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1.
As a part of the safety assessment of the geological disposal of high-level radioactive waste, the effects of dry density and exchangeable cations on the diffusion process of Na+ ions in compacted bentonite were studied from the viewpoint of the activation energy for diffusion. The apparent self-diffusion coefficients of Na+ ions in compacted Na-montmorillonite and in a Na- and Ca-montmorillonite mixture were determined by one-dimensional, non-steady diffusion experiments at different temperatures and dry densities. A unique change in activation energy as a function of dry density was found for the Na+ ions in compacted Na-montmorillonite. The activation energy suddenly decreased from 18.1 to 14.1 kJ mol− 1 as the dry density increased from 0.9 to 1.0 Mg m− 3, whereas it increased to 24.7 kJ mol− 1 as the dry density increased to 1.8 Mg m− 3. Examination of the effect of exchangeable cations on the activation energies determined that the activation energies were almost constant, approximately 25 kJ mol− 1, for the montmorillonite specimens at a dry density of 1.8 Mg m− 3. However, three different activation energy values were obtained at a dry density of 1.0 Mg m− 3. These findings cannot be explained by the conventional diffusion model (the pore water diffusion model), which suggests that the predominant diffusion process alternates among pore water diffusion, interlayer diffusion, and external surface diffusion.  相似文献   

2.
In porous media, chemical species that dissolve in pore water can be transported via diffusion mechanisms or advective fluxes, close to or far away from where precipitation occurs. In the case of a high‐level radioactive waste disposal system, compacted bentonite is used in a buffer material in an engineering barrier system to minimize the amount of specific nuclides that breach into the surrounding host rock. To minimize breaching, it is very important to understand the transport mechanism of multiple chemical species in porous media. In the following research, we introduced FEM analysis methods using the results of the molecular dynamics simulation and homogenization analysis (MD/HA) method. First, the diffusion coefficients of ions (Cl?, I?, and Na+) in different water layers of Na‐beidellite were calculated using the MD/HA procedure under various dry density (1.2, 1.6, and 2.0 Mg/m3) and temperature (293, 323, and 363 K) conditions. Next, using FEM analysis that used the MD/HA results as input parameters, the diffusion behaviors of ions in porous media were calculated. The results indicate that the diffusion coefficients of the interlayer water in Na‐beidellite are different from the diffusion coefficients under dry density conditions. Further, the concentration profiles (Ct/C0) of iodine and chloride are proportional to temperature but inversely proportional to dry density. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

3.
Diffusion of 36Cl− in compacted bentonite was studied using through-diffusion, out-diffusion and profile analysis techniques. Both the bulk dry density of the bentonite and the composition of the external solution were varied. Increasing the bulk dry density of the bentonite resulted in a decrease of both the effective diffusion coefficient and the Cl-accessible porosity. Increasing the ionic strength of the external solutions resulted in an increase of both the effective diffusion coefficient and the Cl-accessible porosity. This can be explained by anion exclusion effects (Donnan exclusion). At high ionic strength values (I ? 1 M NaCl) the Cl-accessible porosity approaches the interparticle porosity. This interparticle porosity is the difference between the total and interlayer porosity of the bentonite. The interlayer porosity was found to depend on the degree of compaction. Up to a bulk dry density of 1300 kg m−3 the interlayer is built up of 3 water layers. Between 1300 and 1800 kg m−3 the interlayer water is reduced from 3 to 2 layers of water. Above 1800 kg m−3 evidence for a further decrease to 1 layer of water was found. These findings are in agreement with X-ray data found in the literature showing a decrease of the basal spacing of montmorillonite (the main clay mineral in bentonite) with increasing degree of compaction. The relationship between the effective diffusion coefficient of Cl− and the diffusion-accessible porosity can be described by an empirical relationship analogous to Archie’s law. To predict the effective diffusion coefficient of Cl− in compacted bentonite, the diffusion coefficient of Cl− in water, the bulk dry density and the ionic strength of the pore water have to be known.  相似文献   

