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

In the aqueous environment, Fe ions enhance the oxidative potential of ozone and hydrogen peroxide by generating the reactive oxoiron species (ferryl ion, FeO) 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 (MeSO) in Fe solutions reacting with O 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 (MeSO, from FeO + MeSO) and methanesulfonate (MeSO, from ·OH + MeSO) over a wide range of Fe and O 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 ∼10 times faster on microdroplets than that in bulk-phase water. Since the production of MeSO is initiated by ·OH radicals at diffusion-controlled rates, experimental ratios of MeSO/MeSO > 10 suggest that FeO is the dominant intermediate under all conditions. MeSO yields in the presence of ligands, L, vary as volcano-plot functions of E(LFeO+ O/LFe + O) reduction potentials calculated by DFT with a maximum achieved in the case of L≡oxalate. Our findings underscore the key role of ferryl FeO intermediates in Fenton chemistry taking place on aqueous microdroplets.