Kinetics and Mechanism of the Iron(III)-catalyzed Autoxidation of Sulfur(IV) Oxides in Aqueous Solution. Evidence for the Redox Cycling of Iron in the Presence of Oxygen and Modeling of the Overall Reaction Mechanism

Abstract

Kinetic traces for the redox decomposition of iron(III)-sulfito complexes exhibit a peculiar break in the presence of oxygen. A detailed kinetic analysis of this feature as a function of [Fe(III)], [Fe(II)], [S(IV)], and [02] at pH 2.5 indicated that this step is a result of the sulfite-induced autoxidation of produced iron(II) in the presence of oxygen. The so observed redox cycling of iron comes to a dead end at the 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 oxidation of iron(II) by S05*-, HS05-, and SO4•-. The latter species are generated in solution via the reaction of S03•-, produced during the reduction of iron(III) by sulfite, with oxygen. Radical scavengers do not 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 sulfite-induced redox cycling of iron(II/III) in the presence of oxygen. Computer simulations based on the proposed reaction mechanism are in good agreement with the observed experimental kinetic traces and indicate that the formation of the SO5•-radical is the main oxygen-consuming step during the overall redox process.

Laser-spectroscopic laboratory studies of atmospheric aqueous phase free radical chemistry

“…T"298 K, pH"4.5 and IP0 M. Former determinations are only available for reaction (R-6) where experimental determinations led to k "3.2 · 10 lmol\ s\ [7] and k 52 · 10 lmol\ s\ [8], respectively. Additionally, from a modelling study a value of k +1 · 10 lmol\ s\ [9] has been derived. In principal, the latter value is confirmed by the present study whereas the two former experimental determinations show considerable deviations from the result of the present determination.…”

Reverse osmosis membrane composition, structure and performance modification by bisulphite, iron(III), bromide and chlorite exposure

“…1:1, 1:2 and 1:3 sulphito complexes can be formed, depending on the total S(IV) 380 concentration used (Kraft and Van Eldik, 1989a), being SO4 2-and S2O6 -the main oxidation 381 products (Kraft and Van Eldik, 1989b). The transition metal-sulphito complexes decompose 382 spontaneously and the sulphite radical, SO3͘ -, and the reduced form of the metal are produced 383 (Brandt et al, 1994). In absence of oxygen, SO4 2-and S2O6 2-are generated; when oxygen is 384 present, peroxomonosulphate radical, SO5͘ -, a strong oxidant, is also formed, which can react 385 with HSO3 -or Fe(II) to produce HSO5 - (Brandt et al, 1994).…”

“…The transition metal-sulphito complexes decompose 382 spontaneously and the sulphite radical, SO3͘ -, and the reduced form of the metal are produced 383 (Brandt et al, 1994). In absence of oxygen, SO4 2-and S2O6 2-are generated; when oxygen is 384 present, peroxomonosulphate radical, SO5͘ -, a strong oxidant, is also formed, which can react 385 with HSO3 -or Fe(II) to produce HSO5 - (Brandt et al, 1994). Greater Fe(III) and S(IV) 386 concentrations result in a higher formation rate of SO3͘ - (Brandt et al, 1994).…”

“…In absence of oxygen, SO4 2-and S2O6 2-are generated; when oxygen is 384 present, peroxomonosulphate radical, SO5͘ -, a strong oxidant, is also formed, which can react 385 with HSO3 -or Fe(II) to produce HSO5 - (Brandt et al, 1994). Greater Fe(III) and S(IV) 386 concentrations result in a higher formation rate of SO3͘ - (Brandt et al, 1994). The ratio of Fe(III), 387…”

“…Fe(II) and S(IV) versus oxygen is important in the overall decay of the iron(III)-sulphite 388 complexes (Brandt et al, 1994), varying the oxidation pathway with the ratio bisulphite/iron 389 (Ziajka et al, 1994). 390…”