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The effect of the pore water composition on the diffusive anion transport was studied for two different argillaceous, low permeability sedimentary rocks, Opalinus Clay (OPA) and Helvetic Marl (HM). The samples were saturated with different solutions with varying molar concentration and different main cations in the solution: NaCl based pore solutions and CaCl 2 based pore solutions. The total porosity was measured by through-diffusion experiments with the neutral tracer HTO. Experiments performed in NaCl solutions resulted in a porosity of 0.12 for OPA and 0.03 for HM, and are consistent with results of the experiments in CaCl 2 solutions. The total porosity was independent of the molar concentration, in contrast to the measured anion porosity, which increased with increasing molar concentration. It could further be observed that the pore solution based on the bivalent cation calcium shielded the negative surface charge stronger than the monovalent cation sodium, resulting in a larger measureable anion-accessible porosity in the case of CaCl 2 solutions. The data was modelled based on an adapted Donnan approach of Birgersson and Karnland (2009). The model had to be adjusted with a permanent free, uncharged porosity, as well as with structural information on the permanent anion exclusion because of so-called bottleneck pores. Both parameters can only be evaluated from experiments. Nevertheless, taking these two adaptions into account, the effect of varying pore water compositions on the anion-accessible porosity of the investigated argillaceous rocks could be satisfactorily described. [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]

Isosaccharinate (ISA[sup -]), a degradation product of cellulose codisposed in low-level nuclear wastes, is expected to be one of the dominant complexing ligands for radionuclides, especially tetravalent actinides. This paper presents a comprehensive thermodynamic model for isosaccharinate reactions with Ca(II) and Np(IV). The model is valid for a wide range of pH values (∼2-14), ISA[sup -] concentrations (ranging up to 0.1 m), and ionic strengths (ranging up to 6.54 m), and is based on (1) NMR investigations of HISA(aq) (α-D-isosaccharinic acid) and ISL(aq) [dehydration product of HISA(aq)], and the solubility of Ca(ISA)[sub 2](c) as a function of pH and concentrations of Ca and ISA[sup -]; (2) NpO[sub 2](am) solubility in a wide range of pH values (∼2-14) and total ISA concentrations of 0.0016 and 0.008 m and at fixed pH values of approximately 5 and 12 with total ISA concentrations ranging from 0.0001 to 0.1 m; and (3) solvent extraction of Np-ISA solutions, containing fixed NaClO[sub 4] concentrations ranging from 0.103 to 6.54 m and at fixed pC[sub H+] values ranging from 1.5 to 1.9, with dibenzoylmethane. Pitzer's ion-interaction approach was used to interpret the data. The different aqueous species required to explain these data included HISA(aq), ISL(aq), Ca(ISA)[sup +], Np(OH)[sub 3] (ISA)(aq), Np(OH)[sub 3](ISA)[sup -, sub 2], Np(OH)[sub 4](ISA)[sup -], and Np(OH)[sub 4](ISA)[sup 2-, sub 2]. The values of equilibrium constants for reactions involving these species and determined from these data provided close agreements between the observed and predicted concentrations in all of the systems investigated in this study and those repotted previously. [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 problem of transient linear 1D diffusion with mass-transfer limitations (boundary layers) at the medium boundaries and variable diffusant concentrations in both perfectly stirred reservoirs is solved in terms of Fourier transforms. This novel solution is especially relevant to the interpretation of measurements of diffusion of adsorbing radio-nuclides in compacted clays where porous filters are routinely used to confine the swelling clay. An analytical expression is obtained for the break-through time with a due account for the mass-transfer limitations at the medium boundaries. An analysis is carried out of the influence of finite inlet volume on the time evolution of diffusant flux into the outlet reservoir. A simple “renormalization” procedure is put forward to make corrections for the decrease in the inlet concentration, which makes applicable the classical procedure of interpretation in terms of stationary flux and break-through time. It is shown that this “renormalization” procedure is fairly accurate in the case of relatively small absolute sorption capacities of medium (as compared to the inlet reservoir), but its accuracy deteriorates with an increase in this parameter. Several examples are considered for the application of these approaches to the interpretation of experimental data on the diffusion of traces of radioactive 22Na through compacted sodium montmorillonite. [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]