4.
Compacted bentonite is used as sealing and buffer material in engineered barrier systems (EBS) of high-level radioactive waste repositories. The chemical characteristics of this clay and its porewater affect the migration of radionuclides eventually released from the waste. They also determine the integrity and long-term performance of the clay barriers. Key features are the structural negative charge and the large proportion of structural (interlayer) water of the main mineral montmorillonite, which leads to exclusion of anions and a surplus of cations in a large part of the porosity space. The objective of this contribution was to assess the impact of different porosity model concepts on porewater chemistry in compacted bentonite in the context of the planned Finnish spent nuclear fuel repository at Olkiluoto. First, a structural model based on well-established crystallographic and electrostatic considerations was set up to estimate the fractions of the different porosity types. In view of the uncertainty related to the chemical properties of the interlayer water, two very different model concepts (anion-free interlayer, Donnan space), together with a well-established thermodynamic model for bentonite, were applied to derive the porewater composition of the bentonite buffer at Olkiluoto. The simulations indicate very similar results in the “free” water composition for the two models and thus support the validity of the reference porewater concept commonly used in performance assessment of waste repositories. Differences between the models are evident in the composition of the water affected by the surface charge (i.e. diffuse double layer and interlayer). These reflect the conceptual uncertainty in current multi-porosity diffusion models.  相似文献   

5.
The through-diffusion of HTO, 22Na+ and 36Cl− in kaolinite, homo-ionic Na-illite and homo-ionic Na-montmorillonite was measured at a high degree of compaction as a function of the salt concentration in the ‘external solution’, i.e. in the solution in contact with the clay sample. The clays were chosen for this study because of their differences in the number and nature of ion exchange sites leading to different proportions of interlayer-, inter-particle and free pore water. It was found that the diffusive mass transfer of Na+ in Na-montmorillonite and Na-illite increased with decreasing external salt concentration, while the opposite trend was observed for the diffusion of Cl−. These trends are more pronounced in the case of Na-montmorillonite than in Na-illite, while almost no salt effect was observed for kaolinite. Similarly no salt effect was observed for the diffusion of HTO through all of the clays tested. These observations are in agreement with a conceptual model where it is assumed that cations diffuse preferentially in the interlayer or diffuse double-layer porosity, while anions are almost completely excluded from these regions. In the case of Na+ diffusion, the salt effects can be explained by an influence on the concentration gradient of diffusing cations, while in the case of Cl− the external salt concentration has an effect on the accessible porosity. Effective diffusion coefficients of Cl− fulfil the same relationship to porosity as those of the uncharged HTO, when using accessible porosities for such a comparison. Furthermore it is shown that pore diffusion coefficients for the three tracers are fairly well correlated with the respective diffusion coefficients in bulk water, if the effective diffusion coefficients for Na+ are derived from calculated tracer concentration gradients in the interlayer or diffuse double-layer porosities.  相似文献   

6.
The aim of the present work was to study the thermodynamic equilibria between water and a homo-ionic montmorillonite SWy-1 saturated by different cations. The choice of this smectite is justified by the large set of experimental data available from the literature for eight different interlayer cations: Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, and Ba2+. In particular, studies by [Cases et al., 1992], [Cases et al., 1997] and [Bérend et al., 1995] are providing heat of adsorption data, pairs of desorption-adsorption isotherms, and information about the partition of adsorption-desorption water molecules between external surfaces and internal spaces. By calculating the effective amount of hydration water as the difference between the so-called gravimetric water and the surface covering water, a thermodynamic model was then developed, based on the concept of Ransom and Helgeson (1994) considering an asymmetric subregular binary solid solution between a fully hydrated and a anhydrous smectite. A set of six thermodynamic parameters ( and four Margules parameters) was extracted by a least square method from measurements of enthalpies of adsorption and paired adsorption-desorption isotherms for each interlayer cation. These six initial parameters were then used to determine a complete set of standard thermodynamic hydration parameters (, heat capacity, molar volume, and number of interlayer H2O) and quantify, for each cation, the number of moles of hydration water molecules as a function of relative humidity and temperature. The validation of the standard state thermodynamic properties of hydration for each end member was carried out using three approaches: (1) a comparison with experimental isotherms obtained on hetero-ionic and homo-ionic SWy-1 smectite at different temperatures; (2) a comparison with the experimental integral enthalpy and entropy of hydration of the SWy-1 smectite; and (3) a comparison with experimental isotherms acquired on various smectites (Upton, MX80, Arizona) with different layer charges.Eventually, the present work demonstrates that, from a limited number of measurements, it is possible to provide the hydration thermodynamic parameters for hydrated smectites with different compositions and under different conditions of temperature and relative humidity, using the newly developed predictive model.  相似文献   

