Through-diffusion Method Research Articles

Diffusion of 99-technetium in compacted bentonite under aerobic and anaerobic conditions

The main aim of this study was to investigate diffusion of technetium 99Tc under different conditions. Because technetium represents one of the most dangerous fission products due to its very long halftime and high mobility in aerobic conditions diffusion experiments of technetium (as 99TcO4− anion) in Czech bentonite from Rokle locality have been carried out. For performance and evaluation of experiments the through-diffusion method was chosen and apparent (Da) and effective (De) diffusion coefficients were evaluated. The effects of particle mesh-size, dry bulk density and aerobic or anaerobic conditions on diffusion were studied. In the presence of oxygen, technetium occurs in oxidation state VII, as an anion, soluble and mobile in the environment. However, under reducing conditions it occurs in a lower oxidation states, mainly as insoluble oxides or hydroxides. Aerobic experiments were carried out under laboratory conditions and anaerobic experiments were performed in a nitrogen atmosphere in a glove box, to simulate the real underground conditions.

A fast and sensitive method for evaluating nuclides migration characteristics in rock medium by using micro-channel reactor concept

Experimental effort to evaluate the barrier performance of geologic disposal requires relatively long testing periods and chemically stable conditions. We have developed a new technique, the micro mock-up method, to present a fast and sensitive method to measure both nuclide diffusivity and sorption coefficient within a day to overcome such disadvantage of the conventional method. In this method, a Teflon plate having a micro channel (10–200 μm depth, 2, 4 mm width) is placed just beneath the rock sample plate, radionuclide solution is injected into the channel with constant rate. The breakthrough curve is being measured until a steady state. The outlet flux in the steady state however does not meet the inlet flux because of the matrix diffusion into the rock body. This inlet–outlet difference is simply related to the effective diffusion coefficient ( D e) and the distribution coefficient ( K d) of rock sample. Then, we adopt a fitting procedure to speculate K d and D e values by comparing the observation to the theoretical curve of the two-dimensional diffusion–advection equation. In the present study, we measured D e of 3H by using both the micro mock-up method and the conventional through-diffusion method for comparison. The obtained values of D e by two different ways for granite sample (Inada area of Japan) were identical: 1.0 × 10 −11 and 9.0 × 10 −12 m 2/s but the testing period was much different: 10 h and 3 days, respectively. We also measured the breakthrough curve of 85Sr and the resulting K d and D e agreed well to the previous study obtained by the batch sorption experiments with crushed samples. The experimental evidence and the above advantages reveal that the micro mock-up method based on the microreactor concept is powerful and much advantageous when compared to the conventional method.

Through-electromigration: A new method of investigating pore connectivity and obtaining formation factors

The retardation of radionuclides and other contaminants in fractured crystalline rock is strongly associated with the diffusive properties of the rock matrix. At present, the scientific community is divided concerning the question of long-range pore connectivity in intrusive igneous rock. This paper presents a fast new method, called the through-electromigration method, of obtaining formation factors and investigating pore connectivity. The method involves the migration of an ionic tracer through a rock sample with an electrical potential gradient as the main driving force. The method is analogous to the through-diffusion method but the experimental time is reduced by orders of magnitude. This enables investigations of pore connectivity, as measurements can be made on longer samples. In a preliminary investigation, the new method is compared to the traditional through-diffusion method as well as to rock resistivity methods. The diffusive properties of nine granitic rock samples from Laxemar in Sweden, ranging from 15 to 121 mm in length, have been investigated and the results are compared.

Contribution of the diffusive exchange method to the characterization of pore-water in consolidated argillaceous rocks

Consolidated argillaceous rocks, potential host rocks for nuclear waste disposal, display particular properties (low water content, very small pore size) that require specific methods, especially for the characterization of their pore-water. The diffusive exchange method was applied to the claystone of Tournemire by taking into account the effect of swelling or micro-cracks induced by drilling. The good agreement of diffusion parameters with those obtained from the through-diffusion method showed that the extent of this disturbance was low. This method led to stable isotope content values in pore-water in good agreement with those measured in the fracture fluids located in the vicinity, validating the method. On the contrary, the vacuum distillation technique gave a systematic depletion in heavy isotopes of water, probably induced by an incomplete extraction of pore-water. For the chloride tracer, the determination of the accessible porosity on each sample allowed the estimate of chloride concentration of pore-water to be improved, by taking into account all the petrophysical heterogeneities.

