The reduction of hexavalent chromium by sulphite in wastewater - An explanation of the observed reactivity pattern

The reduction of Cr(VI) by S(IV) was investigated in buffered aqueous solutions, with the specific goal to establish the reactive Cr(VI) and/or S(IV) species, so that the reactivity pattern of this reaction could be explained. Reaction rates were investigated as a function of [S(IV)], pH and ionic strength. The [S(IV)]-dependence showed that the reaction is second order with resp;ct to the [S(IV)]. From the pH-dependence of the reaction a typical S-shaped curve was obtained. From this curve an apparent pK(a) value was calculated, which indicated that the increase in reactivity at lower pH values can be correlated to the increase in the concentration of the SO2. H2O(aq) species of S(IV). The effect of ionic strength on the reaction rate supported this theory, since it proved that an interaction involving a neutral molecule (i.e. SO2. H2O(aq)) rakes place in the rate determining reaction step. In conclusion it can be said that the reactivity vs, pH pattern of this very important industrial and environmental reaction system can now be explained satisfactory.