Kinetics and mechanism for manganese-catalyzed oxidation of sulfur(IV) by oxygen in aqueous solution | doi.page

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Kinetics and mechanism for manganese-catalyzed oxidation of sulfur(IV) by oxygen in aqueous solution

Inorganic Chemistry1993Vol. 32(21), pp. 4527–4538

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Johan Berglund, Sture Fronæus, Lars I. Elding

Abstract

The kinetics for manganese-catalyzed autoxidation of sulfur(IV) (SO2.nH2O, HSO3-, SO32-) in aqueous solution has been studied spectrophotometrically at 25-degrees-C, 1 (k8) 2Mn(II) + SO3- + H+. The catalytic activity of Mn(III/II) may be explained by formation of an oxo- (or hydroxo-) bridged mixed-valence precursor complex Mn(III)-O-Mn(II)-HSO3, in which bridged electron transfer produces the SO3 radical. When [Fe(III)] < 10(-6) M is added to the reacting system, the oxidation rate becomes much faster than the sum of the individual contributions from the manganese and iron catalyses; i.e., a synergistic effect is displayed. Initiation of the manganese-catalyzed oxidation in the absence of initially added manganese(III) is shown to be due to a trace impurity of ca. (1-2) x 10(-8) M iron(III). The SO3- radical is generated by the oxidation of sulfur(IV) by iron(III). In subsequent steps, manganese(II) is oxidized to manganese(III) by SO5- formed by oxidation of SO3 with dissolved oxygen. Computer simulation of the overall kinetics shows that an iron(III) concentration of 2 x 10(-8) M is indeed sufficient to initiate the manganese-catalyzed oxidation and explains the autocatalysis. At pH 4.0, the first-order rate constant is k(obsd) = (k[Mn(II)] + k[Mn(II)]2)/(A + [Mn(II)]). The quadratic manganese(II) term indicates formation of a sulfito-bridged manganese(II) complex, MnSO3Mn2+, which can also be attacked by manganese(III), forming SO3- radicals. Several discrepancies in previous literature may be explained by the reaction mechanism derived.

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Abstract

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