carbonate (95%) and formate (5%), along with the production of hydrogen. These findings are supported by both experimental and computational evidence. Hydroxide plays dual roles by initiating the reaction as the nucleophile and promoting subsequent steps by maintaining strongly basic conditions. The initial degradation step is rate-determining, with an estimated energy barrier of 27.8 kcal/mol. Similar mechanisms are proposed for reactions of longer-chain perfluoroalkyl sulfonic acids (PFSAs). Finally, the key factors governing PFSA reactivity across chain lengths from C1 to C8 were identified as reaction temperature, nucleophile type and concentration, and reaction time. This study addresses a critical knowledge gap in PFAS hydrothermal reactions and further establishes HALT as an effective technology for PFAS destruction and defluorination.