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Birnessite, pyrolusite and hausmannite were synthesized and tested for the ability to oxidize Cr(Ⅲ) to Cr(Ⅵ). These oxides differed in zero point of charge, surface area, and crystallinity. The kinetic study showed that Cr(Ⅲ) oxidation on the Mn-oxide surface is a first-order reaction. The reaction rate was various for different oxide at different conditions. Generally the reaction by hausmannite, containing Mn(Ⅲ), was faster than the others, and oxidation by pyrolusite was much slower. Solution pH and initial Cr(Ⅲ) concentration had a significant effect on the reaction. Inhibited oxidation at higher pH and initial Cr(Ⅲ) concentration could be due to the chance of Cr(Ⅲ) precipitation or complexing on the oxide surface. Oxidations by birnessite and hausmannite were faster at lower pH, but pyrolusite exhibited increased oxidation capacity at higher pH in the range between 3.0 and 5.0. Reactions were also temperature sensitive. Although calculated activation energies for the oxidation reactions at pH 3.0 were higher than the general activation energy for diffusion, there is no experimental evidence to suggest which reaction is the rate limiting step.
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