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Tc-99 has drawn widespread concern because of its long half-life, high fission yield and high mobility in research of radioactive waste disposal and environmental remediation. TcO 4 − through-diffusion experiments in Opalinus Clay (OPA) were performed under air and under argon No noticeable Tc-breakthrough was observed for over one year while the total Tc concentration in the source reservoir was steadily decreasing under both air and argon atmosphere. The total Tc activity distribution in the clay sample along the diffusion direction was obtained by slicing the OPA clay samples retrieved from the diffusion cells, using the abrasive peeling technique. In the case of diffusion under air atmosphere, almost no Tc was measured in that part of the sample close to source reservoir, while much more Tc was measured under argon atmosphere. A reasonable explanation for this observation is that the reductive retention of Tc plays a significant role during transport. A reactive transport model was constructed to simulate the diffusion process whereby the diffusion of Tc was coupled with redox reactions. Even though reduction of TcO 4 − by aqueous Fe2+ is thermodynamically feasible, it was not observed in the experiment. Furthermore, Fe2+ associated with solid phase was demonstrated to be more active than aqueous Fe2+. Therefore, surface complexation redox reaction was proposed. Dissolution rate of pyrite, equilibrium constant and diffusion coefficient of TcO 4 − were considered as possible factors controlling the redox reaction. Modeling results showed that TcO 4 − diffused into the clay and was partially reduced into surface complexed Tc(IV) by pyrite. When TcO 4 − transported under air atmosphere, O 2 competitively consumed Fe2+ and pyrite, resulting in no Tc immobilization in the related zone. [Display omitted] • 99Tc diffusive mobility in Opalinus clay was investigated. • No 99Tc was observed to diffuse through the 1 cm clay after more than one year. • A reactive transport model coupling diffusion processes and redox reactions with surface complexed Fe was proposed. [ABSTRACT FROM AUTHOR]

Through-diffusion experiments with 36 Cl − , 35 SO 4 2− and HTO in Opalinus Clay (OPA) samples from a deep borehole in North-East Switzerland (Benken; BE) have been performed. The effect of burial depth on the experimental results has been investigated. It could be shown that the effective diffusion coefficients decrease with sample depth for all three tracers. Moreover, there was a good correlation with the texture of the samples. The diffusion coefficients for HTO are the largest (D e  = 5.4–8.8 × 10 −12  m 2  s −1 ), followed by those for 36 Cl − (D e  = 0.7–1.9 × 10 −12  m 2  s −1 ), and finally 35 SO 4 2− (D e  = 0.2–0.6 × 10 −12  m 2  s −1 ). 36 Cl − was partially excluded from the total porosity resulting in an accessible porosity smaller than the total porosity (ε Cl  = 0.041–0.064). 35 SO 4 2− , on the other hand, showed interaction with OPA resulting in a capacity factor (α) larger than the total porosity (ε tot  = 0.13–0.16). Using extended Archie's law the accessible porosity for 35 SO 4 2− was estimated between 0.013 and 0.030. This enabled to evaluate the sorption coefficient of 35 SO 4 2− from the measured capacity factor, resulting in values of K d between 6 × 10 −5 and 9 × 10 −5  m 3  kg −1 . [ABSTRACT FROM AUTHOR]

Abstract: Diffusion is considered the principal transport mechanism of radio-nuclides and other low-molecular-weight pollutants in compacted clays used as barriers at various disposal and storage sites, for example, at projected deep repositories for radioactive waste. Porous filters are routinely used to confine swelling clays in diffusion studies of radio-tracers. The presence of the filter gives rise to considerable mass-transfer limitations at the clay boundary that result in erroneous diffusion parameters. We have solved the problem of in-diffusion with due account for this phenomenon by means of Fourier transforms. By using literature data on the in-diffusion of traces of radioactive cesium in an argillaceous rock (Opalinus clay) and a compacted bentonite (FEBEX bentonite), we have demonstrated that taking into account the mass-transfer limitations considerably improves the quality of the theoretical fit of the time evolution of radio-tracer concentration in the reservoir. Besides that, we have shown that ignoring the mass-transfer limitations leads to a noticeable underestimation of both the effective diffusion coefficient and the specific sorption capacity of the clay. [Copyright &y& Elsevier]

The safety assessment of deep geological disposal of nuclear waste requires a sound understanding of radionuclide retention in the repository host rocks. Here, we investigate the retention behaviour of the safety relevant radionuclide 226Ra in the heterogeneous sandy facies of the Opalinus Clay (OPA-SF) at the Mont Terri underground rock laboratory. Various rock samples were selected from a drill core (BAD-1) located in the lower sandy facies, which were representative for its textural and mineralogical heterogeneity. The samples were carefully characterized with respect to mineralogy and chemistry using X-Ray diffraction and electron microscopy. The amounts of quartz, carbonates and clay minerals in the samples ranged between 58 wt.% – 65 wt.%, 13 wt.% – 35 wt.% and 5 wt.% – 25 wt.%, respectively, underpinning the mineralogical heterogeneity. Rare (Ba,Sr)-sulphate precipitates were identified in the clay matrix by electron microscopy. The uptake behaviour of Ra was evaluated using batch sorption experiments. Corresponding to the mineralogical variability of the rock samples, the measured distribution ratios R d for the 226Ra retention varied between 20 L kg−1 and 260 L kg−1 after 120 days. The experimental results and thermodynamic modelling approaches suggest that various processes contribute to the uptake of 226Ra on different time scales: fast cation exchange and surface complexation reactions by clay minerals and the (slower) formation of solid solutions during recrystallisation of (Ba,Sr)-sulphates as well as retention by carbonate minerals; the latter mechanism still requiring further investigation. [ABSTRACT FROM AUTHOR]

