THE OXIDATION OF CHROMIUM(III) TO CHROMIUM(VI) ON THE SURFACE OF MANGANITE (GAMMA-MNOOH)

The oxidation kinetics of Cr(III) to Cr(VI) on the surface of manganite (gamma-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-mu-mol 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 d[Cr(VI)]/dt = k.([Cr(III)0] - [Cr(VI)]).{gamma-MnOOH} where k is 1.08 x 10(-2) +/- 0.03 x 10(-2) L m-2 s-1 and [Cr(III)0], [Cr(VI)], and {gamma-MnOOH} are the concentrations of initial Cr(III), Cr(VI), and manganite in mol/L and m2/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.