Overlooked Activation Role of Sulfite in Accelerating Hydrated Electron Treatment of Perfluorosulfonates.

Photoexcitation of sulfite (SO 3 2- ) is often used to generate hydrated electrons (e aq - ) in processes to degrade perfluoroalkyl and polyfluoroalkyl substances (PFASs). Conventional consensus discourages the utilization of SO 3 2- concentrations exceeding 10 mM for effective defluorination. This has hindered our understanding of SO 3 2- chemistry beyond its electron photogeneration properties. In contrast, the radiation-chemical study presented here, directly producing e aq - through water radiolysis, suggests that SO 3 2- plays a previously overlooked activation role in the defluorination. Quantitative 60 Co gamma irradiation experiments indicate that the increased SO 3 2- concentration from 0.1 to 1 M enhances the defluorination rate by a remarkable 15-fold, especially for short-chain perfluoroalkyl sulfonate (PFSA). Furthermore, during the treatment of long-chain PFSA (C 8 F 17 -SO 3 - ) with a higher concentration of SO 3 2- , the intermediates of C 8 H 17 -SO 3 - and C 3 F 7 -COO - were observed, which are absent without SO 3 2- . These observations highlight that a higher concentration of SO 3 2- facilitates both reaction pathways: chain shortening and H/F exchange. Pulse radiolysis measurements show that elevated SO 3 2- concentrations accelerate the bimolecular reaction between e aq - and PFSA by 2 orders of magnitude. 19 F NMR measurements and theoretical simulations reveal the noncovalent interactions between SO 3 2- and F atoms, which exceptionally reduce the C-F bond dissociation energy by nearly 40%. As a result, our study offers a more effective strategy for degrading highly persistent PFSA contaminants.