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

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 < pH < 4, [S(IV)] almost-equal-to 2.3 X 10(-5) M, 1 X 10(-6) M less-than-or-equal-to [Mn(II)] less-than-or-equal-to 1.5 X 10(-3) M, [Mn(III)] less-than-or-equal-to 4 X 10(-8) M, [O2] almost 2.5 X 10(-4) M, and low ionic strength. In the absence of added manganese(III), the kinetic traces display an induction period followed by a reaction first-order in sulfur(IV). Addition of a small amount of manganese(III) increases the rate significantly and suppresses the induction period. At pH 2.4, the first-order rate constant is k(obsd) = k[Mn(II)](1 + B[Mn(III)]0)/(A + [Mn(II)]), where A and B are constants and [Mn(III)]0 is the concentration of initially added manganese(III). The experiments are interpreted in terms of a free-radical chain mechanism. The first step, with rate constant k8 = (1.3 +/- 0.6) X 10(6) m-1 s-1, is a reaction between manganese(III) and a manganese(II) hydrogen sulfite complex with stability constant beta1 = (3 +/- 1) X 10(4) M-1: Mn(III) + MnHSO3+ -->(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.