[Show abstract][Hide abstract] ABSTRACT: Bentonite clay is planned to form a part of deep-geological repositories of spent nuclear fuel in several countries. The extremely long operation time of the repository requires an indepth understanding of the structure and properties of used materials. In this work the microstructure of a simplified system of Ca-montmorillonite is investigated using a set of complementary methods: X-ray diffraction, small angle X-ray scattering, nuclear magnetic resonance, transmission electron microscopy and ion exclusion. The paper presents experimental results obtained from compacted, water saturated samples in the dry density range 0.6–5 g/cm3. It can be observed that different methods yield similar quantification of water present in the interlamellar space. Combined results support the multiple porosity concept of the bentonite structure.
[Show abstract][Hide abstract] ABSTRACT: Water-saturated bentonite is planned to be used in many countries as an important barrier component in high-level nuclear waste (HLW) repositories. Knowledge about the microstructure of the bentonite and the distribution of water between interlayer (IL) and non-interlayer (non-IL) pores is important for modelling of long-term processes. In this work the microstructure of water-saturated samples prepared from MX-80 bentonite was studied with nuclear magnetic resonance (NMR) and small-angle X-ray scattering spectroscopy (SAXS) coupled with chloride exclusion modelling. The sample dry densities ranged between 0.7 and 1.6 g/cm3. The NMR technique was used to get information about the relative amounts of different water types. Water in smaller volume domains has a shorter relaxation time than that in larger domains due to the average closer proximity of the water to the paramagnetic Fe at the layer surfaces. The results were obtained using 1H NMR T 1ρ relaxation time measurements with the short inter-pulse CPMG method. The interpretation of the NMR results was made by fitting a sum of discrete exponentials to the observed decay curves. The SAXS measurement on bentonite samples was used to get information about the size distribution of the IL distance of montmorillonite. The chloride porosity measurements and Donnan exclusion calculations were used together with the SAXS results to evaluate the bentonite microstructure. In the model, the montmorillonite layers were organized in stacks having IL water between the layers and non-IL water between the stacks. In the modelling, the number of layers in the stacks was used as fitting parameters which determined the IL and non-IL surface areas. The fitting parameters were adjusted so that the modelled chloride concentration was equal to the measured one. The NMR studies and SAXS studies coupled with the Cl porosity measurements provided very similar pictures of how the porewater is divided in two phases in bentonite.
[Show abstract][Hide abstract] ABSTRACT: The low-content free water and high swelling pressure in compacted bentonite, planned to be used as a buffer in nuclear waste repositories, create adverse conditions for direct measurements of the chemical conditions. This paper presents laboratory results from online measurements with Eh and pH electrodes in water-saturated compacted MX-80 bentonite. The Eh was measured with Au and Pt wires as electrodes, while the pH was determined with IrOx electrodes. The latter were prepared in accordance with the method by Yao et al. . The measurements were carried out in two types of cells: 'squeezing cells' and 'diffusion cells'. The squeezing cell excludes almost completely all chemical interactions between the sample and the surrounding environment outside the cell. The diffusion cell, on the other hand, contains a sample that stays in contact with an external solution and therefore allows following of the physico-chemical interaction between the sample and the external solution. The measuring electrodes were positioned inside the cell in the compacted bentonite, while the reference electrode was positioned outside the cell. (authors)
[Show abstract][Hide abstract] ABSTRACT: The low content of free water and high swelling pressure in compacted bentonite, planned to be used as a buffer in nuclear waste repositories, create adverse conditions for direct measurements of the chemical conditions. This paper presents results from on-line measurements with Eh and pH electrodes in water-saturated compacted MX-80 bentonite. The Eh was measured with Au- and Pt-wires as electrodes, while the pH was determined with IrOx electrodes. The measurements have been carried out in closed bentonite cells and in cells where the bentonite is in contact with an outside solution through a filter plate, so that the experimental conditions can be changed during the experiment. The results suggest that it is possible to measure pH and Eh in compacted bentonite, which improves the possibilities of obtaining information about the chemical processes occurring in the bentonite.
