The reduction of hexavalent chromium by sulphite in wastewater

Abstract

The reduction of hexavalent chromium, Cr(VI), in unbuffered aqueous solution by sulphite was investigated as a function of pH, possible dosage and contact time required to reach equilibrium. Dissolved Na 2 Cr 2 O 7 .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 of the reaction mixture, 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 (∼26 mg/) a sulphite 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 concentrations of sulphite were needed to reduce Cr(VI) effectively. To check the industrial relevance of the results, baghouse dust from a ferrochromium plant was leached to serve as a model source of industrial wastewater containing aqueous Cr(VI), for conducting various comparative experiments. The results clearly indicated that sulphite would be a suitable reductant for Cr(VI) in wastewater under specific conditions (2.0 ≤ pH initial ≤ 5.0 and [sulphite] = 5 x [Cr(VI)] initial ).