Abstract
The reduction of hexavalent chromium, Cr(VI), by hydrogen peroxide in both buffered and non-buffered aqueous solutions was investigated as a function of concentration, pH, ionic strength, effect of radical scavengers, temperature and pressure. The rate of the reaction between Cr(VI) and hydrogen peroxide exhibited a strong dependence on the pH of the reaction mixture, viz. a decrease in reaction rate with increasing pH from 1.0 to 7.0. For a 2.5 x 10-4 mol·dm-3 Cr(VI) solution, a H2O2 concentration of at least 10 times the initial Cr(VI) concentration was required for complete reduction in this pH range. Neither the ionic strength of the reaction mixture, nor the presence of a radical scavenger had an effect on the rate of the rate-determining step. From the temperature dependence of the reaction the activation enthalpy (∆H≠) was calculated to be 10.4 ± 0.5 kJ·mol-1 and the activation entropy (∆S≠) to be -186 ± 3 J K-1·mol-1 for the rate-determining step. The volume of activation (∆V≠) was found to be -6.1 ± 0.5 cm3·mol-1 from the pressure dependence of the reaction rate. The empirical data could be fitted to : kobs = k[H+][H2O2]/(Ka + [H+]), with k = (48.4 ± 1.4) x 103 dm3·mol-1·s-1 and Ka the acid dissociation constant of H2CrO4. A reaction mechanism in which a Cr(VI)/H2O2 adduct is formed in the rate determining step, is proposed. The theoretical rate law that can be derived from this mechanism is in agreement with the empirical rate law.
Highlights
Chromium has found many useful industrial applications, the impact of the chromium industry on the receiving environment is extensive, complicated and not fully quantified
In this study we report the kinetics of the reduction of Cr(VI) by H2O2 over a wide pH range (1.0 to 7.0) as a function of numerous rate-determining factors that include for the first time the pressure dependence from which the volume of activation can be calculated
Hydrogen peroxide would usually not be considered as the primary reductant for the removal of chromates, this study has shown that a hydrogen peroxide treatment of an acid effluent could simultaneously decrease the environmental impact of chromates
Summary
Chromium has found many useful industrial applications, the impact of the chromium industry on the receiving environment is extensive, complicated and not fully quantified. Contaminated landfill sites and industrial effluents are likely to contain only trivalent chromium, Cr(III), and hexavalent chromium, Cr(VI). Cr(II), is unstable and rapidly oxidised to Cr(III). Cr(0), is oxidised to Cr(III) unless it is stabilised by superficial oxidation. Hexavalent chromium is considered toxic and carcinogenic (Yassi and Nieboer, 1988), whereas trivalent chromium is a trace nutrient for humans and animals alike (Wong and Trevors, 1988). The industrial and environmental importance of the reduction of Cr(VI) is emphasised by this fact. In the South African context this reduction process is even more important since South Africa is considered the largest chromite and ferrochromium producing country, as well as the 6th largest stainless-steel producing country in the world. It is estimated that South Africa holds approximately 72% of the world’s chromium reserves (UCT, 2001)
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