Modeling transport of protons and calcium ions in an alginate gel bead system: The effects of physical nonequilibrium and nonlinear competitive sorption

Solute transport in heterogeneous soil systems is determined by a combination of physical and chemical processes. Heterogeneity can cause physical nonequilibrium, which results in asymmetrical breakthrough curves with early initial breakthrough and tailing. In the case of reactive transport, the shape of the breakthrough curve is also effected by the sorption isotherm and by multicomponent effects. The combined effects of physical nonequilibrium and nonlinear multicomponent chemistry are the subject of this study. We show how an experimental system composed of alginate gel beads, together with two-region transport modeling, can be used to study transport phenomena in reactive heterogeneous systems. Proton transport through the gel column with ion exchange of protons and calcium ions was measured and simulated. The results show the combined effects of nonlinear sorption and of competition, which add to the effects of physical nonequilibrium. The results emphasize the importance of a thorough understanding of both physical and chemical processes for a reliable prediction of reactive solute transport.