Chemico-osmosis and solute transport through geosynthetic clay liners.

ABSTRACT Geosynthetic clay liners (GCLs) are increasingly used in bottom liners for landfills because of their low hydraulic conductivity to water and ease of installation. The evaluation of GCL performances as pollutant barriers needs an adequate theoretical approach for modeling the simultaneous migration of water and solutes through bentonite. Bentonite is a clay soil containing typically at least 70% of the three layered (2:1) clay mineral montmorillonite, which is characterized by a very high total specific surface and a negative electric charge. The ultra-fine pore size (< 100 Å) of this clay soil and the electric interaction between montmorillonite particles and ions in pore solution determine macroscopic phenomena that can not be modeled on the basis of the advective-diffusive transport theory, normally used to describe the movement of water and solutes through porous media. Therefore, the equations that describe the macroscopic transport of a solution containing a binary electrolyte through a semi-permeable membrane are derived following the approach of the Thermodynamics of Irreversible Processes. This approach for the description of the macroscopic transport of the solution does not provide any physical interpretation of the phenomena involved. In order to provide a physical interpretation of the phenomenological parameters, a complementary macroscopic transport model is also proposed. Such a model is able to explain the dependency, observed in laboratory experiments, of the chemico-osmotic efficiency coefficient on the solute concentration and the bentonite porosity. A comparison between GCLs and compacted clay liners is presented accounting for semi-permeable membrane behavior, which is shown to play a relevant role at low salt concentrations.