Role of manganese dioxide in the oxidation of aqueous sulphur(IV) in oxic and anoxic suspensions

The kinetics of direct oxidation of sulphur(IV) by MnO2 has been studied in stirred suspensions in nitrogen atmosphere and that of autoxidation reacti on in stirred suspensions in equilibrium with atmospheric oxygen. Sulphate has been detected to be the only oxidation product of sulphur(IV) in both t he cases. In the pH range 4.6-5.8, the kinetics of direct oxidation of sulp hur(IV) in anoxic suspesions agreed with the rate law (I). On the other han d at pH 7.9 for the same reaction a much simpler rate law (II) is obeyed. -d[S(IV)]/dt={(A[H+](-1)+B)[MnO2](0)[S(IV)](0)}/{1+C[S(IV)](0)[H+](-1)} ... (I) -d[S(IV)]/dt = {k(3)K(3)[MnO2](0)[S(IV)](0)}/{1+K-3[S(IV)](0)} ...(II) At 30 degrees C, the values of empirical constants are: A = 4.8x10(-10) mol g(-1)s(-1); B = 2.8x10(-5) L g(-1) s(-1); C = 1.1 x 10(-3); k(3) = 9.3 x 1 0(-7)s(-1) and K-3 = 34. For MnO2 catalysed autoxidation in oxic suspension s the rate data pertaining to pH range 4.6-5.8 fitted Eq. (III) and those -d[S(IV)]/dt = k(5)[MnO2](0)[S(IV)](0)[H+](-0.34) ...(III) -d[S(IV)]/dt = k(8)[MnO2](0)[S(IV)](0)[H+](-0.4) ...(IV) in the pH range 6.5-7.0 fitted Eq. (IV). At 30 degrees C, k(5) and k(8) hav e values of 1.4 x 10(-4) L-0.66 g(-1) mol(-0.34) s(-1) and 3.2 L-1.4 g(-1) mol(-0.4) s(-1). The rate of autoxidation is independent of oxygen partial pressure. The atmospheric conversion rates at 30 degrees C have been calcul ated.