Compacted Montmorillonite Research Articles

Diffusion of acetic acid, butyric acid, and butanol in compacted montmorillonite

Diffusion of acetic acid, butyric acid, and butanol in compacted montmorillonite

Anion Diffusion in Compacted Clays by Pore‐Scale Simulation and Experiments

AbstractAccurate understanding of diffusion of anionic radionuclides in different clays is significant to predict long‐term performance of high‐level radioactive waste (HLW) repositories. The importance of electrical double layer (EDL) on anionic tracer diffusion in different clays has been studied by both through‐diffusion experiments and pore‐scale simulations in this work. The through‐diffusion experiments measured the effective diffusion coefficient and the accessible porosity of Re (VII) in compacted montmorillonite, Na‐bentonite, and illite/smectite mixed layer (I/S) at different salinities. The results showed that the accessible porosity and the effective diffusion coefficient of Re (VII) in montmorillonite and Na‐bentonite were similar but lower than those in I/S under the same conditions. For mechanism analysis and predictions, a pore‐scale modeling was implemented to simulate the diffusion of anions in compacted clays. In the simulations, the characteristics (porosity, density, and total surface area) of microstructures of montmorillonite and I/S were used to regenerate three‐dimensional pore structures numerically by a Quartet Structure Generation Set method. The Re (VII) diffusion was then simulated by directly solving coupled Poisson‐Nernst‐Planck equations via the lattice Boltzmann method. The diminished effect of EDL was therefore calculated and compared with Donnan and multiporosity models. As EDLs overlap in compacted clays, the Donnan model overestimates the influence of EDL on Re (VII) diffusion, while the multiporosity model underestimates it. The pore‐scale modeling, which captures the structure of overlapping EDLs automatically, can simulate the diffusion of anionic radionuclides in compacted clays without any fitting parameters.

Electrokinetic control of the growth of gypsum aggregates in compacted montmorillonite

Mineral precipitation and its impact on the barrier functions of clayey materials have attracted attention in the geological disposal of radioactive wastes. In this study, the growth of gypsum aggregates in compacted montmorillonite was investigated using the electrokinetic method. The results demonstrated that the morphology of aggregates can be partially controlled by adjusting the initial concentration of Ca in Na/Ca mixed montmorillonite. In particular, a lower initial Ca concentration maintained for long periods of time engendered the growth of gypsum as a layer of large aggregates up to 700 μm. The layer of gypsum aggregates was made for a short duration (1 day) without excess salt remaining around the layer. The short duration and the absence of excess salt represent the main advantage of the electrokinetic method compared to a counter diffusion method to form artificial precipitates in clayey materials. Techniques to enhance and control aggregates are useful to prepare altered bentonite, which can be used to evaluate the impact of precipitates and their morphologies on the diffusivity and permeability of compacted bentonite.

Adsorption of humid air in compacted montmorillonite: A Monte Carlo simulation study

Compacted montmorillonite (the most significant constituent of bentonite) was employed as a proxy for bentonite in atomistic Monte Carlo simulations in the grand canonical ensemble. Air, described in this study as a mixture of nitrogen, oxygen, argon, and carbon dioxide, was considered near ambient conditions, and with absolute humidity (i.e. total water content) varying from 0% up to 5% on a molar basis. The adsorption of each species constituting the mixture was computed under these conditions and the selectivity relative to nitrogen was considered. The species of particular interest in this study were water and oxygen, due to their role in the spoilage of foodstuffs. The results of the simulations suggest that molecular shape plays about as much of a role in sorption as dipole moment, with argon and oxygen displaying favourable uptake in bentonite as compared to all other species in the system. Water was somewhat more likely to be adsorbed in bentonite than nitrogen, although the disparity was less marked than for oxygen. The results suggest that bentonite may selectively separate oxygen from noble gases, which can have an impact on its use in packaging materials.

