Clay Barriers Research Articles

Study of thermo-hydro-mechanical processes at a potential site of an Indian nuclear waste repository

A detailed scientific study is required for the disposal of high-level radioactive wastes because they generate extremely high heat during their half-life period. Although, several methods have been proposed for the disposal of nuclear wastes, deep underground repository is considered to be a suitable option.In this paper, field investigation has been done near to Bhima basin of peninsular India. Detailed fracture analysis near the borehole shows very prominent maxima of fractures striking N55∘E coinciding with the trace of master basement cover metasediment fault. Physico-mechanical properties of rocks have been determined in the laboratory.The host rock chosen is granite and engineered barrier near the canister is proposed to be clay. A thermo-hydro-mechanical (THM) analysis has been done to study the effect of heat on deformations, stresses and pore-pressure variation in granite and clay barriers. For this purpose, finite difference method has been used. Suitable rheological models have been used to model elastic canister and elasto-plastic engineered barrier and host rock. It has been found that both temperature and stresses at any point in the rockmass is below the design criteria which are 100∘C for temperature and 0.2 for damage number.

Heavy metal retention properties of kaolin and bentonite in a wide range of concentration and different pH conditions

Accumulation of heavy metals can occur on clay barriers or slurry-trench cutoff wall, resulting in their high concentrations at which the retention reaction takes place. Or high concentration conditions are often adopted to accelerate the reaction process at a laboratory scale for investigating their impacts on engineering behaviors (hydraulic or mechanical) of clays. So far, there is lack of data regarding heavy metal retention behavior on clays under those extreme concentration conditions. This paper aims to mitigate this gap with emphasis on pH effects. To do so, several series of sorption batch tests were conducted to characterize the retention capacity of three ubiquitous heavy metals (Pb, Zn and Cu) on two reference clays (Speswhite kaolin and Wyoming bentonite). A wide range of individual metal concentration varying from 0.1 to 100mM and different pH conditions were tested. Results suggest that the acidifying the metal–clay system can result in a significant decrease in the heavy metal retention, which can be attributed to an increase in competition of H+ with metal ions to the surface sites and a decrease in metal ion precipitation as hydroxides at high pH levels. In addition, the heavy metal retention isotherm over a wide range of concentrations for the bentonite follows Temkin and Langmuir adsorption models while Freundlich and Langmuir models for the kaolin. Interestingly, at a high concentration of 10mM, the Pb precipitation unlikely occurs in the presence of both clays tested over a wide range pH level (<10), whereas it likely takes place in case of Cu and Zn.

Reactivity of Callovo-Oxfordian Claystone and its Clay Fraction With Metallic Iron: Role of Non-Clay Minerals in the Interaction Mechanism

AbstractIn order to better understand the possible interactions between steel canisters and a claystone host rock, in this case the Callovo-Oxfordian rock (COx), the present study investigated in detail, under conditions relevant to high-level radioactive waste repositories (anoxic conditions, temperature of 90°C), the reactions between metallic iron and: (1) COx; (2) the clay fraction extracted from COx (CF); and (3) mixtures of CF with quartz, calcite, or pyrite. Batch experiments were then carried out in the presence of NaCl-CaCl2 background electrolyte, for durations of 1, 3, and 9 months. Solid and liquid end-products were characterized by a combination of techniques including liquid analyses, transmission and scanning electron microscopies, X-ray diffraction, N2 adsorption at 77 K, and Mössbauer spectroscopy. The interaction between CF and metallic iron appeared to proceed by means of pathways similar to those illustrated in previous studies on interactions between metallic iron and purified clays. In spite of the many similarities with previous studies, significant differences were observed between the behavior of COx and CF, particularly in terms of pH and Eh evolution, iron consumption, chemical composition of the neoformed particles, and textural evolution. Such differences demonstrate the important role played by non-clay minerals in reaction pathways. The addition of carbonates or pyrite to CF did not lead to significant change in reactivity. In contrast, under the conditions used in the present study, i.e. for relatively low iron:clay ratios, the presence of quartz strongly influenced reaction pathways. In the presence of quartz, magnetite was observed only in trace abundances whereas the amounts of magnetite were significant in experiments without quartz. Furthermore, filamentous serpentine particles with a small Al:Si ratio appeared which could develop from an FeSiAl gel that only forms in the presence of quartz. Considering that most clay rocks currently being considered for radioactive waste disposal contain significant amounts of quartz, the results obtained in the present study may be of significant interest for predicting the long-term behavior of clay barriers in such sites.

