Reduction of Cr(VI) by hydrazine in solution saturated with KHCO3

Cr(VI) that exists in many industrial wastes is considered highly toxic. One of the most common ways to dispose of these wastes is to reduce Cr(VI) to Cr(III), which is less toxic and can be easily removed or fixed. Reduction of Cr(VI) by hydrazine in aqueous solutions saturated with potassium bicarbonate was studied in this paper. The content of Cr(VI), molar ratio of hydrazine to Cr(VI) and reaction temperature were 200-1000 ppm, 2:1-10:1 and 25-70 °C, respectively. Results showed that the overall reaction was third-order, i.e. first-order with respect to hydrazine and second-order with respect to Cr(VI). The apparent empirical activation energy was 75.6 kJ mol−1. The reaction kinetics could be interpreted by a three-step mechanism. Effect of the oxygen in the solution on the reduction of Cr(VI) was negligible.