Gypsum precipitation enhanced by electrokinetic method and porewater chemistry in compacted montmorillonite

Abstract An understanding of chemical reactions in bentonite is important in completing a reliable safety assessment of the geological disposal of radioactive waste. In the present study, porewater chemistry and calcium reactivity in a compacted Na/Ca-mixed montmorillonite is explored through a gypsum precipitation reaction enhanced using an electrokinetic method. Results showed that CaSO4 was sparsely precipitated as gypsum in up to 300-μm aggregates, and grew into the montmorillonite texture. A threshold ionic equivalent fraction of calcium, XCa, of the Na/Ca-mixed montmorillonite in the gypsum precipitation was experimentally found to be within a range between XCa = 0.1 and XCa = 0.25 under a dry density of 1.0 kg/dm3 saturated with 0.5 M Na2SO4 solution. The saturation indices of gypsum for specimens following the precipitation experiments were evaluated based on a model including anion exclusion and cation exchange (Gains-Thomas selectivity coefficients, KGT) between the montmorillonite and interparticle solution using the PHREEQC code. Gypsum precipitation regions obtained from the precipitation experiments are in good agreement with regions in which the saturation indices are greater than zero when assuming KGT = 200, which is much greater than the KGT = 4.9 reported from the batch experiment. The greater KGT value implied in this study indicates that the thermodynamic properties of confined solutions are different from those of diluted or bulk solutions. The reason for the KGT deviation remains unresolved; however, the precipitation tendency of gypsum may be explained by a model including anion exclusion and cation exchange between the montmorillonite and the interparticle solution when an adequate KGT value is used in the calculation.

Effects of OH− activity and temperature on the dissolution rate of compacted montmorillonite under highly alkaline conditions

AbstractThe highly alkaline environment induced by cementitious materials in a deep geological disposal system of high-level radioactive waste is likely to alter montmorillonite, the main constituent of bentonite buffer materials. Over long time periods, the alteration may cause the physical and/or chemical barrier functions of the buffer materials to deteriorate. In order to evaluate the long-term alteration behaviour, the dissolution rate, RA (kgm−3 s−1), of compacted pure montmorillonite (Kunipia-F) was investigated experimentally under conditions of hydroxide ion concentration of 0.10—1.0 mol dm−3 at temperatures of 50—90°C. The dissolution rate data, including those from a previous study at 130°C, were formulated as a function of the activity of hydroxide ions, aOH− (mol dm−3), and temperature, T (K), and expressed as RA = 104.5·(aOH−)1.3·e−55000/RT by multiple regression analysis, where R is the gas constant. The dissolution rate of montmorillonite was greater in the compacted montmorillonite than in the compacted sand-bentonite mixtures. The difference can be explained by considering the decrease in aOH− in the mixtures accompanied by dissolution of accessory minerals such as quartz and chalcedony. The dissolution rate model developed for pure montmorillonite is expected to be applied to bentonite mixtures if quantification of the decrease in aOH− is achieved somehow.

Diffusion and retention behaviour of Cs in illite-added compacted montmorillonite

AbstractCompacted bentonite is to be used as a component of an engineered barrier system to retard the migration of radionuclides in the geological disposal of radioactive waste. In such an environment, montmorillonite in compacted bentonite might be altered to illite due to the hydrothermal reactions caused by the decay heat of radionuclides. In the present study, the diffusion and retention behaviour of Cs in compacted montmorillonite containing illitewas investigated using through-diffusion experiments. The experimental results showed that the flux of Cs attributed to the surface diffusion was independent of the sorption of Cs on illite, indicating that the Cs sorbed on illite was immobile or considerably less mobile than the Cs sorbed on montmorillonite. Consequently, the illite content in compacted bentonite is expected to enhance the sorption capacity of Cs without causing surface diffusion.

Rheological properties of clay material at the solid/solution interface formed under quasi-free swelling conditions

One scenario of interest for the long-term safety assessment of a spent nuclear fuel repository involves the loss of bentonite barrier material through contact with dilute groundwater at a transmissive fracture interface. Small-scale, flow-through, artificial fracture experiments demonstrated that the quasi-free swelling of compacted montmorillonite into thin, horizontal fractures in the presence of dilute aqueous solutions leads to the formation of an inner, rigid gel-like phase and an outer, sol phase, with a distinct interface separating these two phases. The mechanical strength of the solid phase at this interface was assessed through a set of rheological measurements performed at various salt and montmorillonite solids concentrations. “Sol/gel” phase transformations were identified in suspensions containing between 2 and 4vol.% solids content with electrolyte concentrations below 17mM NaCl. At 4vol.% solids content all of the examined suspensions displayed gel-like properties. The results and analysis further revealed that it can be considered highly unlikely that mechanical forces due to groundwater flow in natural rock fractures would be sufficiently strong to remove any clay material at the solid/solution interface.

