Overlooked Activation Role of Sulfite in Accelerating Hydrated Electron Treatment of Perfluorosulfonates
Zhiwen Jiang, Sergey A. Denisov, Daniel Akuoko Adjei, Mehran Mostafavi, Jun Ma
Abstract
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.