Abstract: Diffusion in compacted clays is often studied in sandwich-like arrangements where the clay is confined by porous filter plates in order to control its swelling. In some clays (for example, Na-montmorillonite) equilibrated with dilute electrolyte solutions, the fluxes of cationic radiotracers can be quite high due to cation-exchange reactions. Accordingly, the diffusion resistance of clay layers can become comparable with or even smaller than the diffusion resistance of porous filters (such layers are called “thin” in this study). In view of the typical uncertainties (ca. 20%) of diffusion permeability of porous filters reported in the literature, the diffusion resistance of clay layers cannot be reliably determined from the steady-state diffusion permeability of the filter–clay–filter “sandwich” in this case. In this study, it is shown that, rather unexpectedly, information on the diffusion permeability of “thin” clay layers can be obtained from the time dependence of diffusant flux into the outlet compartment because at very short times, there is a characteristic flux delay that does not occur in the limiting case of infinitely large diffusion permeability of clay. The flux behavior at longer times is controlled by the diffusion permeability of the filters, which makes possible its determination directly from through-diffusion data and makes superfluous independent diffusion experiments with filters. This approach has been validated via theoretical interpretation of literature data on the diffusion of 22Na radiotracers through confined compacted montmorillonite equilibrated with 0.01M NaClO4 solution. The filter and clay properties estimated in this way are in good agreement with the literature data. [Copyright &y& Elsevier]

Abstract: The development of concentration (membrane) potential upon step-wise change in salt concentration has been studied for diaphragms made of various strongly compacted clays (montmorillonite, illite, kaolinite) equilibrated with 0.1 M NaCl solution. Porous ceramic filters were used to confine the clays mechanically to be able to achieve high extent of compaction (dry density ∼2000 kg/m3). A theoretical analysis has revealed that the relaxation pattern is primarily controlled by the properties of porous filters and only slightly depends on the clay properties. At the same time, quasi-stationary values of concentration potential are directly related to the electrochemical perm-selectivity of clay. This property has revealed considerable differences in the electrochemical behaviour of various clays used in this study. This has been attributed to the differences in the micro-structure of clays, in particular to the existence or nonexistence of the so-called interlayer water where cations may retain some mobility. It has also been shown that in clays with high electrochemical perm-selectivity, one can expect a strong increase in the diffusivity of cationic radio-tracers with decreasing ionic strength of equilibrium electrolyte solution. At the same time, low electrochemical perm-selectivity means no noticeable dependence of this kind. The correctness of this observation has been corroborated by the comparison of our findings with the literature data on the diffusion of cationic radio-tracers through compacted montmorillonite (high perm-selectivity) and kaolinite (low perm-selectivity). To check the self-consistency of our approach, we have also carried out sample measurements of diffusion of cationic and anionic radio-tracers through compacted illite. It has been found that the measured effective diffusion coefficients were in excellent agreement with the electrochemical perm-selectivity estimated for this clay from the measurements of concentration potential. [Copyright &y& Elsevier]

The suitability of swelling clays as a barrier to isolate nuclear waste in deep geological disposal is based on their microstructure, characterized by pore sizes down to the nanometre scale, and by their physico-chemical properties such as an excellent retention capacity for many radionuclides. A process-based understanding of the key features of radionuclide migration in such environments is required to demonstrate a valuable performance under all feasible conditions. In this study, the diffusion of tritiated water (HTO) and 22 Na through Na-montmorillonite was studied as a function of the bulk dry density, the concentration of the background electrolyte (0.1, 1 and 5 M NaClO 4 ) and the temperature (between 0 and 80 °C). We observed that the diffusion of neutral species, such as water, depended on temperature and the bulk dry density only, while the diffusive fluxes of 22 Na + were additionally influenced by the concentration of the background electrolyte. The experimental data were in agreement with both a single porosity and a parallel flux model demonstrating the validity of both approaches for rather low bulk dry densities and high concentrations of the background electrolyte. Some discrepancies between the model predictions and the experimental data were, however, noted. Assuming a reduced molecular mobility in the clay phase strongly reduced those discrepancies. Activation energies measured on samples with high bulk dry density were slightly higher than values measured for bulk water (17 kJ mol −1 ), those on low bulk dry density samples rather smaller. The activation energies of 22 Na + and HTO were similar indicating possibly a dynamical coupling of the diffusion of water and cations. [ABSTRACT FROM AUTHOR]

