pH-Dependent Fe(III) Speciation and Concerted Proton–Electron Transfer Mechanism Accelerated S(IV) Oxidation at the Air–Water Interface

Despite the widespread significance of the Fe(III)-catalyzed oxidation of S(IV) for sulfur chemistry and atmospheric aerosols, its reaction mechanism and accelerated kinetics at the microdroplet surface remain poorly understood. Herein, integrating Born–Oppenheimer molecular dynamic (BOMD) simulations and electron paramagnetic resonance spectrometer (EPR) experiment, the results reveal that the rate-determining SO 3 ·– radical generation exhibits orders of magnitude enhancement at the air–water interface compared to that established in solution-phase kinetics. This interfacial acceleration is progressively amplified under more acidic conditions, as corroborated by both lower calculated free energy changes and enhanced experimentally measured SO 3 ·– signal intensities with decreasing pH. Challenging the traditional Fe(III) solubility-driven paradigm, we demonstrate that the elevated rate mainly stems from highly reactive Fe(III) speciation under low pH conditions, whose reduced molecular orbital energy level improves electron-accepting capacity and thereby accelerates the oxidation reaction as acidity increases. Critically, our simulations establish an unprecedented concerted proton–electron transfer (CPET) mechanism, supported by synchronous proton and electron transfer across all dynamic events. This work elucidates the origin of the high efficiency of Fe(III)-catalyzed S(IV) oxidation in microdroplets and provides fundamental insights into pH-dependent transition-metal ion speciation as a previously under-appreciated factor impacting atmospheric sulfate aerosol formation and sulfur cycling.