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Anions in clay materials : a case study for multi-scale modeling approaches


Swelling property in water endows clay materials with a very low hydraulic conductivity and transport of water and solutes in these media is mainly diffusive, with diffusion coefficients that are much smaller than in bulk water. As a consequence, clay materials are recognized as efficient barriers for waste confinements. For instance, bentonites are already used by solid-waste-disposal operators in the form of compact and/or geosynthetic clay liners while engineered bentonite barriers and natural clay formations are foreseen for radioactive waste confinement in deep geological repositories. Clay material physical and chemical properties result from their small size (typically < 2µm) and structural specificities. Recent developments of computationnal and spectrometric techniques enabled to probe these properties down to the molecular scale, providing accurate descriptions of processes taking place at the solid/water interface. However, for engineering applications, clay properties must be characterised at a large spatial scale (1m-100m) using macroscopic descriptions and often empirical parameters enabling predictive modelling on extended timescales (typically 100 000 years for radwaste repositories). Making the link between these spatial and temporal scales is of paramount importance to assess the robustness of long term predictive modelling results through a detailed justification for the necessary modelling simplifications. The present talk will focus on anions distribution and mobility in montmorillonite and illite clay materials that are representative of natural clay formation mineralogy. Illite and montmorillonite structures have an excess of negative charge that is compensated by cations adsorption in their interlayer space and on their outer surface. Conversely, anions undergo weak or no specific adsorption and are repelled from the negative surface, a phenomenon called anion exclusion. While this process can be adequately predicted by theoretical models on simple static systems (e.g. clay suspension in 1:1 electrolytes), its quantitive prediction on complex systems (compact clay material, multispecies electrolytes) remains problematic as well as its impact on macroscopic anion diffusion parameters. We will review models available from the nano- to the macroscopic scale in order to (i) highlights recent improvements in our understanding of anion distribution and mobility in clay materials and (ii) to identify remaining gaps to be elucidated.
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hal-00599424 , version 1 (09-06-2011)


  • HAL Id : hal-00599424 , version 1


Christophe Tournassat. Anions in clay materials : a case study for multi-scale modeling approaches. Goldschmidt 2011, Aug 2011, Prague, Czech Republic. ⟨hal-00599424⟩


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