Mathematical models to evaluate retention and transport of chromium (VI) in soil

Abstract

Abstract Modeling the transport of reactive heavy metals in soils requires a mathematical description of the kinetics of various sorption processes. A family of models has been developed that can potentially describe the retention and transport of heavy metals in soils. These models include MRM (a multireaction model applicable to batch studies that includes parallel, reversible, nonlinear kinetic and equilibrium retention reactions), MRTM (MRM incorporated into the convection-dispersion equation for solute transport in soils under steady-state water flow), and SOTS (a two-site model applicable to batch and column studies that includes two parallel, reversible, second-order kinetic retention reactions). All the models include an irreversible, first-order kinetic reaction. Batch kinetic and column transport studies were conducted to evaluate the ability of these models to describe the retention and transport of Cr(VI) in three soils. Both MRM and SOTS described kinetic Cr(VI) retention data from batch experiments equally well. However, MRM rate coefficients from batch experiments could not be used to predict Cr(VI) breakthrough curves (BTCs) from column transport experiments when the soil retained a significant amount of solute. This was because MRM rate coefficients were pseudo rate coefficients that varied with initial solute concentration. SOTS rate coefficients from batch experiments could be used to approximate BTCs for Cr(VI). The second-order kinetic model, which accounts for the concentrations of reaction sites in soils, is a more realistic description of apparent retention processes. However, differences in mass transfer processes between batch and column studies will continue to limit the usefulness of batch-derived rate coefficients in predicting transport of reactive solutes in soils.