Bentonite is an important hydraulic barrier material in many geotechnical applications, such as geosynthetic clay liners at solid waste landfills, or as proposed backfill material in engineered barrier systems at nuclear waste repositories. The limited permeability of bentonite is at least partially the result of its low porosity and the swelling of Na-montmorillonite, its major mineralogical component, in water. Due to these characteristics, the transport of contaminants through bentonite layers is expected to be limited and dominated by diffusion processes. In bentonite, the majority of the connected porosity is associated with montmorillonite particles, which consist of stacks of negatively-charged smectite layers. As a result, compacted smectite has two types of porosities: (1) large pores between clay particles, where diffusion is less affected by electric-double-layer forces, and (2) very thin interlayer spaces within individual clay particles, where diffusion is strongly impacted by surface charge and ionic strength. As diffusion is expected to take place differently in these two volumes, this essentially creates two �small-scale diffusion pathways�, where each may become dominant under different system conditions. Furthermore, for surface-reactive solutes, these two porous regimes differ with regards to surface complexation reactions. Electrostatic and hydration forces only are thought to govern interlayer binding, whereas chemical bonding with surface ligands is dominant for reactions at edge sites of layered clay particles and for iron oxide nanoparticles on outer basal planes.We demonstrate the relevance of clay pore structure and diffusion-accessible porosity for solute diffusion rates, using experimental data, molecular dynamics simulations, and diffusive transport modeling. The importance of solute charge for solute diffusion rates is demonstrated with the results of a calcium-bromide (CaBr2) through-diffusion experiment in Na-montmorillonite and modeling simulations of the system. In the experiment, purified Na-montmorillonite was packed into a cell, allowed to equilibrate with background electrolyte (pH=7, I=0.1 M NaCl), and then exposed to a constant CaBr2 concentration gradient between two solution reservoirs. After reaching steady-state conditions for the diffusive fluxes of Ca and Br, cumulative mass data could be used to compute effective and apparent Ca and Br diffusion coefficients. Diffusion data for tritiated water allowed the calculation of clay porosity, and the determination of Ca and Br sorption distribution coefficients. Our results indicate that Ca diffusion in Na-montmorillonite is slower than for the non-reactive tracer, due to Ca exchange reactions within the clay interlayers. In contrast, rates of Br diffusion are faster than the tracer, and MD simulations show that there are solute-specific differences in diffusion-accessible porosities that govern the observed behaviors.

Impacts of pore structure and diffusion-accessible porosity for calcium-bromide diffusion in sodium-montmorillonite

248th National Meeting (2014)

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