Fe-catalyzed S(IV) oxidation revisited: Roles of iron ligands and dissolved oxygen

Fe(III)-catalyzed sulfite oxidation is a critical process that influences the sulfur transformation in atmospheric waters and contributes to acid precipitation. Despite being an old topic, it has not yet succeeded to establish a universally accepted reaction model since real-time data on dissolved oxygen (DO) consumption is not incorporated. Utilizing an advanced fibre optic oxygen meter, kinetics of Fe(III)-catalyzed sulfite oxidation were revisited. In contrast to the promoting role of oxalate in traditional photochemical Fe(III)-S(IV) system, the presence of either oxalate or malonate significantly diminished the rates of the sulfite oxidation and the oxygen consumption under dark conditions, by inhibiting inner-sphere complexation of Fe(III) with sulfite, with the kinetic rate ratios (k₁/k₂) were 282 for the ferric sulfate system, 0.13 for the Fe(III)-oxalate system and 67 for the Fe(III)-malonate system. This study further elucidated the central role of dissolved oxygen, which acts as the terminal electron acceptor facilitating the redox cycling of Fe(II)/Fe(III), thereby regulating the free radical chain reaction pathway from SO₃^•-to SO₄^2-. The present work is significant for advancing the understanding of the redox reactions of the aqueous Fe-S(IV)-DO system and for establishing kinetic models in aquatic oxygenation reactions by filling the missing link of oxygen.