Effect of clay silylation on curing and mechanical and thermal properties of unsaturated polyester/montmorillonite nanocomposites

This work focuses on the chemical modification of montmorillonite (MMT) (Cloisite® Na) with compatible silanes, vinyltriethoxysilane (CVTES) and γ-methacryloxypropyltrimethoxysilane (CMPS) in order to prevent agglomeration and to improve montmorillonite interaction with an unsaturated polyester resin matrix seeking to achieve a multifunctional composite. Clays were dispersed in the resin by mechanical stirring and sonication and the nanocomposites were prepared by resin transfer into a mold. The mechanical, morphological, thermal and flammability properties of the obtained composites were compared with those prepared using commercial Cloisite® 30B (C30B) and Cloisite® 15A (C15A) clays. Advantages of using silane-modified clays (CVTES and CMPS) as compared with organic-modified clays (C30B and C15A) can be summarized as similar flexural strength and linear burning rate but higher storage modulus and improved adhesion to the polyester resin with consequent higher thermal deflection temperature and reinforcement effectiveness at higher temperatures. However, organic modified clays showed better dispersion (tendency to exfoliate) and consequently delayed thermal volatilization due to the clay barrier effect.

A clay permeable reactive barrier to remove Cs-137 from groundwater: Column experiments

Clay minerals are reputed sorbents for Cs-137 and can be used as a low-permeability material to prevent groundwater flow. Therefore, clay barriers are employed to seal Cs-137 polluted areas and nuclear waste repositories. This work is motivated by cases where groundwater flow cannot be impeded. A permeable and reactive barrier to retain Cs-137 was tested. The trapping mechanism is based on the sorption of cesium on illite-containing clay. The permeability of the reactive material is provided by mixing clay on a matrix of wood shavings. Column tests combined with reactive transport modeling were performed to check both reactivity and permeability. Hydraulic conductivity of the mixture (10−4 m/s) was sufficient to ensure an adequate hydraulic performance of an eventual barrier excavated in most aquifers. A number of column experiments confirmed Cs retention under different flow rates and inflow solutions. A 1D reactive transport model based on a cation-exchange mechanism was built. It was calibrated with batch experiments for high concentrations of NH4+ and K+ (the main competitors of Cs in the exchange positions). The model predicted satisfactorily the results of the column experiments. Once validated, it was used to investigate the performance and duration of a 2 m thick barrier under different scenarios (flow, clay content, Cs-137 and K concentration).

Review of Risk Status of Groundwater Supply Wells by Tracing the Source of Coliform Contamination

Coliform source tracking was undertaken on 48 water sources of which 42 are potable in 26 water supply systems spread across South Australia. The water sources in the study vary from unprotected springs in creek beds to deep confined aquifers. The frequency analysis of historical coliform detections indicate that aquifer types, depth to water and casing depth are important considerations; whilst maintaining well integrity and the presence of low permeable clay layers above the production zone are the dominant parameters for minimizing coliform contamination of water supply wells. However, in karst and fractured rock aquifers, pathways for coliform transport exist, as evidenced in the &gt;200 MPN/100 mL level of coliform detection. Data indicate that there is no compelling evidence to support the contention that the wells identified as low risk are contaminated through geological strata and clay barriers. However, data strongly supports the suggestion that coliform detection from sample taps and wellheads stem from the surrounding groundwater and soil-plant sources as a result of failed well integrity, or potentially from coliform bacteria that can persist within biofilms formed on well casings, screens, pump columns and pumps. Coliform sub-typing results show that most coliform bacteria detected in town water supply wells are associated with the soil-water-plant system and are ubiquitous in the environment: Citrobacter spp. (65%), Enterobacter spp. (63%), Pantoea spp. (17%), Serratia spp. (19%), Klebsiella spp. (34%), and Pseudomonas spp. (10%). Overall, 70% of wells harbor detectable thermotolerant coliforms (TTC) with potentially 36% of species of animal origin, including Escherichia coli species found in 12% of wells.

