REACTION OF PEROXOMONOSULFATE RADICAL WITH MANGANESE(II) IN ACIDIC AQUEOUS-SOLUTION - A PULSE-RADIOLYSIS STUDY

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. [GRAPHIC] 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 x 10(8)-2 x 10(10) 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.