A second‐order kinetic approach for modeling solute retention and transport in soils

We present a second‐order kinetic approach for the description of solute retention during transport in soils. The basis for this approach is that it accounts for the sites on the soil matrix which are accessible for retention of the reactive solutes in solution. This approach was incorporated with the fully kinetic two‐site model where the difference between the characteristics of the two types of sites is based on the rate of kinetic retention reactions. We also assume that the retention mechanisms are site‐specific, e.g., the sorbed phase on type 1 sites may be characteristically different in their energy of reaction and/or the solute species from that on type 2 sites. The second‐order two‐site (SOTS) model was capable of describing the kinetic retention behavior of Cr(VI) batch data for Olivier, Windsor, and Cecil soils. Using independently measured parameters, the SOTS model was successful in predicting experimental Cr breakthrough curves (BTC's). The proposed second‐order approach was also extended to the diffusion controlled mobile‐immobile or two‐region (SOMIM) model. The use of estimated parameters (e.g., the mobile water fraction and mass transfer coefficients) for the SOMIM model did not provide improved predictions of Cr BTC's in comparison to the SOTS model. The failure of the mobile‐immobile model was attributed to the lack of nonequilibrium conditions for the two regions in these soils.