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

Photoexcitation of sulfite (SO32–) is often used to generate hydrated electrons (eaq–) in processes to degrade perfluoroalkyl and polyfluoroalkyl substances (PFASs). Conventional consensus discourages the utilization of SO32– concentrations exceeding 10 mM for effective defluorination. This has hindered our understanding of SO32– chemistry beyond its electron photogeneration properties. In contrast, the radiation-chemical study presented here, directly producing eaq– through water radiolysis, suggests that SO32– plays a previously overlooked activation role in the defluorination. Quantitative 60Co gamma irradiation experiments indicate that the increased SO32– 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 (C8F17–SO3–) with a higher concentration of SO32–, the intermediates of C8H17–SO3– and C3F7–COO– were observed, which are absent without SO32–. These observations highlight that a higher concentration of SO32– facilitates both reaction pathways: chain shortening and H/F exchange. Pulse radiolysis measurements show that elevated SO32– concentrations accelerate the bimolecular reaction between eaq– and PFSA by 2 orders of magnitude. 19F NMR measurements and theoretical simulations reveal the noncovalent interactions between SO32– 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.