Role of Ferryl Ion Intermediates in Fast Fenton Chemistry on Aqueous Microdroplets

Abstract

In the aqueous environment, Fe^(II) ions enhance the oxidative potential of ozone and hydrogen peroxide by generating the reactive oxoiron species (ferryl ion, Fe^(IV)O²⁺) and hydroxyl radical (·OH) via Fenton chemistry. Herein, we investigate factors that control the pathways of these reactive intermediates in the oxidation of dimethyl sulfoxide (Me₂SO) in Fe^(II) solutions reacting with O₃ in both bulk-phase water and on the surfaces of aqueous microdroplets. Electrospray ionization mass spectrometry is used to quantify the formation of dimethyl sulfone (Me₂SO₂, from Fe^(IV)O²⁺ + Me₂SO) and methanesulfonate (MeSO₃⁻), from ·OH + Me₂SO) over a wide range of Fe^(II) and O₃ concentrations and pH. In addition, the role of environmentally relevant organic ligands on the reaction kinetics was also explored. The experimental results show that Fenton chemistry proceeds at a rate ∼10⁴ times faster on microdroplets than that in bulk-phase water. Since the production of MeSO₃⁻ is initiated by ·OH radicals at diffusion-controlled rates, experimental ratios of Me₂SO₂/MeSO₃⁻ > 10² suggest that Fe^(IV)O²⁺ is the dominant intermediate under all conditions. Me₂SO₂ yields in the presence of ligands, L, vary as volcano-plot functions of E⁰(LFe^(IV)O²⁺ + O₂/LFe²⁺ + O₃) reduction potentials calculated by DFT with a maximum achieved in the case of L≡oxalate. Our findings underscore the key role of ferryl Fe^(IV)O²⁺ intermediates in Fenton chemistry taking place on aqueous microdroplets.

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