Diffusion and sorption of Cs+, Na+, I− and HTO in compacted sodium montmorillonite as a function of porewater salinity: Integrated sorption and diffusion model

Montmorillonite clay is an important component of barrier materials such as bentonites and argillaceous rocks for the safe geological disposal of radioactive waste. The diffusion and sorption behaviors of cationic Cs+ and Na+, anionic I− and neutral tritiated water (HTO) tracers in compacted montmorillonite were investigated as a function of porewater salinity by experimental and modeling approaches. The effective diffusivities of these tracers were measured by the through-diffusion method coupled with multiple curve analysis including depth profiles, and typical salinity-dependent cation excess and anion exclusion effects were shown. The distribution coefficients for Cs+ and Na+ obtained by through-diffusion tests were sensitive to salinity because of competitive ion exchange and were in good agreement with those obtained by batch sorption tests. However, trends in the apparent diffusivities of Cs+ and Na+ were mostly independent of salinity.A diffusion model based on a simplified pore structure and electrical double layer theory, describing the change of ionic concentration and viscoelectric effects caused by the electrostatic interaction with negatively charged clay surfaces, could quantitatively account for the cation excess and anion exclusion behaviors and their salinity dependence. The trends in the distribution coefficients predicted by the ion exchange sorption model show good agreement with the measured values as a function of salinity. Salinity-independent apparent diffusivities could be interpreted as results of the coupled effects of the drastic dependence of diffusion and sorption on salinity. The difference in the diffusion and sorption behaviors between Cs+ and Na+ can be explained by the difference in the sorption mechanism (inner- and outer-sphere) and their representation in the diffusion model (cation excess diffusion). This integrated sorption and diffusion model was further demonstrated as being applicable to the diffusion data of these monovalent cations and anions in compacted bentonites over a wide range of compaction.

Dissolution of compacted montmorillonite at hyperalkaline pH and 70°C: in situ VSI and ex situ AFM measurements

AbstractIn situ measurements were carried out to quantify montmorillonite dissolution rates at a compaction pressure ranging from 0.04 to 10.00 MPa and temperature of 70°C in 0.3 M NaOH solution (pH 12.1 at 70°C) using vertical scanning interferometry (VSI) and an auto-compaction cell. Ex situ measurements of the reacted samples using atomic force microscopy (AFM) were performed to quantify the ratio of edge surface area (ESA) to total surface area (TSA) (XESA = ESA/TSA). Accordingly, the actual ESA for the montmorillonite examined by in situ VSI could be estimated. The XESA value increases as a function of run duration or compaction pressure. At atmospheric pressure, XESA is approximately 0.0054 and converges to ∼0.0107 at 10 MPa, An expression that relates reactive surface area and montmorillonite compaction (XESA/XESA initial = kXESA, k: variable factor) is kXESA = 1.0 + 0.64628 P0.1527 where P is in MPa. Using the calculated XESA, dissolution rates from the in situ VSI measurements are obtained. The early dissolution (<1500 min) at less compaction pressure tends to show faster rates (>1.0 × 10-11 mol/m2/s) than that at higher compaction pressure. The rates after >1500 min are slower, with values of less than 3×6 10-12 mol/m2/s, but there is no significant dependency on the density in the range from 1.0 to 1.7 Mg/m3. These observed rates for compacted montmorillonite are two-orders of magnitude slower (2.63×10-13 mol/m2/s) than dissolution rates in the suspended state.

Buffering of pH conditions in sodium bentonite

ABSTRACTA bentonite buffer is a part of the engineering barrier system of the geological disposal concept for spent nuclear fuel in Finland. The chemical conditions in the bentonite porewater determine the solubility, speciation and diffusion/sorption behaviour of the radionuclides in the bentonite. The OH-groups on the edge sites of the montmorillonite, the main component of bentonite, can buffer the pH conditions especially in the case that the buffering capacity of the accessory minerals is small. In this work, the pH conditions created by the interaction of sodium montmorillonite and an acetate buffer solution were studied experimentally in batch experiments and in compacted sodium montmorillonite. In the experiment with the compacted sample, the ends of an 18.4 mm long cylinder were exposed to an external solution of 0.3 M NaCl and 0.1 M acetate adjusted to pH 5 with NaOH. The effects were monitored by measuring the pH in the montmorillonite sample at 5 mm and 9.2 mm from the solution-montmorillonite interface as a function of time. The batch experiment was modelled considering the surface reactions of montmorillonite and the dissolution reactions of calcite and the added constituents to explain the observed phenomena.