[Show abstract][Hide abstract] ABSTRACT: The KBS-3 underground nuclear waste repository concept designed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) includes a bentonite buffer barrier surrounding the copper canisters and the iron insert where spent nuclear fuel will be placed. Bentonite is also part of the backfill material used to seal the access and deposition tunnels of the repository. The bentonite barrier has three main safety functions: to ensure the physical stability of the canister, to retard the intrusion of groundwater to the canisters, and in case of canister failure, to retard the migration of radionuclides to the geosphere. Laboratory experiments (< 10 years long) have provided evidence of the control exerted by accessory minerals and clay surfaces on the pore water chemistry. The evolution of the pore water chemistry will be a primordial factor on the long-term stability of the bentonite barrier, which is a key issue in the safety assessments of the KBS-3 concept. In this work we aim to study the long-term geochemical evolution of bentonite and its pore water in the evolving geochemical environment due to climate change. In order to do this, reactive transport simulations are used to predict the interaction between groundwater and bentonite which is simulated following two different pathways: (1) groundwater flow through the backfill in the deposition tunnels, eventually reaching the top of the deposition hole, and (2) direct connection between groundwater and bentonite rings through fractures in the granite crosscutting the deposition hole. The influence of changes in climate has been tested using three different waters interacting with the bentonite: present-day groundwater, water derived from ice melting, and deep-seated brine. Two commercial bentonites have been considered as buffer material, MX-80 and Deponit CA-N, and one natural clay (Friedland type) for the backfill. They show differences in the composition of the exchangeable cations and in the accessory mineral content. Results from the simulations indicate that pore water chemistry is controlled by the equilibrium with the accessory minerals, especially carbonates. pH is buffered by precipitation/dissolution of calcite and dolomite, when present. The equilibrium of these minerals is deeply influenced by gypsum dissolution and cation exchange reactions in the smectite interlayer. If carbonate minerals are initially absent in bentonite, pH is then controlled by surface acidity reactions in the hydroxyl groups at the edge sites of the clay fraction, although its buffering capacity is not as strong as the equilibrium with carbonate minerals. The redox capacity of the bentonite pore water system is mainly controlled by Fe(II)-bearing minerals (pyrite and siderite). Changes in the groundwater composition lead to variations in the cation exchange occupancy, and dissolution-precipitation of carbonate minerals and gypsum. The most significant changes in the evolution of the system are predicted when ice-melting water, which is highly diluted and alkaline, enters into the system. In this case, the dissolution of carbonate minerals is enhanced, increasing pH in the bentonite pore water. Moreover, a rapid change in the population of exchange sites in the smectite is expected due to the replacement of Na for Ca.
Journal of Contaminant Hydrology 10/2008; 102(3-4):196-209. · 2.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Materials Research Society Symposium Proceedings. Vol. 1124 : Scientific Basis for Nuclear Waste Management XXXII. Materials Research Society. Warrendale, PA, USA, 251-256 After closure, the near-field of a nuclear waste repository contains large amounts of oxygen in tunnels and deposition holes. The bentonite buffer/backfill will contain oxygen as a gas phase in unsaturated pores as well as dissolved gas in porewater. The redox conditions in the bentonite filling after post-closure will change towards reducing conditions. In the initial stage, the development of the redox state is mainly governed by the depletion of oxygen. The main mechanisms of oxygen depletion in the bentonite are: 1) diffusion into the surrounding rock and 2) reactions with accessory minerals and by microbial aerobic consumption of organic matter [1,2]. The reactions leading to oxygen depletion are not, however, well understood. The objective of this work was to gather new information concerning oxygen depletion in MX-80. This was done by measuring oxygen depletion and changes in the redox state in suspensions of 1) MX-80, 2) a heavy fraction of MX-80, or 3) a light fraction of MX-80. 2008 MRS Fall Meeting. Boston, MA, USA, 1-5 Dec. 2008
[Show abstract][Hide abstract] ABSTRACT: In the present work, the effect of homogenization on the microstructure and exclusion of chloride in compacted bentonite was studied. The experiments were performed with MX-80 bentonite, from which easily dissolving components, e.g. chlorides and gypsum, had been removed to ensure that the excess ions in the pore water came from the external solution alone. In the first case, the bentonite was homogenized by dispersing the clay powder in deionized water with ultrasound. The extra water was subsequently squeezed out in order to obtain the desired clay density. In the second case, the samples were compacted to the desired density at the outset and then saturated with deionized water. Thereafter, both sample types were equilibrated with 0.1 M NaCl solution through filter plates. In the third case, the compacted samples were equilibrated directly with 0.1 M NaCl solution. The concentrations in the pore water at the end of equilibration were determined by dispersing the bentonite samples in deionized water. The concentrations in the pore water of the samples saturated first with deionized water were clearly lower than those saturated directly with 0.1 M NaCl solution. This probably reflects the effect of homogenization and a smaller size of the external pores on the chloride concentrations. The model results calculated with the Donnan model by assuming a homogenous bentonite structure were lower than the measured values. Excellent agreement between experimental and model results could be obtained by assuming a dual-porosity structure.
