[Kinetic characteristics of Cr(III) oxidation by delta-MnO2].

The kinetics of Cr(III) oxidation by synthesized vernadite(delta-MnO2) was investigated through magnetic stirring and fractional centrifugation. The oxidation procedure of Cr(III) by delta-MnO2 could be divided into two first-order reactions, a fast reaction followed by a slow one. In high Cr(III) concentration solution, the reaction was also well sub-simulated with diffusion equation and Elovich equation according to the reaction phases. The rate constants markedly increased with temperature rising. The ratio of Cr(VI)/Mn(II) gradually decreased and eventually reached the theoretical value (0.667) with the reaction. Cr(VI), Cr(III) and Mn(II) adsorbed on delta-MnO2 only accounted 0.1 to 3% of the total amounts. The Cr(VI)/Mn(II) ratio on the surface of MnO2 was much smaller than the theoretical value due to the release of Mn(II) to the solution lagged behind Cr(VI). When Cr(III) was in low concentration, Cr(III) oxidation was controlled by the diffusion and adsorption of Cr(III). While in high Cr(III) concentration, the key step was the diffusion of Mn(II) to solution.