The reduction of hexavalent chromium by sulphite in wastewater

The reduction of hexavalent chromium, Cr(VI), in unbuffered aqueous solutio n by sulphite was investigated as a function of pH, Possible dosage and con tact time required to reach equilibrium Dissolved Na2Cr2O7.2H(2)O served as the aqueous source of Cr(VI). The rate of the reaction between Cr(VI) and sulphite exhibited a very strong dependence on the pH di the reaction mixtu re, with a decrease In reaction rate with an increase in pH from 2.0 to 5.0 . For a 5.0 x 10(-4) mol-dm(-3) Cr(VI) solution (similar to 26 mg/l) a sulp hite concentration of at least 5 times the initial Cr(VI) concentration was needed for complete reduction in this pH range. Above pH 6 very large conc entrations of sulphite were needed to reduce Cr(VI) effectively. To check t he industrial relevance of the results, baghouse dust from a ferrochromium plant was leached to serve as a model source of industrial wastewater conta ining aqueous Cr(VI), for conducting Various comparative experiments. The r esults clearly indicated that sulphite would be a suitable reductant for Cr (VI) in wastewater under specific conditions (2.0 less than or equal to pH( initial) less than or equal to 5.0 and [sulphite] = 5 x [Cr(VI)](initial)).