The reduction of hexavalent chromium by sulphite in wastewater
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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 Na2Cr2O7.2H2O 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 < pHinitial < 5.0 and [sulphite] = 5 x [Cr(VI)]initial).