A review of porosity and Diffusion in Bentonite (pdf) (2.4 MB) - Posiva

1ABSTRACTPorosity in bentonite can be subdivided in free porewater, diffuse double layer (DDL)water, and interlayer water in montmorillonite, the most important mineral of bentonite.The proportions of these water types can be calculated from the internal and externalspecific surface area of montmorillonite and the width of interlayer- and DDL-water.Stacking of the TOT layers in montmorillonite in the c-direction is the master variablethat determines the specific internal and external surface areas. The thickness of the waterlayers depends on the ionic strength and the composition of the free porewater, andon the packing density of the bentonite.When the proportions of the water types change, the diffusion of cations, anions andneutral molecules is affected in different ways since the diffusional properties vary. Diffusivefluxes are proportional with accessible porosity, chemical potential gradient, anddiffusion coefficient, but not with concentration as has been proposed often in diffusionmodels. The equations for calculating the diffusive flux through interlayer water are derived.The potential gradient here is given by the gradient of the equivalent (or molar)fraction of the cation in the cation exchange capacity. The latter is expressed as concentrationin the interlayer water. An example calculation illustrates that the flux in interlayerwater can be dominant and opposite to the one in free porewater.Retardation by ion exchange is an important process in bentonite that can be modeled ifthe concentrations of major ions are known. Unfortunately, the analyses are almost neverdone in diffusion experiments.Keywords: Bentonite, montmorillonite, porosity, free porewater, diffuse double layer,interlayer, specific surface area, water layer, diffusivity, diffusive flux.