Importance of Mn(III) Availability on the Rate of Cr(III) Oxidation on δ-MnO2

The oxidation of Cr(III) to Cr(VI) significantly increases its mobility and toxicity and therefore its environmental hazard. The oxidation rate of aqueous Cr(III) in a suspension of δ-MnO2 (birnessite) was investigated at different pyrophosphate concentrations and pH values. Pyrophosphate, which is known to form strong complexes with Mn(III), was added to the suspension in order to sequester available Mn(III) and thereby demonstrate the importance of Mn(III) availability on the rate of Cr(III) oxidation. Pyrophosphate concentrations of 3 μmol/L, equivalent to approximately 14% oxide surface site complexation, resulted in a greater than 45% reduction in k obs. Dramatic rate reductions were observed in the 0 to 7 μmol/L pyrophosphate region. At concentrations greater than 7 μmol/L, additional pyrophosphate had little effect on the reaction rate. On the basis of experimental results a two reaction pathway model in which the rate of one pathway scales with free Mn(III) concentration was created and fit to experimentally determined k obs. The fit to experimental data was excellent. This study indicates that Mn(III) availability may be an important factor in the oxidation of Cr(III) to Cr(VI).