Chromium transport, oxidation, and adsorption in manganese-coated sand

We examine how the processes of advection, dispersion, oxidation-reduction, and adsorption combine to affect the transport of chromium through columns packed with pyrolusite (beta -MnO2)-coated sand. We find that beta -MnO2 e ffectively oxidizes Cr(III) to Cr(VI) and that the extent of oxidation is s ensitive to changes in pH, pore water velocity, and influent concentrations of Cr(III). Cr(III) oxidation rates, although initially high, decline well before the supply of beta -MnO2 is depleted, suggesting that a reaction pr oduct inhibits the conversion of Cr(III) to Cr(VI). Rate-limited reactions govern the weak adsorption of each chromium species, with Cr(III) adsorptio n varying directly with pH and Cr(VI) adsorption varying inversely with pH. The breakthrough data on chromium transport can be matched closely by calc ulations of a simple model that accounts for (1) advective-dispersive trans port of Cr(III), Cr(VI), and dissolved oxygen, (2) first-order kinetics ads orption of the reduced and oxidized chromium species. and (3) nonlinear rat e-limited oxidation of Cr(III) to Cr(VI). Our work supplements the limited database on the transport of redox-sensitive metals in porous media and pro vides a means for quantifying the coupled processes that contribute to this transport. (C) 2001 Elsevier Science B.V. All rights reserved.