The oxidation of chromium(III) to chromium(VI) on the surface of manganite (γ-MnOOH)

Abstract

The oxidation kinetics of Cr(III) to Cr(VI) on the surface of manganite (γ-MnOOH) have been investigated as a function of Cr(III) and manganite concentration, pH, ionic strength, and temperature. The reaction is first order with respect to the manganite adsorption density and also to Cr(III) concentration up to a critical adsorption density (0.2 μmol m⁻²). Above this concentration the reaction appears to be inhibited. The reaction is independent of pH and ionic strength. The rate law can be written as $\text{d[Cr(VI)]}\text{dt}\text{= k · ([Cr(III)}{}_0\text{] − [Cr(VI)]) · {γ-MnOOH}}$ where k is 1.08 × 10⁻² ± 0.03 × 10⁻² L m⁻² s⁻¹ and [Cr(III)₀], [Cr(VI)], and {γ-MnOOH} are the concentrations of initial Cr(III), Cr(VI), and manganite in $\text{mol}\text{L}$ and $\text{m}{}^2\text{L}$, respectively. Approximately three Mn(II) ions are released for every Cr(III) ion which is oxidised. In the presence of organic ligands the reaction proceeds more slowly.