Subsurface Barriers for Hazardous Waste Containment

Industrial and waste management activities have contaminated soil and groundwater. Subsurface contamination poses a continuing risk to human health and environment. Liquid contaminants can migrate through the soil matrix and leach into the ground water, while solid and semi-solid pollutants may be transported and dispersed through the subsurface. Because clean-up technologies, when available for subsurface containment can be costly and time consuming, it is necessary to examine other, possibly cheaper, ways to reduce the risk and protect human health and the environment at contaminated sites. Subsurface engineered barriers have been used to isolate hazardous waste from contact, precipitation, surface water and groundwater. Barriers are currently used for the containment of contaminated waste, as an interim step while final remediation alternatives are developed or decided and in coordination with treatment technologies. Engineered barriers are constructed containment systems that control horizontal migration of groundwater. Vertical barriers typically used to control sources of hazardous waste are soil-bentonite, soil-cement bentonite, cement bentonite, sheet pile (steel or high density polyethylene - HDPE) and clay barriers.

The Role of Smectite Clay Barriers for Isolating High-Level Radioactive Waste (HLW) In Shallow and Deep Repositories

The major engineered barriers to migration of radionuclides from HLW in repositories are the canister and surrounding smectite clay. They interact physically and chemically by which the properties of both are changed, especially of the smectite “buffer” clay that is examined in the paper. The canisters are made of copper-lined iron according to the Swedish and Finnish concepts, steel being an alternative. The function of the host rock is of importance and the paper examines the role of two repository concepts with long subhorizontal or deep vertical holes for placing the waste.The hydraulic conductivity of the canister-embedding smectite clay can be significantly raised by high temperature and temperature gradients, which generate precipitation of salt and silica in different parts of the buffer clay. The impact of the degrading processes on the waste-isolating capacity is different for shallow repositories in permeable rock and for very deep disposal with higher temperature. The latter has stagnant groundwater as major barrier to the migration of radionuclides.

Analytical model for 1-D contaminant diffusion through clay barriers

Studies on diffusion transport of contaminants through clays are vitally important for performance assessment of waste disposal facilities. Laboratory through-diffusion (double-reservoir) experiment represents a well-established and routinely used technique for the determination of design mass transport parameters in a single test. Availability of accurate and simple theoretical models is valuable for simulating diffusion transport of contaminants through clays for the design and evaluation processes of many waste disposal facilities. The theoretical solutions for this problem are commonly obtained by numerically solving the governing diffusion equation in combination with finite mass boundary conditions. The lack of analytical solutions to this problem encouraged the development of numerical solutions. However, a high computational cost is associated with such time-marching numerical solutions. Closed-form analytical solutions are proposed, in this work, for one-dimensional (1D) transient through diffusion transport of sorptive contaminants through homogeneous clays. Proposed analytical solutions are compared with the solution obtained using existing methods on experimental data. The proposed analytical solutions are simple and accurate for theoretically estimating transient diffusion transport of sorptive contaminants through clays. The proposed model has ready application for analysing routine laboratory through-diffusion experimental data.

Electric–hydraulic–chemical coupled modeling of solute transport through landfill clay liners

Pollutant migration in dense clay barriers appears to be strongly influenced by the electric double layer of colloidal surfaces. Osmosis that resulted from chemical potential or electric potential difference across the clay membrane has been successively described in a number of theoretical works. Streaming potential (SP) which is present in charged porous medium under hydraulic gradient has been recognized as a significant factor governing the mass migration in compacted clays. However, few studies have been carried out in geo-environmental area with regard to this physical phenomenon. A coupled model was proposed to account for the effects of electrical, chemical and fluidic fields on solute transport in porous medium in this study. The electrical field deals with both the streaming potential and the externally applied electrical potential. The coupled nonlinear partial differential equations are numerically simulated by finite element method. Both the steady state solution and the time-dependent solution were investigated with the consideration of a series of influential factors. The streaming potential coefficient and the electro-osmotic coefficient were found to control the solute transport process. The potential application of the materials with tendency of producing SP was discussed. With appropriate selection of materials and parameters, optimum barring effect could be obtained for soil barriers in waste containment applications.