Donnan equilibrium in Na-montmorillonite from a molecular dynamics perspective

We present molecular dynamics simulations of the Donnan equilibrium principle in compacted montmorillonite with three hydration layers of water in the interlayer. This draws attention to the misconception, frequently seen in the literature, stating that anions cannot enter interlayer space due to electrostatic repulsion forces, sometimes referred to as anion exclusion. However, the calculations presented here show that excess salt, i.e., both anions and cations enter interlayer space to the extent predicted by Donnan equilibrium. Thus the excess salt concentration is reduced in the interlayer in comparison to the external electrolyte but not totally excluded.

Ca/Na selectivity coefficients from the Poisson–Boltzmann theory

As a model for ion equilibrium in montmorillonite, the Poisson–Boltzmann (PB) equation was solved for two parallel charged surfaces in contact with an external NaCl/CaCl 2 mixed solution. The ion concentration profiles in the montmorillonite interlayer were obtained from the PB equation and integration of those gave the occupancy of Na + and Ca 2+ in the clay. That information together with the composition of the external electrolyte were then used for the calculation of the Gaines–Thomas selectivity coefficient K GT . The predictions from the model were compared to experimental data from batch as well as compacted conditions, and the agreement was generally good. With a surface layer-charge density of one unit charge per 145 Å 2, which is close to the value for Wyoming-type montmorillonite, the calculated selectivity coefficients were found to vary from about 4 in batch to 8 in compacted montmorillonite with dry density ∼1700 kg/m 3. From the point of view of assessing the evolution, with regard to sodium–calcium ion exchange, of the bentonite buffer in a repository for spent nuclear fuel, these results justify the use of data obtained in batch experiments.

Consistent interpretation of the results of through-, out-diffusion and tracer profile analysis for trace anion diffusion in compacted montmorillonite

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.

A Study on Hydraulic Properties of Compacted Fe(III)-montmorillonite

Hydraulic conductivities were determined for compacted Fe(III)-montmorillonite sample. The Fe(III)-montmorillonite sample used in this study, which was prepared by the ion-exchange treatment of Namontmorillonite in FeCl3 solution, contains less than 20% of Na+ ions as exchangeable cations and negligibly small amounts of iron precipitates. The measured hydraulic conductivities of compacted Fe(III)-montmorillonite were determined to be the order of 10−9, 10−11, and 10−13 ms−1 at dry densities of 0.80, 1.01, and 1.20 Mgm−3, respectively. These values were found to be kept constant during the experimental period, suggesting no significant change in the physicochemical properties of Fe(III)-montmorillonite during the experiment. When compared with Na-montmorillonite, remarkably high values of hydraulic conductivities were found for Fe(III)-montmorillonite at the dry densities of 0.8 and 1.0Mgm−3. On the other hand, almost the same value of hydraulic conductivity was obtained at the dry density around 1.2Mgm−3. This different effect of exchangeable cations on the hydraulic conductivity of montmorillonite could be attributed to the different sizes of macropores in compacted montmorillonite and/or the different thicknesses of electrical double layers formed over montmorillonite sheets.

Diffusion and sorption of neptunium(V) in compacted montmorillonite: effects of carbonate and salinity

AbstractDiffusion and sorption of radionuclides in compacted bentonite/montmorillonite are key processes in the safe geological disposal of radioactive waste. In this study, the effects of carbonate and salinity on neptunium(V) diffusion and sorption in compacted sodium montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (De) and distribution coefficients (Kd) of237Np(V) in sodium montmorillonite compacted to a dry density of 800 kg m−3were measured under four chemical conditions with different salinities (0.05/0.5 M NaCl) and carbonate concentrations (0/0.01 M NaHCO3).Devalues for carbonate-free conditions were of the order of 10−10–10−11 m2s−1and decreased as salinity increased, and those for carbonate conditions were of the order of 10−11–10−12 m2s−1and showed the opposite dependence. Diffusion-derivedKdvalues for carbonate-free conditions were higher by one order of magnitude than those for carbonate conditions. Diffusion and sorption behaviors were interpreted based on mechanistic models by coupling thermodynamic aqueous speciation, thermodynamic sorption model (TSM) based on ion exchange, and surface complexation reactions, and a diffusion model based on electrical double layer (EDL) theory in homogeneous narrow pores. The model predicted the experimentally observed tendency ofDeandKdqualitatively, as a result of the following mechanisms; 1) the dominant aqueous species are NpO2+and NpO2CO3−for carbonate-free and carbonate conditions, respectively, 2) the effects of cation excess and anion exclusion result in opposite tendencies ofDefor salinity, 3) higher carbonate in solution inhibits sorption due to the formation of carbonate complexes.

