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(II I/II)-catalysed autoxidation of S(IV) in acidic aqueous solution. In t he 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 p ulse radiolysis. The study was performed in the presence of excess S(I V). Under these conditions Mn(II) is distributed among the complexes M n2+ (aq), [Mn(HSO3)]+ and [Mn(SO3)Mn]2+. The rate of reaction decrease s as a function of increasing [Mn(II)]total which is rationalized qual itatively by a mechanism involving three parallel reactions between SO 5- and the Mn(II) complexes, with rate constants k16, k17 and k18, res pectively. [GRAPHIC] For increasing total concentrations of Mn(II), fo rmation 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 obs erved following the formation of Mn(III). These reactions have been at tributed to the disproportionation of Mn(III) and reactions between th e Mn(III) species and excess S(IV). The implications of the present re sults for the Mn(III/II) catalysed autoxidation of S(IV) are discussed .