Effects of Chemical Structure on the Stability of Smectites in Short-Term Alteration Experiments

AbstractBecause of their isolating capacity, smectite-rich clays have been proposed as buffer and backfill materials in high-level radioactive waste repositories. These repositories have to guarantee long-term safety for ~1 million years. Thermodynamics and kinetics of possible alteration processes of bentonite determine its long-term performance as a barrier material. Smectites in 25 different clays and bentonites were investigated in order to identify possible differences in their rates of alteration. These samples were saturated for 30 days in 1 M NaCl solution and deionized water, and then overhead rotated at speeds of 20 rpm and 60 rpm. Depending on the octahedral and interlayer composition, each of the smectites studied had specific rate of alteration, a so-called specific dissolution potential of smectite. The bentonites were classed as ‘slow-reacting bentonite’, ‘moderate-reacting bentonite’, or ‘fast-reacting bentonite’ corresponding to a relatively low (ΔP specific dissolution potential — &lt;-5%), moderate (-5% &lt; ΔP &lt; -20%), or high specific dissolution potential (ΔP &gt; -20%), respectively. The larger the amount of octahedral Fe and Mg compared to octahedral Al, the greater the specific dissolution potential. The present study found that the interlayer composition has a discernible impact on the rate of alteration. In experiments with rotation speeds of 60 rpm and a 1 M NaCl solution, Na+ was found to be the stabilizing cation in the interlayers of all the smectites. The Na-stabilizing mechanism was identified in only some of the smectites (type A) in experiments with 20 rpm (1 M NaCl solution). A second stabilization mechanism (by interlayer cations; Ca and Mg) was identified for other smectites (type B). Each bentonite has a specific rate of alteration. ‘Slow-reacting bentonite’ and clay with smectite-illite interstratifications are recommended as potential clay barriers in HLW repositories. The experimental and analytical procedures described here could be applied to potential barrier materials to identify ‘slow-reacting bentonite’.

Bentonite evolution at elevated pressures and temperatures: An experimental study for generic nuclear repository designs

Geologic disposal of spent nuclear fuel in high-capacity metal canisters may reduce the repository footprint, but it may yield high-thermal loads (up to 300 °C). The focus of this experimental work is to expand our understanding of the hydrothermal stability of bentonite clay barriers interacting with metallic phases under different geochemical, mineralogical, and engineering conditions. The hydrothermal experiments were performed using flexible Au/Ti Dickson reaction cells mounted in an externally heated pressure vessel at 150-160 bars and temperatures up to 300 °C for five to six weeks. Unprocessed Wyoming bentonite, containing primarily montmorillonite with minor amount of clinoptilolite, was saturated with a K-Ca-Na-Cl-bearing water (~1900 mg/L total dissolved solids) at a 9:1 water:rock mass ratio. The bentonite and solution combination contained either steel plates or Cu-foils and were buffered to low Eh using magnetite and metallic iron. During reactions, pH, K+, and Ca2+ concentrations decreased, whereas SiO2(aq), Na+, and SO42- concentrations increased throughout the experiments. Pyrite decomposition was first observed at ~210 °C, generating H2S(aq,g) that interacted with metal plates or evolves as a gas. The aqueous concentrations of alkali and alkaline earth cations appear to be buffered via montmorillonite and clinoptilolite exchange reactions. Illite or illite/smectite mixed-layer formation was significantly retarded in the closed system due to a limited K+ supply along with high Na+ and SiO2(aq) concentrations. Precursor clinoptilolite underwent extensive recrystallization during the six weeks, 300 °C experiments producing a Si-rich analcime in addition to authigenic silica phases (i.e., opal, cristobalite). Analcime and feldspar formation partially sequester aqueous Al3+, thereby potentially inhibiting illitization. Associated with the zeolite alteration is a ~17% volume decrease (quartz formation) that translates into ~2% volume loss in the bulk bentonite. These results provide chemical information that can be utilized in extending the bentonite barriers’ lifetime and thermal stability. Zeolite alteration mineralogy and illitization retardation under these experimental conditions is important for the evaluation of clay barrier long-term stability in a spent nuclear fuel repository.

