Reaction of peroxomonosulfate radical with manganese(II) in acidic aqueous solution. A pulse radiolysis study

Abstract

The reaction between the SO₅^– radical and Mn^II has been proposed to be the most important process for regeneration of Mn^III in the Mn^III/II-catalysed autoxidation of S^IV 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 S^IV. Under these conditions Mn^II is distributed among the complexes Mn²⁺(aq), [Mn(HSO₃)]⁺ and [Mn(SO₃)Mn]²⁺. The rate of reaction decreases as a function of increasing [Mn^II]_total which is rationalized qualitatively by a mechanism involving three parallel reactions between SO₅^– and the Mn^II 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 Mn^II, 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 Mn^II species is predominant. Subsequent slow processes are observed following the formation of Mn^III. These reactions have been attributed to the disproportionation of Mn^III and reactions between the Mn^III species and excess S^IV. The implications of the present results for the Mn^III/II catalysed autoxidation of S^IV are discussed.