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

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