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 solut ions, 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 s trength. The [S(IV)]-dependence showed that the reaction is second order wi th 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 c alculated, which indicated that the increase in reactivity at lower pH valu es can be correlated to the increase in the concentration of the SO2. H2O(a q) species of S(IV). The effect of ionic strength on the reaction rate supp orted this theory, since it proved that an interaction involving a neutral molecule (i.e. SO2. H2O(aq)) rakes place in the rate determining reaction s tep. In conclusion it can be said that the reactivity vs, pH pattern of thi s very important industrial and environmental reaction system can now be ex plained satisfactory.