Reduction of chromate by granular iron in the presence of dissolved CaCO3

Column experiments were conducted to determine the effects of dissolved CaCO3 on the reactivity of Fe towards Cr(VI) reduction and hydraulic conductivity. To provide mechanistic descriptions of reaction progress, Fe corrosion potential was measured continuously, and surface film composition was determined by Raman spectroscopy. Results showed that in the absence of CaCO3, Cr(VI) was removed primarily by precipitation of chromite (FenCrmO4 spinel) and Fe(III)-Cr(III) mixed (oxy)hydroxides. However, the precipitates caused positive shifts in Fe corrosion potential. The reactivity of Fe towards Cr(VI) reduction progressively decreased, resulting in a gradual migration of the Cr removal front. In the presence of CaCO3, the increase in pH due to Fe corrosion and Cr(VI) reduction was buffered, and relatively low corrosion potentials were maintained: therefore, the reactivity of the Fe towards Cr(VI) reduction was not significantly affected. A multi-component reactive transport model that incorporates reactivity loss of Fe due to accumulation of various mineral precipitates reasonably reproduced the experimental data. The simulation results suggest that the migration of Cr(VI) profiles was governed by the formation of Fe(III)-Cr(III) precipitates rather than carbonate minerals. The formation of carbonate minerals, however, significantly affected other geochemical parameters and caused porosity changes. In addition, the model was used to estimate the longevity of Fe PRBs in the absence and in the presence of dissolved CaCO3. The simulations showed that Cr(VI) can be treated effectively for substantial periods of time. However, the loss of porosity may compromise the long-term performance in situations where dissolved CaCO3 concentrations and Fe corrosion rates are high. (C) 2008 Elsevier Ltd. All rights reserved.