Reaction of Peroxomonosulfate Radical with Manganese(II) in Acidic Aqueous Solution. A Pulse Radiolysis Study

The reaction between the SO5– radical and MnII has been proposed to be the most important process for regeneration of MnIII in the MnIII/II-catalysed autoxidation of SIV in acidic aqueous solution. In the present study, the second-order rate constant for this reaction has been determined at pH 3. 0 and 10 mmol dm–3 ionic strength by use of pulse radiolysis. The study was performed in the presence of excess SIV. Under these conditions MnII is distributed among the complexes Mn2+(aq), [Mn(HSO3)]+ and [Mn(SO3)Mn]2+. The rate of reaction decreases as a function of increasing [MnII]total which is rationalized qualitatively by a mechanism involving three parallel reactions between SO5– and the MnII complexes, with rate constants k16, k17 and k18, respectively. Mn2++ SO5– [graphic omitted] Mn3++ HSO5–(16), [Mn(HSO3)]++ SO5– [graphic omitted] [Mn(HSO3)]2+ HSO5–(17), [Mn(SO3)Mn]2++ SO5– [graphic omitted] [Mn(SO3)Mn]3++ HSO5–(18), For increasing total concentrations of MnII, formation of the sulfito-bridged complex is favoured which implies that k18 < k16, k17. Values of the second-order rate constant in the range 2 × 10 108–2 × 1010 dm3 mol–1 s–1 have been determined, depending on which MnII species is predominant. Subsequent slow processes are observed following the formation of MnIII. These reactions have been attributed to the disproportionation of MnIII and reactions between the MnIII species and excess SIV. The implications of the present results for the MnIII/II catalysed autoxidation of SIV are discussed.