A Simple Kinetic Model for Autoxidation of S(IV) Oxides Catalyzed by Iron and/or Manganese Ions

Abstract

The reaction kinetics of S(IV) autoxidation catalyzed by single metal ions of Mn(II) and Fe(II) or Fe(III) and by a mixture of Mn(II) and Fe(II) under the conditions representative for acidified atmospheric liquid water was investigated. A simple power law kinetic model based on the stability constants for metal-sulfito complexes formed during the first step of a radical chain mechanism predicts well the kinetics for the reactions catalyzed by single metal ions. The calculated stability constants for iron (5.7 × 10 3 dm 3 mol −1 ) and manganese (10 × 10 4 dm 3 mol −1 )s ulf ito complexes are close to those reported in the literature. The catalytic synergism between Mn(II) and Fe(II) was confirmed. For this system the following power law rate equation was suggested: rtot = SFe · rFe + SMn · rMn , where rFe and rMn are the reaction rates in the presence of Fe(II) and Mn(II), respectively. SFe and SMn are proportional factors, which account for the synergistic effect. The proposed power law rate equation predicts the reaction kinetics very well. The values of SFe (1.35) and SMn (15) indicate that the influence of Fe(II)/Fe(III) on Mn(II)/Mn(III) cycling is larger than, vice versa, agreeing with the reaction mechanism proposed for the S(IV) autoxidation catalyzed by mixed metal ions.