Physics and Chemistry of the Earth Parts A/B/C 01/2007; 32:485-490. · 1.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This work describes the development of an on-line method for studying the chemical conditions in compacted water-saturated bentonite. The equipment includes a squeezing cell, measurement electrodes, a reference electrode, and a data-logging system. IrOx wire was used for the pH measurements and Au and Pt wires for the redox measurements. The electrodes were made in-house at VTT. The reference electrode was a commercial leak-free electrode. The measurement electrodes were installed in the bentonite, while the reference electrode was placed in water outside the clay. The contact between the reference electrode and the measurement electrodes was established via the solution in a tube.The work included first a test and calibration of the electrodes in various standard solutions. The performance of the electrodes was subsequently studied in compacted MX-80 in squeezing cell experiments. The squeezing cell initially contained MX-80 mixed with different solutions at a dry density of 0.8–0.9Mg/m3. The pore water was then slowly squeezed out from the clay until the final density of 1.5Mg/m3 was reached. The measurement electrodes were then placed in the clay and the potential measurements were started. At the end of the experiments the electrodes were removed from bentonite and recalibrated. The pH and Eh in the squeezed pore waters were determined for comparison.The results obtained so far have led to the conclusion that it is possible to perform pH measurements in compacted bentonite by using IrOx electrodes. The electrode can survive the swelling pressure inside compacted water-saturated clay without losing its ability to function as a pH electrode. It is also possible to perform Eh measurements in compacted clay by using Au and Pt electrodes.
Physics and Chemistry of the Earth Parts A/B/C 01/2007; 32(1):241-246. · 1.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The concentrations caused by the external solution into the porewater were studied with compacted bentonite (MX-80), from which easily dissolving components had been removed in order to ensure that the ions in the porewater came from the external solution. The dry densities of the samples varied from 700 to 1700 kg/m3 and NaCl solutions of 0.1–3 M were used as the external solution for saturation. The concentrations in the porewater were determined by the direct analysis of the squeezed porewaters and by dispersing the sample in deionized water.At high concentrations, the Donnan model can predict the concentrations in the porewater rather well. At low concentrations, where the ion exclusion is stronger, the measured concentrations are clearly higher than the modelled values. One possible explanation for this discrepancy is the microstructure of the bentonite, and an attempt to couple the effects of the microstructure and the Donnan model was made. It was assumed that there are two pore types, interlamellar pores in the montmorillonite stacks and large pores in the gel between the stacks. The dimensions of the microstructure were obtained from SAXS and BET(N2) measurements. In this case, the fitting is much better, which supports the assumption of different pore types in bentonite.
Physics and Chemistry of the Earth Parts A/B/C 01/2004; 29(1):119-127. · 1.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A geochemical modelling of one of the experimental parcels of the LOT experiment (Äspö,
Sweden) is presented. The model consists on a 2D reactive transport model of a perpendicular
section of the experimental borehole with a stationary temperature gradient ranging from 130 to
15ºC. The results of the model show that temperature has an important effect on the geochemical
evolution of the system. Due to the temperature effect, anhydrite replaces gypsum and hematite
precipitates instead of Fe(OH)3(am). Calcite and siderite dissolve in the warmer part of
bentonite; whereas a major replacement of Na by Ca in the bentonite takes place close to the
bentonite-granite boundary. pH is buffered by the equilibrium with calcite, although the effect of
temperature leads to a decrease in pH.
