Bentonite colloid diffusion through the host rock of a deep geological repository
Introduction
Compacted bentonite is the main engineering barrier considered in a high level radioactive waste repository (HLWR) in granite. Bentonite is able to retard the migration of radionuclides (RN) to the geosphere because it has very low permeability, limits the water income to the waste, and presents high sorption capability for many radionuclides. Nevertheless, for a long-term performance assessment of a HLWR, it is necessary to evaluate all the possible processes occurring at the bentonite barrier that may affect the RN transport. In particular, this work focuses on the possible contribution of bentonite colloids generated at the host rock/bentonite interface of the repository. Colloids are particles with diameters ranging from 1 nm to 1 μm that have high surface area that may affect contaminant transport in the subsurface (Abdel-Salam and Chrysikopoulos, 1995a, Abdel-Salam and Chrysikopoulos, 1995b, Tatalovich et al., 2000, James et al., 2005). Because of their high sorption capability colloid-borne migration can be particularly relevant for low solubility radionuclides, that otherwise could be immobilized in the rock surface (Swanton, 1995).
Prior to a study of the colloid transport mechanisms, the stability and mobility of colloids in a given environment are the first issues to be evaluated. In addition, the RN sorption irreversibility must be proved in order to assess the relevance of colloid-mediated radionuclide transport (Miller et al., 1994).
Recent studies have demonstrated that, in favorable conditions, colloids could be detached at this interface and that in low ionic strength and alkaline waters could be stable and mobile in the environment (Missana et al., 2003).
The transport of colloids in fractured rocks is expected to be controlled by advection and dispersion in water if a fracture network exists, and it is generally tackled by a theoretical point of view (Ibaraki and Sudicky, 1995, James and Chrysikopoulos, 1999, Skagius, 1992). The theoretical description of colloid transport in a fractured medium usually includes surface retention in fracture walls and matrix diffusion, as possible mechanisms that eliminate colloids from the aqueous phase. All phenomena that eliminate colloid from the aqueous phase are usually included in the term filtration. The scarce experimental colloid transport studies, performed by batch or column laboratory experiments (Ryan and Elimelech, 1996, Smith and Degueldre, 1993, Keller et al., 2004, Anders and Chrysikopoulos, 2005) or also in situ in a granitic massif (Möri et al., 2003), emphasized the need of identifying and quantifying the colloid filtration mechanisms to predict the colloid-mediated radionuclide transport, pointing out the strong lack of experimental data. Great uncertainties still exist on the actual contribution of these mechanisms (Ryan and Elimelech, 1996). In that sense, obtaining experimental parameters, such as colloid diffusion coefficients, for inclusion in theoretical transport codes can overload the main filtration mechanisms. One of these parameters is precisely the diffusion coefficient of colloid in the rock matrix.
The aim of the present work was, thus, to assess whether bentonite colloids are able to diffuse within the granite. A methodology based on the ion nuclear beam technique Rutherford backscattering spectrometry (RBS) was selected to perform these colloid diffusion studies. The RBS is a technique widely applied in material science to study diffusion processes at the micrometer scale. RBS is not usually applied to study geological materials because of their high heterogeneity, but some applications in earth sciences are reported in the literature (Dran et al., 1988, Ryan, 2004, Toulhoat et al., 1996, Trocellier et al., 1999). In previous works an adequate characterization of granite was carried out to study diffusion processes within the rock (Alonso et al., 2003a, Alonso et al., 2003b).
One should note that in order to determine bentonite colloid diffusion within the granite by the RBS, tracing the colloids was necessary. The clay and the granite matrix are, in fact, mainly composed of light elements. Europium was examined as a possible tracer because of being a heavy element, thus imparting RBS sensitivity to the system. Complementary batch sorption experiments of Eu on bentonite colloids were required previous to diffusion studies. Eu presented almost 100% sorption onto the clay colloids and practically no desorption in the presence of granite was observed within the time scales of diffusion experiments. Thus Eu was considered a suitable tracer for these experiments.
Then RBS spectra performed on granite samples, with Eu as a solute and adsorbed onto bentonite colloids, were compared. Undoubtedly, Eu diffusion profiles were detected. The demonstration of Eu as a suitable tracer of bentonite colloids enables one to guarantee that the determined Eu diffusion coefficients can be taken as an estimation of a colloid diffusion coefficient within a crystalline rock. This is the first time that such a parameter was experimentally determined.
Section snippets
Materials
A spanish granite was selected for these studies. It is granodiorite type and its mineral composition is basically quartz (33–35%), plagioclase (29–32%), K-feldspars (26–28%), muscovite (5–6%) and biotite (2–3%). Additionally it presents corderite, andalucite, illmenite and zircon (Buil, 2002).
The clay used to prepare the bentonite colloids is a Spanish clay (FEBEX bentonite), basically comprised of smectite (93 ± 2%), with small percentages of quartz (2 ± 1%), plagioclase (3 ± 1%), cristobalite (2 ±
Sorption studies
Fig. 2 shows the logarithm of distribution coefficient (in ml/g) of the Eu adsorbed onto the bentonite colloids as a function of time ([Eu] = 4.6 × 10−6 M, pH 5 and ionic strength 10−3 M). It can be appreciated that, within experimental error, after only 5 min the log (Kd) value measured was 6.0 ± 0.4 ml/g, which is equivalent to a percentage of Eu adsorbed onto the colloids up to 98 ± 2% as can be seen in the inset of Fig. 2. The Eu adsorption on the tube surfaces, under these conditions, was accounted for
Conclusions
The diffusion of bentonite colloids within granite was experimentally studied by means of a novel approach applying the RBS technique, and compared with the diffusion of Eu as solute. The methodology carried out in this work allowed the observation of Eu profiles within the granite in both cases that presented time dependence, in agreement with a diffusion process. The diffusion coefficient calculated for the Eu as solute (10−14 m2/s) is in the same range as those reported in the literature,
Acknowledgements
This work was partially supported by the EU within the FUNMIG (Fundamental Processes of Radionuclide Migration) project (Ref: FP6-516514), by the INFN-LNL within the MIRACOL (Migration of Radionuclides and Colloids) project (USP Ref. 0003446/04) and in the framework of the CIEMAT-ENRESA association. The authors are grateful to Prof. Stucki and D. Ghosh for their helpful revisions that significantly improved the final manuscript.
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