Diffusion of cations in homo-ionic swelling clays as a function of the water content

Document available in extended abstract form only. In the past years, dielectric spectroscopy has been recognized as a powerful tool for studying the dynamics of cations in aluminosilicate compounds such as zeolites or clays minerals. In the case of nuclear waste storage, clays show remarkable properties for confinement: swelling capacity and cation retention. Indeed, exchangeable cations are more weakly linked to the adjacent atoms than framework ions and thus are more mobile. They consequently contribute to the whole dielectric signal, assuming that the network can be considered as rigid. Furthermore, polar molecules, such as water, that strongly interact with the cations or/and with the structural aluminosilicate network, have relatively large dielectric effects. It is thus possible to investigate adsorption process with dielectric spectroscopy. Besides, in the case of clays their swelling capacity complicates the interpretation of results and modifies the activation energy for cation diffusion. The present work reports conductivity data, by means of Complex Impedance Spectroscopy (CIS), recorded on clays samples: homo-ionic montmorillonites (i.e. saturated by alkaline or alkaline-earth cations) and mixed Na-Ca montmorillonite. Purified MX-80 bentonite is considered in our study as a model for montmorillonite. The dynamics of exchangeable cations, in terms of the activation energy ΔE required for the charge carrier displacement, is discussed as a function of: i) the nature of the clays, ii) the nature of the exchangeable cations (Li+, Na+, K+, Cs+ and Ca2+) and iii) the hydration state. In the latter, CIS results are interpreted in the light of the data recorded from Sample Controlled Thermal Analysis (SCTA). It is thus shown that the comparison between ?E obtained on dry solids and ΔE determined on water saturated systems is an efficient way to estimate the hydration energy and the swelling process. The values are compared with theoretical hydration energy values based on electrostatic calculations. It follows that the agreement and the differences obtained between the two approaches allows us to distinguish different behaviors as a function of the nature of the interlayer cations. Another powerful result is the estimation of the diffusion coefficients of the interlayer cations. Conductivity measurements as a function of the relative humidity are combined with a recent approach used to determine the specific surface area. It is then possible to extract cations diffusion coefficients as a function of the relative humidity. The evolution of these diffusion coefficients clearly shows the influence of the hydration state of the cations as well as the opening of the interlayer space. It results that for some cations (Li+ and Na+), diffusion coefficients can reach very high values at high relative humidity, while for larger cations (Cs+ in particular), the diffusion coefficients stay at a very low value. These results are in agreement with the classical description of hydration in swelling clays where the smaller cations can hydrate with a complete water layer, in contrast with the larger cations which are strongly bonded to the clay layer, even at high hydration states. (authors)