Catalysis of HSO3-/SO32- oxidation by manganese ions

Among transition metal ions, Mn(II) ions are considered to be the most active catalysts of HSO3-/SO2- oxidation. All published data on the kinetics of the catalytic oxidation of HSO3-/SO2- by manganese ions are analyzed in this review. In addition to primary experimental data, information on the rate constants of the reactions of SO3-5 and HSO5- with Mn(II)/Mn(III) or Fe(II)/Fe(III) ions was also engaged in this analysis. Unavoidable impurities of iron ions in aqueous solutions were found to be responsible for the observed catalytic activity of manganese ions. Manganese ions are highly efficient activators of iron ions. Manganese ions are catalytically inert in the absence of iron ions. These metal ions form a synergistic pair in which Mn(II) ions increase the catalytic properties of iron ions many times. This effect results from the catalysis of a rate-limiting step of chain propagation in sulfite oxidation and from a shift in the distribution of iron species toward Fe(III), which participates in a step of chain initiation.