KINETICS AND MECHANISM OF THE IRON(III)-CATALYZED AUTOXIDATION OF SULFUR(IV) OXIDES IN AQUEOUS-SOLUTION - EVIDENCE FOR THE REDOX CYCLING OFIRON IN THE PRESENCE OF OXYGEN AND MODELING OF THE OVERALL REACTION-MECHANISM

Kinetic traces for the redox decomposition of iron(III)-sulfito comple xes exhibit a peculiar break in the presence of oxygen. A detailed kin etic analysis of this feature as a function of [Fe(III)], [Fe(II)], [S (IV)], and [O2] at pH 2.5 indicated that this step is a result of the sulfite-induced autoxidation of produced iron(II) in the presence of o xygen. The so observed redox cycling of iron comes to a dead end at th e point in time when all the oxygen in the solution has been used up. The kinetic traces can be interpreted in terms of a first-order decay of the iron(III)-sulfito complexes and a reverse pseudo-zero-order oxi dation of iron(II) by SO5.-, HSO5-, and SO4.-. The latter species are generated in solution via the reaction of SO3.-, produced during the r eduction of iron(III) by sulfite, with oxygen. Radical scavengers do n ot affect the first-order decay but inhibit the pseudo-zero-order step . The results reveal no evidence for the formation of an intermediate oxygenated complex, and clearly indicate the important role of the sul fite-induced redox cycling of iron(II/III) in the presence of oxygen. Computer simulations based on the proposed reaction mechanism are in g ood agreement with the observed experimental kinetic traces and indica te that the formation of the SO5.- radical is the main oxygen-consumin g step during the overall redox process.