Impact of basin burial and exhumation on Jurassic carbonates diagenesis on both sides of a thick clay barrier (Paris Basin, NE France)

Several diagenetic models have been proposed for Middle and Upper Jurassic carbonates of the eastern Paris Basin. The paragenetic sequences are compared in both aquifers to propose a diagenetic model for the Middle and Late Jurassic deposits as a whole. Petrographic (optical and cathodoluminescence microscopy), structural (fracture orientations) and geochemical (δ18O, δ13C, REE) studies were conducted to characterize diagenetic cements, with a focus on blocky calcite cements, and their connection with fracturation events. Four generations of blocky calcite (Cal1–Cal4) are identified. Cal1 and Cal2 are widespread in the dominantly grain-supported facies of the Middle Jurassic limestones (about 90% of the cementation), whereas they are limited in the Oxfordian because grain-supported facies are restricted to certain stratigraphic levels. Cal1 and Cal2 blocky spars precipitated during burial in a reducing environment from mixed marine-meteoric waters and/or buffered meteoric waters. The meteoric waters probably entered aquifers during the Late Cimmerian (Jurassic/Cretaceous boundary) and Late Aptian (Early Cretaceous) unconformities. The amount of Cal2 cement is thought to be linked to the intensity of burial pressure dissolution, which in turn was partly controlled by the clay content of the host rocks. Cal3 and Cal4 are associated with telogenetic fracturing phases. The succession of Cal3 and Cal4 calcite relates to the transition towards oxidizing conditions during an opening of the system to meteoric waters at higher water/rock ratios. These meteoric fluids circulated along Pyrenean, Oligocene and Alpine fractures and generated both dissolution and subsequent cementation in Oxfordian vugs in mud-supported facies and in poorly stylolitized grainstones. However, these cements filled only the residual porosity in Middle Jurassic limestones. In addition to fluorine inputs, fracturation also permitted inputs of sulphur possibly due to weathering of Triassic or Purbeckian evaporites or H2S input during Paleogene times.

Tunable staged release of therapeutics from layer-by-layer coatings with clay interlayer barrier

In developing new generations of coatings for medical devices and tissue engineering scaffolds, there is a need for thin coatings that provide controlled sequential release of multiple therapeutics while providing a tunable approach to time dependence and the potential for sequential or staged release. Herein, we demonstrate the ability to develop a self-assembled, polymer-based conformal coating, built by using a water-based layer-by-layer (LbL) approach, as a dual-purpose biomimetic implant surface that provides staggered and/or sustained release of an antibiotic followed by active growth factor for orthopedic implant applications. This multilayered coating consists of two parts: a base osteoinductive component containing bone morphogenetic protein-2 (rhBMP-2) beneath an antibacterial component containing gentamicin (GS). For the fabrication of truly stratified composite films with the customized release behavior, we present a new strategy—implementation of laponite clay barriers—that allows for a physical separation of the two components by controlling interlayer diffusion. The clay barriers in a single-component GS system effectively block diffusion-based release, leading to approximately 50% reduction in bolus doses and 10-fold increase in the release timescale. In a dual-therapeutic composite coating, the top GS component itself was found to be an effective physical barrier for the underlying rhBMP-2, leading to an order of magnitude increase in the release timescale compared to the single-component rhBMP-2 system. The introduction of a laponite interlayer barrier further enhanced the temporal separation between release of the two drugs, resulting in a more physiologically appropriate dosing of rhBMP-2. Both therapeutics released from the composite coating retained their efficacy over their established release timeframes. This new platform for multi-drug localized delivery can be easily fabricated, tuned, and translated to a variety of implant applications where control over spatial and temporal release profiles of multiple drugs is desired.

Molecular Dynamics Simulations of Water and Sodium Diffusion in Smectite Interlayer Nanopores as a Function of Pore Size and Temperature

The diffusion coefficients (D) of water and solutes in nanoporousNa-smectite clay barriers have been widely studied because of their importancein high-level radioactive waste (HLRW) management and ...

