Oxidation rate of sulfite ion (S(IV)) in rain water-measurement and estimation based on Fe³⁺ and Mn²⁺ concentrations and pH

The oxidation of S(IV) ion in rain water containing Fe³⁺ and Mn²⁺ was investigated in test solutions, and rain water samples. The rate of oxidation reaction of S(IV) ion both in test solutions and rain water was of the first order. The rate of oxidation of S(IV) ion was 0.12~1.6 x 10^-6 s^-1 without the catalyst, while it became larger, 1.7~16 x 10^-4 s^-1, by the order of 2 to 4 by the existence of Fe³⁺ and Mn²⁺. Therefore, Fe³⁺ and Mn²⁺ strongly contribute as catalysts for the oxidation of S(IV), and the positive correlationship was found between the concentrations of Fe³⁺ and/or Mn²⁺ and the oxidation rate. The increase of the oxidation rate at lower pH of solution was also found at pH 4 to 6. This could be due to the change of chemical form of each catalytic ion by the pH of solution. As the results of these experiments, catalytic oxidation reaction of S(IV) ion was expressed as a function of concentrations of Fe³⁺ and Mn²⁺ and pH of solution. The oxidation rate of S(IV) ion in 11 rain water samples collected at Hiyoshi, Yokohama, May through November, 1985 was 3.4~92 x 10^-5 s^-1. These measured values were almost agreed with the values of 3.5~140 x 10^-5 s^-1, calculated by the experimental equation.