A high-resolution abrasive method for determining diffusion profiles of sorbing radionuclides in dense argillaceous rocks

The diffusion of 134Cs + and 22Na + in Opalinus Clay (OPA) was studied by in-diffusion laboratory experiments. The diffusive tracer profiles in the rock were determined using a high-resolution abrasive peeling method. The radionuclide activities in the grinding swarf were measured directly via γ -spectrometry. By choosing the appropriate abrasive paper, a resolution down to 15 μm can be achieved. This is important when analysing strongly sorbing radionuclides such as tri- and tetravalent actinides that show steep, shallow diffusion profiles. In this study, a resolution between 20 and 90 μm was obtained which was sufficient for a good spatial resolution of the diffusion profiles. Both the effective diffusion coefficients and the distribution coefficients of the radionuclides could be determined by applying a single reservoir with decreasing source concentration analysis for a semi-infinite case. In the case of 22Na +, effective diffusion coefficients of D e = 2.0 × 1 0 - 11 m 2 s - 1 and D e = 1.5 × 1 0 - 11 m 2 s - 1 for Benken (Zürcher Weinland) OPA and Mont Terri OPA, respectively, were derived. The distribution coefficients were K d = 3.1 × 1 0 - 4 and 0.9×10 −4 m 3 kg −1, respectively. For 134Cs + the effective diffusion coefficients were higher, i.e. D e = 3.1 × 1 0 - 11 m 2 s - 1 for OPA from Benken and D e = 3.0 × 1 0 - 11 m 2 s - 1 for OPA from Mont Terri. The distribution coefficients determined were K d = 0.16 m 3 kg - 1 for Benken and 0.23 m 3 kg −1 for Mont Terri. Comparison of the data obtained for the weakly sorbing 22Na + with those from earlier through-diffusion experiments showed that there is good agreement between the two methods. In the case of 134Cs + such a comparison was not possible because through-diffusion data are not available. Because through-diffusion methods cannot be applied to strongly sorbing tracers in reasonable time periods, in-diffusion combined with high-resolution abrasive peeling offers an excellent alternative for measuring the diffusion properties of strongly sorbing tracers in dense argillaceous rocks.

Cesium sorption and diffusion in Bure mudrock samples

This work deals with cesium diffusion and sorption in rock samples from Bure (Meuse/Haute-Marne, France). Four different samples extracted from the Callovo–Oxfordian layer (between −423 m and −478 m) were investigated to quantify the Cs diffusion parameter range. HTO diffusion properties were also evaluated. The through-diffusion method was used in both cases. The steady-state was reached, allowing a direct access to the values of effective diffusion coefficients ( D e). Major attention was paid to experimental chemical conditions. Cs and HTO diffusion parameters were quite homogeneous for the different investigated rock samples. D e(Cs) was found about one order of magnitude higher than D e(HTO). Such observation was already reported in the literature mainly for compacted bentonite. Results presented here show this trend also exists in mudrocks. Batch tests were performed in order to characterize cesium sorption on ground Bure mudrock samples. The isotherms were fitted with a 2-site ion exchange model. This model was extrapolated to intact rock samples and coupled with diffusion equations, using the measured D e(Cs) values. This coupled model was tested against experimental data measured in the transient part of Cs diffusion experiments. A fairly good agreement between model and experimental data was found.

Study of sorption and diffusion processes in natural bentonites

Distribution coefficients (Kd) have been determined for 134Cs and 85Sr by the batch method. Various factors affecting the Kd value as water-to-bentonite ratio and composition of the aqueous phase have been evaluated and will be discussed. Effective (De) and apparent (Da) diffusion coefficients for 85Sr in the compacted bentonite have been determined using the through-diffusion method. Dry bulk density and porosity of the dry bentonite and the water content in the saturated bentonite have been measured as well. Distribution coefficients calculated from the diffusion data using the dry bulk density and porosity values for given conditions have been compared and discussed with those determined by the batch method.