Effects of compression on porous texture of clay powder: Application to uranium diffusion

This paper presents the results of tests conducted on the compaction properties of clay. Swelling clays of Algeria are being examined as components of the buffer material. Knowledge of the porous texture in clay samples is a useful element in the microstructural characterization of such materials. The prepared materials were characterized by X-ray diffraction, Differential Thermal and Thermo Gravimetric Analysis, Laser Granulometry, Scanning Electron Microscopy, mercury porosimetry and nitrogen adsorption/desorption to obtain information about their structure and surface texture. The cationic exchange of clay minerals, in particular smectites, influences the cation retention and diffusion processes. These processes influence the migration of contaminants from waste disposal sites through clay barrier. In the present work we studied the effect of cation exchange and compaction on the textural properties of clay. Pellets of the Na-saturated and bulk clay samples were prepared by compression of the powder samples using uniaxial stress at different pressures in order to obtain different densities. The pressures of 31–127MPa are applied for the production of cylindrical samples with diameter of 20mm and heights of 15–30mm. The effect of compaction of clay on the diffusion behavior of uranium was studied for the safety assessment of the radioactive waste. The diffusion process of uranium in compacted clay has been modeled by second Fick's law taking into account the effect of sorption and considering the non-steady state. The diffusion coefficients and profiles concentration values of uranium were calculated by computational method using numerical program based on Newton–Raphson algorithm.

Competitive effect of the metallic canister and clay barrier on the sorption of Eu3+ under subcritical conditions

An in depth knowledge and understanding of high activity radionuclide (HLRW) immobilization processes on the materials composing the engineered barrier (clay and metallic canister) is required to ensure the safety and the long-term performance of radioactive waste disposal procedures. Therefore, the aim of this study was to understand the mechanisms involved in the retention of Eu3+ by two components of the multibarrier system, the bentonite barrier and the canister. As such, a comparative study of the interaction of trivalent Eu3+, used to simulate trivalent actinides, with both bentonite and a metallic canister has been undertaken in this work. To this end, we designed a minireactor into which the bentonite was introduced and compacted. The minireactor-bentonite system was then submitted to a hydrothermal reaction with a 7.9×10−2M solution of Eu3+ at 300°C for 4.5days. SEM and XRD results revealed that both bentonite and the container were involved in the immobilization of europium by the formation of insoluble europium silicate phases. The presence of europium silicate adsorbed on the surface of the metallic canister indicates the competitive effect of both components of the engineered barrier (bentonite and metallic canister) in HLRW immobilization. These results suggested that the canister could play a role in the HLRW immobilization even during its corrosion process.

폴리스티렌-클레이 나노 복합재료의 합성 및 차단 특성에 관한 연구

고성능 고분자/클레이 나노 복합재료의 제조 과정에는 친수성을 보이는 클레이 원료 물질인 $Na^+$ -MMT (sodium monmorilonite)를 친유성을 갖도록 유기화된 계면활성제로처리하여 개질하는 과정이 필수적이다. 이를 위하여 이 연구에서는 VDAC (vinylbenzyldimethyl-dodecylammonium chloride)를 간단한 화합물로부터 합성하였고 이를 이용하여 양이온 교환반응에 의하여 $Na^+$ -MMT를 개질한 후 $VDA^+$ -MMT를 제조하였다. 이를 스티렌과 혼합하여 in-situ 중합에 의하여 나노복합재료를 제조하였고 클레이의 분산성 및 차단특성을 연구하였다. 연구 결과 PS/ $VDA^+$ -MMT 나노 복합재료의 경우 클레이의 분산이 $Na^+$ -MMT와 비교할 때 현저히 증가함을 확인하였고 이로 인해 유기 용매에 대한 차단 특성이 매우 우수함을 확인하였다. 【In prepaparation of the high performance polymer/clay nanocomposite, it is essential to modify the hydrophillic $Na^+$ -MMT to hydrophobic alkyl ammonium-MMT via organic surfactant. The organic surfactant, VDAC (vinylbenzyldimethyl-dodecylammonium chloride) was synthesized from two primary chemicals and $VDA^+$ -MMT was prepared from $Na^+$ -MMT through a cation exchange reaction between $Na^+$ and $VDA^+$ (vinylbenzyldimethyl- $dodecylammonium^+$ ) cation. $VDA^+$ -MMT was then dispersed in styrene and polystyrene/ $VDA^+$ -MMT nanocomposite was fabricated by in-situ polymerization reaction. The clay dispersion and barrier property of the nanocomposite were investigated. From the investigations, it was confirmed that dispersion of the $VDA^+$ -MMT was enhanced compared with that of $Na^+$ -MMT and as a consequency of better dispersion, barrier property of organic solvent was improved in a great extent.】