The pore size distribution of two natural argillaceous rock samples, Opalinus Clay (OPA) and Helvetic Marl (HM) was investigated with five different methods: NMR, NMR cryoporometry, mercury intrusion porosimetry and CO 2 adsorption, as well as N 2 adsorption. Due to different physical principles of these methods different ranges of pore width could be detected, from micropores (< 2 nm) to mesopores (2–50 nm) and macropores (> 50 nm). The aim was to shed light on the role of small pores on the transport properties of natural argillaceous rocks, in particular to explain the differences of anion diffusion in the two argillaceous rock samples. Knowing that Helvetic Marl exhibits a stronger anion exclusion than Opalinus Clay it was hypothesized that HM (with its smaller phyllosilicate and smectite content compared to OPA) has more interlayer equivalent (ILE) pores than OPA. ILE pores are defined as pores so narrow (< 0.5 nm) that diffuse double layers, formed at negatively charged surfaces, are overlapping. Accordingly, ILE pores behave similarly as interlayer pores and may block the anion diffusion. This study could not confirm the hypothesis that HM has more ILE pores. Similar pores size distributions were determined for both materials, even with a tendency of a larger fraction of small pores in OPA as compared to HM. However, all methods have limitations in the range of very small (nm) pores. [ABSTRACT FROM AUTHOR]

Abstract: Literature data for anion diffusion in compacted swelling clays contain systematic inconsistencies when the results of through-diffusion tests are compared with those of out-diffusion or tracer profile analysis. In the present work we investigated whether these inconsistencies can be explained by taking into account heterogeneities in the compacted samples; in particular increased porosities at the clay boundaries. Based on the combined results of out-diffusion, tracer profile analysis and the spatial distribution of the electrolyte anion in the clay, we conclude that the inconsistencies can indeed be resolved by taking into account a heterogeneous distribution of the total and the anion-accessible porosity. This, by definition, leads to a position dependence of the effective diffusion coefficient. Neglecting these effects results in a rather subordinate systematic error in the determination of effective diffusion coefficients of anions from through-diffusion tests with clay thicknesses in the centimetre range. However, stronger errors in terms of absolute values and conceptual interpretation may be introduced in out-diffusion tests and profile analyses of the diffused tracer. We recommend that anion diffusion tests should be accompanied by measurements of the total and anion-accessible porosity as a function of position in the direction of diffusion. [Copyright &y& Elsevier]

A mechanistic understanding of transport phenomena in compacted clays is essential for the use of such materials as engineered barrier systems for the safe geological disposal of radioactive wastes. The present contribution is a first step in the development of an integrative treatment of the properties of tracer cations in compacted bentonites with respect to diffusion and sorption. The diffusion of 22Na and 85Sr in highly compacted montmorillonite and kaolinite is investigated as a function of the "external salt concentration" (NaClO4), i.e., of the solution in equilibrium with the clay. Consistent results were obtained from through-diffusion experiments and tracer profile analysis. Knowledge of genuine diffusion coefficients of the filter plates turned out to be crucial in cases where the diffusive resistance of the filter plates was similar to that of the clay. Diffusion coefficients formally calculated on the basis of the tracer concentration gradient in the external aqueous phase, and the sorption distribution ratios were found to decrease with increasing external salt concentration in the case of montmorillonite. In a logarithmic representation of these data, a slope of -1 was obtained for the monovalent 22Na, whereas the slope was -2 for the divalent 85Sr. In the case of kaolinite, diffusion coefficients were independent of the external salt concentration. It is postulated that the diffusion of the tracer cation through the interlayer water is the dominant pathway in compacted swelling clays under the experimental conditions tested. Effective diffusion coefficients, based on a tracer concentration gradient in the interlayer water of the clay, were found to be independent of the composition of the external aqueous phase. The latter gradient is assumed to be a function of the external salt concentration, according to a calculated distribution of the tracer cation between free pore water and the interlayer water via cation exchange. [ABSTRACT FROM AUTHOR]

During the last two decades, the Mont Terri rock laboratory has hosted an extensive experimental research campaign focusing on improving our understanding of radionuclide transport within Opalinus Clay. The latest diffusion experiment, the Diffusion and Retention experiment B (DR-B) has been designed based on an entirely different concept compared to all predecessor experiments. With its novel experimental methodology, which uses in-situ X-ray fluorescence (XRF) to monitor the progress of an iodide plume within the Opalinus Clay, this experiment enables large-scale and long-term data acquisition and provides an alternative method for the validation of previously acquired radionuclide transport parameters. After briefly presenting conventional experimental methodologies used for field diffusion experiments and highlighting their limitations, this paper will focus on the pioneer experimental methodology developed for the DR-B experiment and give a preview of the results it has delivered thus far. Image 1 • Novel experimental set-up for the study of iodide ion diffusion in compacted clay. • In-situ downhole quantification of trace metal amounts. • Visualization of the impact of small-scale heterogeneities on transport properties. • Long-term and large-scale iodide diffusion data acquisition. • Merging of mineralogical, chemical and structural elements for data interpretation. [ABSTRACT FROM AUTHOR]