Diffusivity of U, Pu and Am carbonate complexes in a granite from Inada, Ibaraki, Japan studied by through diffusion

The diffusivities of uranium, plutonium and americium in Inada granite have been determined using through-diffusion method. Experiments were performed at (25±1)°C in a 0.1 mol l −1 NaHCO 3 solution where the actinides are present as carbonate or hydroxy–carbonate complexes. Effective diffusivity ( D e) values of (1.42±0.24)×10 −13 m 2 s −1 and (5.1±2.0)×10 −14 m 2 s −1 were obtained for uranium and plutonium, respectively. Diffusion through 5 mm thick granite was not observed for americium within the experimental period of 366 days. The D e value for uranyl carbonate species obtained in these experiments was found to be four times higher than the previously obtained D e value for the uncomplexed uranyl ion.

Experimental and modelling studies on diffusion of Cs, Ni and Sm in granodiorite, basalt and mudstone

Through-diffusion experiments were carried out for Cs, Ni and Sm which can have a valence of I, II and III, respectively, through granodiorite, basalt and mudstone which have been considered candidates as natural barriers at 25°C under ambient conditions. The tracer solution was prepared as a mixture of Cs, Ni and Sm. The experiments were continued for a maximum of 596 days. During the experiments, the pH was monitored. The tortuosities of the rocks were measured using a NaCl tracer and a through-diffusion method. The porosities, pore size distributions, specific surface area and dry densities of the rocks were measured by a water saturation method and a mercury porosimetry. Effective and apparent diffusion coefficients ( ϵDp, Da) were obtained for each element. The ϵDp and Da values ranged from 0.57 to 1.4 × 10 −12 and 1.0 × 10 −11 m 2 s −1 for granodiorite, respectively. Those for mudstone ranged from 0.53 to 4.8 × 10 −13 and from 2.6 to 3.9 × 10 −13 m 2 s −1, respectively, and those for basalt ranged from 0.28 to 1.5 × 10 −13 and from 1.6 to 2.1 × 10 −13 m 2 s −1, respectively. The ϵDp and Da values of Cs were the highest of the three elements for all rocks. Both diffusion coefficients for all elements were in the order: granodiorite > mudstone > basalt. The pore size was found to be relatively large for each rock compared with the ionic radius. The ϵDp values were predicted, based on the formation factors by taking into account the porosity and tortuosity. The predicted values were in relatively good agreement with the measured values with deviations of less than about five times.

Diffusive transport through compacted Na- and Ca-bentonite

The effect of exchangeable cation — Na + and Ca 2+ — on the diffusive transport of I −, Sr 2+ and 3H (as HTO) in compacted bentonite was examined using a through-diffusion method. Total intrinsic diffusion coefficients, D i, were determined from the steady-state flux of the diffusants through the clays, and apparent diffusion coefficients, D a, were obtained from the time lag technique. The clays were compacted to a dry bulk density of 1.3 Mg/m 3, and Na-bentonite was saturated with a solution of 100 mol NaCl/m3 and Ca-bentonite with one of 50 mol CaCl 2/m 3. The D i values for all diffusants are 2 to 6 times higher in the Ca- than Na-clay. We attribute this to the larger quasicrystal, or particle, size of Ca- compared to Na-bentonite. Hence, Ca-bentonite has a greater proportion of relatively large pores; this was confirmed by Hg intrusion porosimetry. This means the diffusion pathways in Ca-bentonite are less tortuous than those in Na-bentonite. Moreover, in some cases the effective porosity, or the porosity available for diffusive transport, may be greater in Ca-bentonite. The D a values are inversely proportional to the distribution coefficients of the diffusants with the clays.

Facilitated Transport of Charged Colloids in Geologic Media

AbstractDiffusion of a charged colloidal particle in a two-dimensional simple shear flow was studied by means of Monte Carlo calculations and the effects of a bounding wall and charge of a particle on convective diffusion were elucidated. Taking charge effects into account has a marked effect on the diffusion behavior of the particle, increasing the migration distance. Diffusion of latex colloidal particles in a quartz-powder packed cell was also studied by through-diffusion methods. For the large latex colloidal particles, the effective diffusion coefficients measured are larger than those estimated by Stokes-Einstein equation.

