Effect of Water Saturation on the Diffusion/Adsorption of ²²Na and Cesium onto the Callovo-Oxfordian Claystones

Abstract

The diffusion and adsorption behaviors of sodium and cesium were investigated in the Callovo-Oxfordian claystones (France) under unsaturated conditions. Through-, out-, and in-diffusion laboratory experiments were performed on intact and compacted samples. These samples were partially saturated using an osmotic method for imposed suction up to 9 MPa. This specific technique enabled us to obtain water saturation degree ranging from 81% to 100% for intact samples and from 70% to 100% for compacted materials. The results showed a very low impact of water saturation on the extent of adsorption for 22 Na and cesium, onto intact and compacted materials. Such observations suggest that the saturation degrees were not low enough to limit the access of cations to adsorption sites on clay surfaces. At full saturation, enhanced diffusion for 22 Na and cesium was clearly evidenced onto intact and compacted samples. Under unsaturated conditions, the diffusion behavior for Cs and 22 Na was not only slower but also distinct as compared to fully saturated samples. For the intact rock and under suction of 1.9 MPa, the Cs diffusivity was reduced by a factor of 17, whereas for sodium, it was reduced by a factor of 5. Explanation was then proposed to explain such a difference.

Water retention and diffusion in unsaturated clays: Connecting atomistic and pore scale simulations

“…For instance, the relative hydraulic conductivity and the specific storage coefficient decreased strongly with decreasing saturation of the samples. Savoye et al (2010Savoye et al ( , 2012Savoye et al ( , 2014Savoye et al ( , 2017 investigated diffusion of a water tracer and of anions and cations through partially saturated samples of Callovo-Oxfordian claystone as well as through compacted clays with variable clay content. All diffusion coefficients D e were clearly reduced in partly saturated samples compared to fully saturated ones.…”

Influence of Water Saturation Level on Electrical Double Layer Properties in a Clay Mineral Mesopore: A Molecular Dynamics Study

“…Diffusion experiments have shown that anion diffusion is more strongly reduced than cation diffusion in the presence of water unsaturated conditions. Numerous processes were put forward to explain this observation, including differences in diffusion pathway disruptions among cations, anions, and neutral molecules. − Our study emphasizes that mechanisms taking place at the surface, such as formation and immobilization of ion pairs, may also be partly responsible for the observed differences between anion and cation diffusion property evolution with desaturation. As for water diffusion, our study corroborates the fact that a limited desaturation is not responsible for a decrease of water molecules self-diffusion coefficient.…”

Water and ion diffusion in partially-water saturated compacted kaolinite: Role played by vapor-phase diffusion in water mobility

“…Note that Churakov (2013) reports also significant increase of self-diffusion coefficient of water for similar conditions. This behavior combined with data obtained from other clayey materials (Savoye et al, 2017) allow us to explore the waterenhanced mobility assumption. This can be also justified due to the very low Rd values measured for ions under unsaturated conditions compared to full saturated one.…”

“…They showed that the De value decreased by 22% when saturation decreased from 100% to 93.3%. Savoye et al developed an original approach to perform diffusion experiments under partially saturated conditions in illite-sand mixtures (Savoye et al, 2014) and in Callovo-Oxfordien claystones, envisaged to host a French disposal facility for high-and intermediate level longlived radioactive waste (Savoye et al, 2017(Savoye et al, , 2012(Savoye et al, , 2010. The osmotic method was used to control the partial saturated conditions of the clayey samples over the duration of diffusion experiments.…”

“…On the one hand, the extent of surface diffusion may be attenuated when dehydration occurs in claystones, reducing the enhanced diffusion phenomenon for cation species (Savoye et al, 2012). On the other hand, in addition to HTO diffusing in the liquid phase, HTO diffusing in vapor form may contribute to the relatively high (compared to sodium) HTO diffusive rate, even at 81% water saturation (Savoye et al, 2017). This latter process was proposed by Smiles et al (1995) and more recently by Maples et al (2013) to explain the anomalously widespread distribution of HTO in layers adjacent to lowlevel radioactive waste burial facilities.…”