Rapport final du projet européen CatClay sur les processus de migration des cations dans les roches argileuses indurées

International audienceIn the framework of the feasibility studies on the radioactive waste disposal in deep argillaceous formations, it isnow well established that the transport properties of solutes in clay rocks, i.e. parameter values for Fick’s law, are mainlygoverned by the negatively charged clay mineral surface. While a good understanding of the diffusive behaviour of non-reactiveanionic and neutral species is now achieved, much effort has to be placed on improving understanding of coupledsorption/diffusion phenomena for sorbing cations. Indeed, several cations known to form highly stable surface complexes withsites on mineral surfaces migrate more deeply into clay rock than expected. Therefore, the overall objective of the EC CatClayproject is to address this issue, using a ‘bottom-up’ approach, in which simpler, analogous systems (here a compacted clay,‘pure’ illite) are experimentally studied and modelled, and then the transferability of these results to more complex materials, i.e.the clay rocks under consideration in France, Switzerland and Belgium for hosting radioactive waste disposal facilities, isverified. The cations of interest were chosen for covering a representative range of cations families: from a moderately sorbingcation, the strontium, to three strongly sorbing cations, Co(II), Zn(II) and Eu(III). For the 4 years of this project, much effort wasdevoted to developing and applying specific experimental methods needed for acquiring the high precision, reliable data neededto test the alternative hypotheses represented by different conceptual-numerical models. The enhanced diffusion of the sorbingcations of interest was confirmed both in the simpler analogous illite system for Sr2+, Co(II) and Zn(II), but also in the naturalclay rocks, except for Eu(III). First modelling approach including diffusion in the diffuse double layer (DDL) promisinglysucceeded in reproducing the experimental data under the various conditions both in illite and clay rocks, even though someassumptions made have to be verified. In parallel, actual 3D geometrical pore size distributions of compacted illite, and in lessextent, clay rock samples, were successfully determined by combining TEM and FIB-nt analyses on materials maintained in awater-like saturation state by means of an extensive impregnation step. Based on this spatial distribution of pores, first numericaldiffusion experiments were carried at the pore scale through virtual illite, enabling a better understanding of how transferpathways are organized in the porous media. Finally, the EC CatClay project allowed a better understanding of the migration ofstrongly sorbing tracers through low permeability ‘clay rock’ formations, increasing confidence in our capacity to demonstratethat the models used to predict radionuclide migration through these rocks are scientifically sound

The Effect of Monovalent Electrolytes on the Deprotonation of MAl12 Keggin Ions

We study the deprotonation of MAl12 Keggin ions in monovalent electrolyte solutions of varying composition and concentration by potentiometric titration. The structures exhibit very steep deprotonation, where the singly coordinated aquo groups lose protons within a narrow pH range. Once the deprotonation is substantial, the Keggin ions start to aggregate by dehydration and linkage of terminal functional groups into hydroxo-bridges. In the present study, we address three aspects with our experiments. We test the cation-specificity, the anion-specificity and the overall effect of electrolyte concentration with respect to the deprotonation behavior. Our results show that the cation series in chloride systems does not show any ion-specificity and all the curves coincide for the 100 mM solutions, whereas the anion series in sodium systems does. The most structure-making anion (bromate) used in this study causes aggregation of the Keggins prior to the onset of aggregation in the presence of border-line and structure-breaking anions (chloride, nitrate and perchlorate). For the latter, no significant difference is observed. The fluoride ion causes a completely different behavior. No significant deprotonation pH effect is observed where deprotonation occurs in the absence of fluoride. At even higher pH, massive consumption of hydroxide occurs. Some possible scenarios for the behavior of the fluoride-containing systems are discussed

The Effect of Monovalent Electrolytes on the Deprotonation of MAl12 Keggin Ions

We study the deprotonation of MAl Keggin ions in monovalent electrolyte solutions of varying composition and concentration by potentiometric titration. The structures exhibit very steep deprotonation, where the singly coordinated aquo groups lose protons within a narrow pH range. Once the deprotonation is substantial, the Keggin ions start to aggregate by dehydration and linkage of terminal functional groups into hydroxo-bridges. In the present study, we address three aspects with our experiments. We test the cation-specificity, the anion-specificity and the overall effect of electrolyte concentration with respect to the deprotonation behavior. Our results show that the cation series in chloride systems does not show any ion-specificity and all the curves coincide for the 100 mM solutions, whereas the anion series in sodium systems does. The most structure-making anion (bromate) used in this study causes aggregation of the Keggins prior to the onset of aggregation in the presence of border-line and structure-breaking anions (chloride, nitrate and perchlorate). For the latter, no significant difference is observed. The fluoride ion causes a completely different behavior. No significant deprotonation pH effect is observed where deprotonation occurs in the absence of fluoride. At even higher pH, massive consumption of hydroxide occurs. Some possible scenarios for the behavior of the fluoride-containing systems are discussed. 1