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

In the aqueous environment, Fe-II ions enhance the oxidative potential of ozone and hydrogen peroxide by generating the reactive oxoiron species (ferryl ion, (FeO2+)-O-IV) and hydroxyl radical (center dot 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 O-3 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 (FeO2)-O-IV+ + Me2SO) and methanesulfonate (MeSO3-, from center dot OH + Me2SO) over a wide range of FeII and O-3 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 similar to 10(4) times faster on microdroplets than that in bulk-phase water. Since the production of MeSO3- is initiated by center dot OH radicals at diffusion-controlled rates, experimental ratios of Me2SO2/MeSO3- > 10(2) suggest that (FeO2+)-O-IV is the dominant intermediate under all conditions. Me2SO2 yields in the presence of ligands, L, vary as volcano-plot functions of E-0((LFeO2+)-O-IV+ O-2/LFe2+ + O-3) reduction potentials calculated by DFT with a maximum achieved in the case of L=oxalate. Our findings underscore the key role of ferryl (FeO2+)-O-IV intermediates in Fenton chemistry taking place on aqueous microdroplets.

Automated summary

The study reveals that Fenton chemistry proceeds at a rate approximately 10^4 times faster on aqueous microdroplets than in bulk-phase water. The dominant intermediate under all conditions is found to be (FeO2+)-O-IV. Organic ligands also have an impact on the reaction kinetics.