7.
It is important to clarify the migration behavior of hydrogen gas dissolved in water-saturated, compacted bentonite, which is a promising material for geologic disposal of high-level waste and TRU waste disposal. The diffusion coefficients of helium, which can be detected under extremely low background conditions, in water-saturated, compacted Na-montmorillonite were determined as a function of temperature by a transient diffusion method. The activation energies for diffusion of helium were then obtained. The activation energies were from 6.9 ± 4.8 to 19 ± 2.8 kJ mol− 1 and were regarded to be independent of dry density. The activation energies of helium in water-saturated Na-montmorillonite were roughly equal to those in bulk water, 14.9 kJ mol− 1, and in ice, from 11 to 13 kJ mol− 1. It is possible that helium diffuses not only in pore water but also in interlayer water.  相似文献   

8.
Diffusion and sorption behaviors of cationic Cs+, anionic I− and neutral HTO in samples of the Wakkanai Formation from the Horonobe underground research laboratory (URL), Japan, were investigated as a function of ionic strength (I) of synthetic groundwater by through-diffusion and batch sorption experiments and mechanistic modeling. The effective diffusivities (De) measured by through-diffusion experiments showed cation excess and anion exclusion effects, which were strongly dependent on I; De for Cs+ decreased as I increased, De for I− showed the opposite dependency and De for HTO showed no dependence. The sorption of Cs+ measured by through-diffusion and batch sorption experiments were described by Freundlich isotherms with consistent parameters and decreased with I as a result of competitive ion exchange.Diffusion and sorption behaviors were interpreted by assuming the clay components of illite and smectite control diffusion and sorption mechanisms. The component additive (CA) sorption model, which includes illite and smectite contents and their ion exchange constants, provided a reasonable account of the Cs+ sorption trends measured as functions of I and Cs concentration. The diffusion model was developed by coupling the electrical double layer (EDL) model, describing the change of ionic concentrations (cation excess and anion deficit) and viscoelectric effects caused by electrostatic interaction at negatively charged clay surfaces, and a simplified pore model assuming one type of pore shape and includes their size distribution. When averaging the electrostatic effects by using the pore surface area distribution, the model could predict the cation excess and anion exclusion effects, and its dependence on I reasonably well. This result implies the nanoscale pores dominating the pore surface area can strongly impact on ionic diffusion in argillaceous rocks. The clay-based modeling approach described here provides a useful tool to predict ionic diffusion and sorption in argillaceous rocks.  相似文献   

