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 SO₃^·- 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 SO₃^·- 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.