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 δ-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 solid–solid proximity in Cr(OH)3–MnO2 interactions at pH 5. The different rates and extents of Cr(VI) production in the mu...