9.
Diffusion experiments with HTO, 36Cl−, Br−, I−, 22Na+, 85Sr2+ and 134Cs+ at trace concentrations in a single sample of Opalinus Clay are modeled with PHREEQC’s multicomponent diffusion module. The model is used first in a classical approach to derive accessible porosities, geometrical factors (the ratio of pore tortuosity and constrictivity) and sorption behavior of the individual tracers assuming that the clay is homogeneous. The accessible porosity for neutral species and cations is obtained from HTO, the anion exclusion volume from 36Cl− and Br−, and the cation exchange capacity from 22Na+. The homogeneous model works well for tritium, the anions and 22Na+. However, the 85Sr2+ and 134Cs+ experiments show an early arrival of the tracer and a front-form that suggest a dual porosity structure. A model with 10% dead-end pores, containing 19% of the total exchange capacity, can satisfactorily calculate all the experimental data. The Cs+ diffusion model builds on a 3-site exchange model, constructed from batch sorption data. The excellent agreement of modeled and measured data contradicts earlier reports that the exchange capacity for Cs+ would be smaller in diffusion than in batch experiments.The geometrical factors for the anions are 1.5 times larger than for HTO, and for the cations 2-4 times smaller than for HTO. The different behavior is explained by a tripartite division of the porespace in free porewater, diffuse double layer (DDL) water, and interlayer water in montmorillonite. Differences between estimated and observed geometrical factors for cations are attributed to increased ion-pairing of the divalent cations in DDL water as a result of the low relative dielectric permittivity. Interlayer and/or surface diffusion contributes significantly to the diffusive flux of Cs+ but is negligible for the other solutes. The geometrical factors for anions are higher than estimated, because pore constrictions with overlapping double layers force the anions to take longer routes than HTO and the cations. Small differences among the anions can also be attributed to different ion-pairing in DDL water.  相似文献   

10.
This paper presents an upscaling concept of swelling/shrinking processes of a compacted bentonite/sand mixture, which also applies to swelling of porous media in general. A constitutive approach for highly compacted bentonite/sand mixture is developed accordingly. The concept is based on the diffuse double layer theory and connects microstructural properties of the bentonite as well as chemical properties of the pore fluid with swelling potential. Main factors influencing the swelling potential of bentonite, i.e. variation of water content, dry density, chemical composition of pore fluid, as well as the microstructures and the amount of swelling minerals are taken into account. According to the proposed model, porosity is divided into interparticle and interlayer porosity. Swelling is the potential of interlayer porosity increase, which reveals itself as volume change in the case of free expansion, or turns to be swelling pressure in the case of constrained swelling. The constitutive equations for swelling/shrinking are implemented in the software GeoSys/RockFlow as a new chemo‐hydro‐mechanical model, which is able to simulate isothermal multiphase flow in bentonite. Details of the mathematical and numerical multiphase flow formulations, as well as the code implementation are described. The proposed model is verified using experimental data of tests on a highly compacted bentonite/sand mixture. Comparison of the 1D modelling results with the experimental data evidences the capability of the proposed model to satisfactorily predict free swelling of the material under investigation. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

11.
Here we present a long-term nonisothermal reactive transport model for the interactions of the corrosion products of a carbon-steel canister and the compacted bentonite of the engineered barrier of a high-level radioactive waste repository in granite. Canister corrosion causes an increase in the pH and the concentration of dissolved Fe2+ of the bentonite porewater. Iron precipitates as magnetite and siderite and sorbs via cation exchange and surface complexation on weak sites. Magnetite precipitation reduces significantly the porosity of the bentonite near the canister. The thickness of the zone of reduced porosity is 7 cm at t = 1 Ma. This thickness increases significantly when the dependence of the corrosion rate on the chemical conditions is considered and decreases 3 cm when smectite dissolution and analcime precipitation are taken into account. Model results are not significantly sensitive to the thermal transient and the effect of temperature on the corrosion rate. The conclusions of our simulations are consistent for the most part with those reported by others for engineered barrier systems at similar chemical conditions.  相似文献   

12.
Lake Turkana, in northwestern Kenya, is a closed-basin, alkaline (pH = 9.2) lake of moderate salinity (TDS = 2500 ppm). Principal ions are Na+, HCO?3 and CI?. The lake is essentially polymictic in the northern basin and little compositional variation occurs in surface waters. The Omo River is the principal influent, providing some 80–90% of water input to the lake. Chloride has an apparent accumulation time of about 2500 years after accounting for burial of interstitial water.The bottom sediments are predominantly detrital and fine-grained, yet mineral-water reactions are very important for the geochcmical budget. Ca2+ is precipitated as calcite; Na+ is removed as an exchangeable cation on smectite; Mg2+ is probably incorporated into a Mg-silicate phase, most likely poorlycrystalline smectite, as it enters the lake water; K+ may be used in illite regradation. Cation exchange is a very important process in the mass balance of this lake. Over 40% of incoming Na is removed as an exchangeable cation. After cation exchange and interstitial water burial, Na has a response time of 2650 years, which compares favorably with that of chloride. These processes seem to occur rapidly within the water mass of the lake: other reactions may be important in regulating interstitial water compositions.Several changes occur in the upper 3m of sediment: interstitial-water pH drops to 8.3 and alkalinity increases slightly with depth, SO2?4 decreases slightly, and amorphous silica saturation is approached. These changes are a response to organic matter oxidation and the dissolution of unstable silicates rather than a reversal of reactions occurring in the lake water. High rates of sedimentation (up to 1 cm per year) may minimize the effects of diffusion between the interstitial waters and the lake water, although burial of interstitial water assumes considerable importance.  相似文献   