Homogenization in clay barriers and seals: Two case studies

The paper presents two case studies that provide information on the process of homogenization of initially heterogeneous clay barriers and seals. The first case is the canister retrieval test performed in the Aspö Hard Rock Laboratory (Sweden). The heterogeneity arises from the use of a combination of blocks and pellets to construct the engineered barrier. The degree of homogenization achieved by the end of the tests is evaluated from data obtained during the dismantling of the test. To assist in the interpretation of the test, a fully coupled thermo-hydro-mechanical (THM) analysis has been carried out. The second case involves the shaft sealing test performed in the HADES underground research laboratory (URL) in Mol (Belgium). Here the seal is made up of a heterogeneous mixture of bentonite pellets and bentonite powders. In addition to the full scale test, the process of homogenization of the mixture has also been observed in the laboratory using X-ray tomography. Both field test and laboratory tests are successfully modelled by a coupled hydro-mechanical (HM) analysis using a double structure constitutive law. The paper concludes with some considerations on the capability of highly expansive materials to provide a significant degree of homogenization upon hydration.

Modeling specific pH dependent sorption of divalent metals on montmorillonite surfaces. A review of pitfalls, recent achievements and current challenges

Within the context of the clay barrier concept for underground nuclear waste disposal, montmorillonite and bentonite have been widely used as reference materials for sorption. In some cases, accompanying modeling work aims at understanding and predicting sorption in complex natural systems where clays are assumed to be representative of the most reactive phases. This bottom-up approach relies heavily on good confidence in the mechanistic understanding of sorption phenomena. The present study aims at reviewing experimental and modeling work on montmorillonite with a focus on divalent metals experiencing pH dependent specific sorption. Current knowledge points out distinct sorption mechanisms on three types of sites: cation exchange on basal planes and surface complexation on edge surfaces with two types of sites: high energy (or strong) sites (HES) with high affinity for metals but low site density and low energy (or weak) sites (LES) with lower affinity for metals but high site density. Based on this current knowledge, criteria are given to select data relevant for surface complexation model calibration (especially ionic strength, pH, clay preparation and characterization, metal to clay ratio and solubility limits), with an emphasis on data uncertainties and reproducibility. Problematic experimental features are highlighted, especially those related to the reversibility of sorption and to the effect of the solid to liquid ratio (R~SL~) on sorption distribution coefficients. Guidelines for data acquisition and selection are proposed. Surface complexation models available in the literature are then tested in terms of efficiency (data fit) and mechanistic likelihood. None of the currently available models is able to satisfy both aspects. Models directly adapted from oxide surface complexation models fail in both aspects. The most efficient model (in terms of simplicity and accuracy) is a non-electrostatic model. It is the only one that reproduces pH dependent specific sorption data at a low metal clay ratio (\<0.001 mol/kg~clay~; HES) in all selected experimental conditions, as well as data obtained at medium metal to clay ratio (∼0.01-0.05 mol/kg~clay~; low energy sites). To account for physical mechanisms, an electrostatic surface complexation model has been developed. It takes into account the spill-over effect of negatively charged basal surfaces over edge surfaces, a typical feature of montmorillonite, and is able to reproduce sorption data for LES but not for HES. The reasons for this failure are explained through the mathematical derivations of model equations. This approach shows that it is impossible to reconcile HES properties with an oxide-like surface complexation electrostatic model. Amongst other alternatives, a successful electrostatic surface substitution model, which is compatible with current knowledge on HES structural properties, is proposed.