Determination of diffusion and sorption parameters of thin confined clay layers by direct fitting of through-diffusion flux

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 0.01 M NaClO 4 solution. The filter and clay properties estimated in this way are in good agreement with the literature data.

Diffusion of Cesium and Iodine in Compacted Sodium Montmorillonite Under Different Saline Conditions

Diffusion and sorption of cesium (Cs) and iodine (I) were investigated in a purified and moderately compacted sodium montmorillonite (dry density of 800 kg m-3) saturated with 0.01, 0.1 and 0.5M NaCl solutions. The effective diffusivity (De) and capacity factor (α) for Cs and I were measured by through-diffusion experiments, coupled with multiple curve analyses, including tracer depletion, breakthrough and depth concentration curves, which could be fitted with a conventional diffusion model using only one set of parameters. The De values obtained for Cs were of the order of 10-9-10-10 m2 s-1 and decreased as salinity increased, and those for I were of the order of 10-11-10-12 m2 s-1 and showed the opposite dependency. The distribution coefficient (Kd) of Cs decreased from the order of 100 to 10-2 m3 kg-1 as salinity increased. Diffusion and sorption parameters for Cs were also obtained by in-diffusion and batch sorption experiments and showed good agreement with those obtained by the through-diffusion experiments. The diffusion model, based on homogeneous pore structure and electrical double layer (EDL) theory, predicted the salinity dependence of De reasonably well, showing the effect of cation excess and anion exclusion as a function of salinity. The apparent diffusivity (Da), which includes sorption effects, was also interpreted by a coupled sorption model.

Analysis of microstructural images of dry and water-saturated compacted bentonite samples observed with X-ray micro CT

Compacted bentonite, of which the major clay mineral is montmorillonite, is a candidate buffer material for geological disposal of high-level radioactive waste. In this study, a microfocus X-ray computed tomography (micro-CT, X-ray microscope), which enables non-destructive, three-dimensional observation of the interior microstructure of a sample with high resolution (several microns), examined compacted montmorillonite samples under dry and water-saturated states. The images thus obtained were analyzed by a computer code developed for this study to obtain the information on the size and shape of montmorillonite grains in the samples before and after the water saturation. From the results of the image analysis, it can be supposed that the outer montmorillonite sheets of grains swelled and formed a gel, whereas the inner montmorillonite sheets did not change significantly in the water-saturation process.

Self-diffusion of water and its dependence on temperature and ionic strength in highly compacted montmorillonite, illite and kaolinite

abstract The effect of temperature and ionic strength on the diffusion of HTO parallel to the direc-tion of compaction through 5 highly compacted clay minerals (bulk dry density, q b,d = 1.90 ± 0.05 Mg/m 3 ), namely montmorillonite (Na- and Ca-form), illite (Na- and Ca-form), and kaolinite, was studied. The diffusion experiments were carried out at tempera-tures between 0 C and 60 C and at ionic strengths of 0.01 M and 1 M NaCl for the Na-formclays and kaolinite, and of 0.005 M and 0.5 M CaCl 2 for the Ca-form. The ionic strength hadan insignificant influence on the values of the effective diffusion coefficient (variation byless than 10%) for the clays under study at this degree of compaction. The effectivediffusion coefficients followed the order Na-montmorillonite < Ca-montmorillonite < Ca-illite < Na-illite 6 kaolinite. It is thought that the differences between Na- and Ca-montmo-rillonite originate from the larger size particles, and thus the lower tortuosity of the latter;whereas the differences between Na- and Ca-illite are related to the different degree of sol-vation of the Na and Ca cations. The activation energies were successfully calculated usingthe Arrhenius law. Swelling clays (Na- and Ca-montmorillonite) had slightly larger activa-tion energy values (20 kJ/mol) compared to bulk water (17 kJ/mol); Ca-illite (16 kJ/mol),Na-illite (13 kJ/mol) and kaolinite (14.4 kJ/mol) lower values than that of bulk water. Thelow activation energies of the last three clays may be related to weaker H-bonds betweenwater and the clay surfaces compared to those in bulk water. 2008 Elsevier Ltd. All rights reserved.Contents1. Introduction . . . . . . . . . . . . . 38412. Materials. . . . . . . . . . . . . . . . 38423. Methods . . . . . . . . . . . . . . . . 38434. Modelling . . . . . . . . . . . . . . . 38445. Results and discussion. . . . . 38455.1. Clay porosities and densities . . . . 38455.2. Effective diffusion coefficients at 20 C 38465.3. Temperature dependency . . . . . . . 38486. Summary and conclusions. . 3849