13.
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14.
R. W. RENAUT 《Sedimentology》1993,40(2):271-301
Late Quaternary fluviolacustrine siltstones, mudstones and claystones (Loboi Silts) on the northern margins of the saline, alkaline Lake Bogoria in the Kenya Rift Valley contain up to c. 40% authigenic analcime and minor natrolite. The zeolitic sediments are reddish brown and up to 1 m thick. The amount of analcime increases upward in the profile, but decreases with distance from the lake. The altered sediments show many pedogenic features including zeolitic root mats, rootmarks, concretions and carbonate rhizoliths. Residual patches of calcrete locally cap the zeolitic rocks. The profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake. The analcime occurs as submicroscopic (0–5–2–5 μ.m) subhedral and euhedral crystals, which have an average Si/A 1 ratio of 2–33 (as determined by X-ray microanalysis) or 2–18 (d-value of 639 analcime peak). The analcime formed in lake marginal sediments (soils) by reaction of silicate detritus with Na2CO3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration. Poorly ordered clay minerals were probably the main reactants. Authigenic illite may have been a by-product of the reactions. Chemical analyses suggest that pore waters supplied some of Na+, and possibly K+ and SiO2. The associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration. The Ca2+ may have originated from infiltrating dilute runoff and groundwater. Authigenic smectite was precipitated in open porosity following analcime formation. The zeolitic alteration at Lake Bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline, alkaline lake sediments.  相似文献   

15.
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16.
The Wyoming bentonite was saturated with alkali and alkaline earth cations and allowed to react hydrothermally for one month at 300°C, and one week and one month at 400°C. Generally, the rate of formation of collapsed layers for the alkali clays was inversely related to interlayer hydration energy. This pattern of reaction is expected if layer dehydration results from an attraction between negatively charged 2:1 layers and the positive interlayer cation. In this mechanism, the greater the hydration energy of the interlayer region, the greater the charge that must develop on the 2:1 layers to cause dehydration, and, therefore, the slower the reaction rate. Reaction rate for the alkaline earth clays was directly related to interlayer hydration energy. Clays saturated with cations of hydration energy equal to or greater than Na+ did not react at 300°C, and ceased to react at 50% expandable layers at 400°C. Those saturated with cations of lower hydration energy continued to form mica-like layers with increasing run time at both 300° and 400°C. Clays saturated with monovalent cations reacted by a process of gradual transformation of smectite layers into collapsed layers, whereas those with divalent cations reacted directly from the smectite structure to form a rectorite-like phase.  相似文献   

17.
The degree of saturation of compacted bentonite buffer in deep geological repositories is subject to alterations from infiltration of groundwater and heat emanated from the waste canisters. The matric suction (ψ)–degree of saturation (S r ) relations of unsaturated clays is represented by soil–water characteristics curves (SWCC) that are influenced by soil structure, initial compaction condition and stress history. Infiltration of groundwater besides increasing the degree of saturation can also alter the pore water chemistry; the associated changes in cation hydration and diffuse double layer thickness could impact the micro-structure and matric suction values. This study examines the influence of infiltrating sodium chloride solutions (1,000–5,000 mg/L) on the transient ψ–S r relations of compacted bentonite–sand specimens. Analysis of the ψ–S r plots, and X-ray diffraction measurements indicated that infiltration of sodium chloride solutions has progressively less influence on the micro-structure and SWCC relations of bentonite–sand specimens compacted to increasingly higher dry densities. The micro-structure and SWCC relations of specimens compacted to 1.5 Mg/m3 were most affected, specimens compacted to 1.75 Mg/m3 were less affected, while specimens compacted to 2 Mg/m3 remained unaffected upon infiltration with sodium chloride solutions.  相似文献   

