Transport of ions in physically heterogeneous systems; convection and diffusion in a column filled with alginate gel beads, predicted by a two-region model

An experimental flow system was developed to simulate solute transport in heterogeneous porous media, in which physical nonequilibrium occurs. The system consists of a column filled with spherical alginate gel beads. The gel acts as a stagnant water phase, in which ions can only move by diffusion. First, the release of nitrate and phosphate ions from the gel beads by diffusion was measured separately, in a batch system. Model calculations showed that it is possible to accurately predict the release of ions from the gel beads, by applying Fick’s law for diffusion and reported diffusion coefficients of the ions in water. Subsequently, the transport of ions in the column was tested against model calculations. The breakthrough of nitrate from a bead filled column was measured at different flow rates. The breakthrough curves were successfully predicted with a two-region model. In this model the solution in the beads is represented by an immobile region and the solution in the pores by a mobile region. Mass transport takes place by convection in the mobile region, and by diffusion in the immobile region. The model only requires the following independent input parameters: the flow rate, the diffusion coefficient in water, the column dimensions and the bead radius.