Understanding the Roles of Dissolution and Diffusion in Cr(OH)3 Oxidation by δ-MnO2

Manganese oxides are the major oxidants of Cr(III) to Cr(VI) in natural environments. This study evaluated the rate and extent of oxidation of Cr(III) released from Cr(OH)(3) by delta-MnO2 from pH 5 to 9 with a particular focus on quantifying the rate constant for Cr(III) oxidation on the MnO2 surface. The Cr(III) oxidation rate was initially fast, but it then slowed and ceased for pH 5 to pH 7, which agrees with previously reported inhibition of the redox reaction above pH 4 by precipitation of Cr(III) on the MnO2 surface. Above pH 7, overall Cr(VI) production was higher than at lower pH even though the dissolved Cr(III) concentration in equilibrium with Cr(OH)(3) was lower. This is probably due to the reoxidation of aqueous Mn(II) by dissolved oxygen at higher pH, which made more manganese oxide available to oxidize Cr(III) to Cr(VI). Multichamber experiments were used to assess the role of solidsolid proximity in Cr(OH)(3)MnO2 interactions at pH 5. The different rates and extents of Cr(VI) production in the multichamber reactor and completely mixed batch reactor indicate that mixing of Cr(OH)(3) and MnO2 solids plays a role in the rate and extent of Cr(VI) generation. Further Cr(VI) generation in this systems strongly depends on pH. Because of the importance of mass transfer of Cr(III) to MnO2 solids for the overall Cr(III) oxidation process, the net rate of Cr(VI) generation in natural environments with Cr(III)-containing solids and MnO2 can be orders of magnitude slower than observed in well-mixed laboratory-scale batch experiments.