Julio Gonçalvès

Chapter 8 - Semipermeable Membrane Properties and Chemomechanical Coupling in Clay Barriers

Abstract In this chapter, the semipermeable behavior of clay-rocks is mainly exemplified on fluid flow. For such media and besides pressure gradients, other driving forces not accounted for in the classical form of Darcys law describes fluid flow. In the context of coupled transport processes, the additional driving forces are chemical, electrical potential and thermal gradients. Consequently, the resulting so-called osmotic fluxes must be considered in clay media for hydrodynamic and transport calculations. All available osmotic conductivity data have been collected here, including the most recent values acquired in the course of ongoing research on nuclear waste confinement in clay-rock formations. By means of adjusted curves, these data can be directly used as abacuses. The data are also used to support some theoretical calculations, some of which are presented and discussed here. These predictive models (theoretical expressions or abacuses) for osmotic parameters make it possible to perform some fluid and transport calculations within clay-rock formations. Therefore, they can be conveniently used for pore pressure profiles interpretation within clay stratigraphic layer, calculation of characteristic times of solute migration through Peclet number characterization, or even steady or transient state direct transport calculations. For transient state hydrodynamics, the identification of the required chemomechanical coupling coefficient is analyzed in the light of the most recent theoretical work.

Analytical Expressions for Thermo‐Osmotic Permeability of Clays

In this study, a new formulation for the thermo‐osmotic permeability of natural pore solutions containing monovalent and divalent cations is proposed. The mathematical formulation proposed here is based on the theoretical framework supporting thermo‐osmosis which relies on water structure alteration in the pore space of surface‐charged materials caused by solid‐fluid electrochemical interactions. The ionic content balancing the surface charge of clay minerals causes a disruption in the hydrogen bond network when more structured water is present at the clay surface. Analytical expressions based on our heuristic model are proposed and compared to the available data for NaCl solutions. It is shown that the introduction of divalent cations reduces the thermo‐osmotic permeability by one third compared to the monovalent case. The analytical expressions provided here can be used to advantage for safety calculations in deep underground nuclear waste repositories.