The measurement of apparent diffusion coefficients in compacted clays: an assessment of methods

Diffusion coefficients are critical parameters for predicting migration rates and fluxes of contaminants through dense clay-based barrier materials used in many waste containment strategies. A number of methods have been developed to measure diffusion coefficients in porous media. Here we compare three methods, involving both in-diffusion (ID) and through-diffusion (TD) techniques, for measuring the apparent diffusion coefficient, D, of I − and Cs + in dense saturated bentonite. The clay was compacted to a dry density of about 1.3 Mg/m 3 and saturated with a Na-Ca-Cl-dominated solution having an ionic strength of 220 mol/m 3. The D values obtained are designated D p (from the concentration profile developed in ID studies), D g (from the concentration gradient measured in the TD method), and D t (from the “time-lag” technique in TD experiments). For I −, the mean D p value was 200 μm 2/s, D g was 300 μm 2/s and D t was 130 μm 2/s. The corresponding values for Cs + were 1.2 μm 2/s, 0.54 μm 2/s and 1.9 μm 2/s. The D values are lower for Cs + than for I − mainly because Cs + is sorbed to a greater extent on the clay. The D values obtained by the three methods for both diffusants are in reasonable agreement, and we conclude that any of the methods examined here can be used reliably to obtain D in compacted bentonite.

Consideration on Effective Diffusivity of Strontium in Granite

Abstract Diffusive behavior of strontium and certain kinds of divalent cations in Inada granite were studied by a through-diffusion method. In order to examine the effect of sorption onto overall diffusive behavior, two kinds of solutions were used: 0.1M KCl solution and deionized water. The effective diffusion coefficient (De ) and rock capacity factor (a) were (2.0–3.6) xl0–13 m2/s and less than 0.022 in 0.1M KCl solution and (0.32–1.7) x 10 −13m2/s and 1.5–2.4 in deionized water respectively. The De , and α in deionized water were much larger than those in 0.1M KCl solution. These results are well explained by taking into account the diffusion of sorbed ion or the surface diffusion. In support of this mechanism, most De , values of Sr reported for various rocks are found proportional to the sorptivity ( ρRd )-In the case that the sorptivity is low, De of Sr depends on porosity like that of nonsorbed iodide. The effective diffusion coefficient of Sr in rocks was well explained by taking into account pore and surface diffusion and was expressed as De=2.1 xl0−10 ϵ 1.3+3.5xl0″−12 ρRd . The effective diffusion coefficient of divalent cations in the granite was found proportional to their diffusion coefficients in bulk solution.

Consideration on Effective Diffusivity of Strontium in Granite.

Abstract Diffusive behavior of strontium and certain kinds of divalent cations in Inada granite were studied by a through-diffusion method. In order to examine the effect of sorption onto overall diffusive behavior, two kinds of solutions were used: 0.1M KCl solution and deionized water. The effective diffusion coefficient (De ) and rock capacity factor (a) were (2.0–3.6) xl0–13 m2/s and less than 0.022 in 0.1M KCl solution and (0.32–1.7) x 10 −13m2/s and 1.5–2.4 in deionized water respectively. The De , and α in deionized water were much larger than those in 0.1M KCl solution. These results are well explained by taking into account the diffusion of sorbed ion or the surface diffusion. In support of this mechanism, most De , values of Sr reported for various rocks are found proportional to the sorptivity ( ρRd )-In the case that the sorptivity is low, De of Sr depends on porosity like that of nonsorbed iodide. The effective diffusion coefficient of Sr in rocks was well explained by taking into account por...

Radionuclide Migration Studies on Tonalite

ABSTRACTMigration of water, chloride, sodium, and calcium in tonalite was studied, using dynamic column and static through-diffusion methods. Autoradiography of rocks impregnated with 14C-methylmethacrylate was introduced in order to determine the spatial porosity distribution, as well as to identify and visualize the migration pathways of non-sorbing radionuclides in tonalite matrix at the mm-cm scale. The migration routes of sorbing radionuclides and the sorptive minerals in tonalite were determined by autoradiographic methods, using 45Ca as a tracer. Transport of radionuclides was interpreted, using models for hydrodynamic dispersion with diffusion into the rock matrix. In tonalite, porous minerals were distributed homogeneously in matrix and, therefore, retardation capacity of the rock matrix was found to be high.