18.
The acid chemistry of confined waters in smectite interlayers have been investigated with first principles molecular dynamics (FPMD) simulations. Aiming at a systematic picture, we establish the model systems to take account of the three possible controlling factors: layer charge densities (0 e, 0.5 e and 1.0 e per cell), layer charge locations (tetrahedral and octahedral) and interlayer counterions (Na+ and Mg2+). For all models, the interlayer structures are characterized in detail. Na+ and Mg2+ show significantly different hydration characteristics: Mg2+ forms a rigid octahedral hydration shell and resides around the midplane, whereas Na+ binds to a basal oxygen atom and forms a very flexible hydration shell, which consists of five waters on average and shows very fast water exchanges. The method of constraint is employed to enforce the water dissociation reactions and the thermodynamic integration approach is used to derive the free-energy values and the acidity constants. Based on the simulations, the following points have been gained. (1) The layer charge is found to be the direct origin of water acidity enhancement in smectites because the neutral pore almost does not have influences on water dissociations but all charged pores do. (2) With a moderate charge density of 0.5 e per cell, the interlayer water shows a pKa value around 11.5. While increasing layer charge density to 1.0 e, no obvious difference is found for the free water molecules. Since 1.0 e is at the upper limit of smectites’ layer charge, it is proposed that the calculated acidity of free water in octahedrally substituted Mg2+-smectite, 11.3, can be taken as the lower limit of acidities of free waters. (3) In octahedrally and tetrahedrally substituted models, the bound waters of Mg2+ show very low pKa values: 10.1 vs 10.4. This evidences that smectites can also promote the dissociations of the coordinated waters of metal cations. The comparison between the two Mg2+-smectites reveals that different layer charge locations do not lead to obvious differences for bound and free water acidities.  相似文献   

19.
We propose a thermodynamic approach to model the stepwise dehydration with increasing temperature or decreasing H2O activity of K, Na, Ca and Mg-smectite. The approach relies on the relative stability of the different solid-solutions that describe the hydration of di- or trioctahedral-smectites containing 0, 1, 2 or 3 interlayer water layers. The inclusion of anhydrous mica end-members makes it possible to cover, with the same solid-solution model, the entire range of composition from low-charge smectite to mica, through high-charge smectite and illite. Non-ideal Margules parameters were used to describe the non-ideality of the solid solutions between the hydrated and dehydrated smectite end-members. Standard state properties of all smectite end-members as well as Ca- and Mg-muscovite and -phlogopite were initially estimated by oxide summation. These values were then refined and the other non-ideal interactions were estimated on the basis of different experimental data. The stepwise dehydration of smectite, and its stability and compatibility relations were calculated by Gibbs free energy minimising. Our results account for the progressive evolution of smectite to interlayered illite/smectite and then to mica, as observed in nature and experiments, and our model provides an explanation for the thermodynamic stability of smectite and illite/smectite compared to mica + kaolinite or pyrophyllite assemblages. The results suggest that the enthalpic contribution of interlayer water is a function of the ionic potential of the interlayer cation and the number of interlayer water molecules. This evolution makes possible to estimate the standard-state thermodynamic parameters and hydration-temperature behaviour of smectite of virtually all possible compositions. For the four-interlayer cations considered in the study, our model reproduces the 3 → 2 → 1 water-layer transitions that accompany a reduction of water activity or an increase of temperature at ambient pressure. The range of water content and interlayer distance calculated for the 3w, 2w and 1w states are also in fair agreement with the experimental values at ambient pressure.  相似文献   

20.
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