Modeling Solute Transport through Semipermeable Bentonite-Based Barriers under Transient Conditions

Bentonite-based barriers can act as semipermeable membranes, partially inhibiting the passage of charged solutes (i.e., ions). This behavior also induces chemico-osmotic, whereby water migrates from a location of lower solute concentration to a location of higher solute concentration. To assess the performance of such barriers, a one-dimensional advection-dispersion-reaction equation (ADRE) is derived, focusing specifically on a single salt composed of monovalent ions, such as NaCl or KCl. The equation is formulated using both actual and virtual ion concentrations. In the first case, the boundary conditions account for the ion partition effect, which is determined by the interaction between the solute charge and the solid skeleton charge. In the second case, a virtual salt concentration is obtained by imposing the equality between the electrochemical potentials of a hypothetical external bulk solution and the solution that is contained in the pores of the bentonite-based barrier. Numerical analyses are performed to demonstrate the significance of the semipermeable membrane behavior and its relevance for accurately predicting barrier performance.