[Show abstract][Hide abstract] ABSTRACT: The porewater chemistry in compacted bentonite was studied in solution–bentonite interaction experiments. The parameters varied in the experiments were the bentonite density, bentonite-to-water ratio (B/W), ionic strength of the solution, and the composition of bentonite. The bentonite types used in the experiments were Volclay MX80 and artificial bentonites prepared from purified MX-80 in sodium form where CaCO3 and CaSO4 were added. At the end of the experiment, the equilibrating external solution and the porewater squeezed out of the bentonite were analyzed to give information for interpretation of the interaction. The equilibrium was modelled with the HYDRAQL code.The evolution of porewater chemistry was determined by the dissolving components initially present in the bentonite together with the ions entering with water from the surroundings. Ion-exchange processes occurred between the bentonite and the porewater. The concentrations in the external solution and porewater strongly depended on the B/W used. The concentrations in the squeezed porewaters were clearly lower than in the equilibrating waters. The modelling results reasonably fit the experimental data.
[Show abstract][Hide abstract] ABSTRACT: The information available from the open literature and our studies on exclusion, sorption and diffusion mechanisms of ionic and neutral species in bentonite has been compiled and re-examined in relation to the microstructure of bentonite. The emphasis is placed on a more thorough understanding of the diffusion processes taking place in compacted bentonite. Despite the scarcity of experiments performed with neutral diffusants, these imply that virtually all the pores in compacted bentonite are accessible to neutral species. Anion exclusion, induced by the overlap of electrical double layers, may render the accessible porosity for anions considerably less than the porosity obtained from the water content of the clay. On the basis of the compiled data, it is highly probable that surface diffusion plays a significant role in the transport of cations in bentonite clays. Moreover, easily soluble compounds in bentonite can affect the ionic strength of porewater and, consequently, exclusion, equilibrium between cations, and surface diffusion.
[Show abstract][Hide abstract] ABSTRACT: The nature of diffusivity and porosity in crystalline rock was studied by electrical conductivity measurements, steady-state diffusion experiments, saturation-leaching of tracers with cylindrical rock samples and analysis of the concentrations of different elements from core samples or pore water near fractures. The phenomena of main interest were dead-end porosity, ion-exclusion, sorption, and the continuity of pore networks. The modelling of experimental results was based on a modified Fick's second law for diffusion, which was solved either by analytical or numerical methods. The measured De and ε were found to statistically follow an exponential presentation: Archie's law. The existence of ion-exclusion for anions was confirmed. The connectivity of the pore network extended in the laboratory experiments at least six centimetres, in coarse-grained granite in nature several metres but in fine-grained rock samples of a uranium deposit the element mobilization effects could be seen only to the depth of 2–3 centimetres.
[Show abstract][Hide abstract] ABSTRACT: Alteration of compacted sodium-bentonite (Volclay MX-80) caused by groundwater in simulated repository conditions for high level radioactive waste, was studied in an experiment where bentonite (wrapped by a copper cylinder) was let to react with two types (A,B) of synthetic granitic groundwater that are distinguished by their initial concentration of potassium and chloride. The reaction took place in ambient conditions at a temperature of 75 °C and proceeded during several time intervals up to 36 months.At the end of each time interval the water was chemically analysed for determination of possible changes in composition. Chemical and mineralogical changes in the bentonite were investigated by using NH4Cl extractions, XRD and microprobe (SEM, EDS) analyses and were studied as a function of the reaction time (months) as well as of the distance (mm) from the contact front with water.Sodium ions were found to migrate out from the bentonite while being replaced by other cations such as calcium, magnesium and to some extent, particularly during the reaction of the bentonite with water B, by potassium. No clear evidence was found for the fixation of potassium ions in the interlayer position of montmorillonite clay and the transformation to illite. The main mineralogical change in the bentonite was from Na- to Ca-rich montmorillonite. Secondary processes were the dissolution-precipitation of sulphur compounds, dissolution of gypsum and carbonates and the dissolution-precipitation of copper compounds.
[Show abstract][Hide abstract] ABSTRACT: Scientific Basis for Nuclear Waste Management XII. Boston, USA, 27 - 30 Nov. 1989. Pittsburgh 1990, Materials Research Society Symp. Proc. Ed. by V. M. Oversby & P. W. Brown, 641 - 647