Clay Barriers Research Articles

Technetium stabilization in Portland cement and bentonite clay barriers by thiourea

Technetium stabilization in Portland cement and bentonite clay barriers by thiourea

Controlling metal ion migration in contaminated groundwater with Iraqi clay barriers for water resource protection.

This study investigates the effectiveness of using Iraqi clay as a low-permeability layer to prevent the migration of lead and nickel ions in groundwater-aquifers. Tests of batch operation have been conducted to determine the optimal conditions for removing Pb2+ ions, which were found to be 120 minutes of contact time, a pH of 5, 0.12 g of clay per 100 mL of solution, and an agitation of 250 rpm. These conditions resulted in a 90% removal efficiency for a 50 mg L-1 initial concentration of lead ions. To remove nickel ions with an efficiency of 80%, the optimal conditions were 60 minutes of contact time, a pH of 6, 12 g of clay per 100 mL of solution, and an agitation of 250 rpm. Several sorption models were evaluated, and the Langmuir formula was found to be the most effective. The highest sorption capacities were 1.75 and 137 mg g-1 for nickel and lead ions, respectively. The spread of metal ions was simulated using finite element analysis in the COMSOL multiphysics simulation software, taking into account the presence of a clay barrier. The results showed that the barrier creates low-discharge zones along the down-gradient of the barrier, reducing the rate of pollutant migration to protect the water sources.

Clay–rock fracturing risk assessment under high gas pressures in repository systems

At the interface between the steel canister and the bentonite in a nuclear waste repository, we expect generation of hydrogen gas because of corrosion processes. The pressurized gas might fracture the engineered or natural clay barrier system, enhancing radionuclide transport into the geosphere. To assess the long-term integrity of the clay host rock under various conditions and scenarios, we need a large number of numerical simulations. However, a simulation tool for complex fracture propagation is often prohibitively expensive to run many realizations. Here, we developed a risk analysis tool based on the Design of Experiments to overcome the computational challenges by generating a computationally inexpensive proxy fracture model using a set of critical factors known as heavy hitters. We provided parameters and their probability distributions that are subject to uncertainty, as well as an objective function that assesses the risk of fracturing due to high gas pressures. Through various scenarios, we found that the fracture toughness dominates the impact on the risk.

Modeling gas breakthrough and flow phenomena through engineered barrier systems using a discrete fracture approach

Compacted clays are being considered to build engineered barrier systems (EBS) intended for the safe isolation of high-level nuclear waste (HLW) and spent nuclear fuel (SNF). The corrosion of the metallic canister containing the HLW/SNF will lead to the generation and buildup of the gas pressure in the more internal part of the clay buffer. This phenomenon would eventually trigger the formation and propagation of fractures in the clay barrier, jeopardizing its safety functions. In this work we propose to use the fragmentation technique (MFT) to model evolving fractures in clays triggered by gas pressurization. The MFT has been successfully used to model the formation of fractures in concrete, drying cracks in soil, hydraulic and thermo-fractures in rocks. In this work, we extend the MFT to deal with multiphase fluid flow in deformable porous media, and we upgraded a fully coupled computer finite element code using the extended technique. The proposed approach is first verified against analytical solutions and is then applied to model gas breakthrough experiments in clays. A very satisfactory performance of the method is observed in all the analyses, showing the potential of the MFT to tackle multiphase flow problems in deformable porous media with evolving discontinuities.

Cassava peel ash modified black cotton soil as material for hydraulic barriers in municipal solid waste containment facility

This study investigates the potential of cassava peel ash (CPA) modified black cotton soil as a compacted clay barrier in a waste containment facility. Classification of the soil sample and chemical composition of cassava peel ash used were determined. Laboratory determination of some design parameters such as hydraulic conductivity, volumetric shrinkage and unconfined compressive strength of CPA-black cotton soil mixture were carried out using three compactive methods (Standard Proctor, SP, West African Standard, WAS and Modified Proctor, MP) at a stepped increment of 4% up to 24% CPA by dry weight of compacted soil prepared with water contents 2% dry of OMC, optimum moisture content (OMC), 2 and 4% wet of OMC. The cation exchange capacity (CEC) of the natural and treated specimens was determined via batch equilibrium adsorption test, and finally, the microstructure of optimally blended specimens was studied through scanning electron microscopy. The natural black cotton soil was grouped as A-7-6 (31) based on the AASHTO system and as CH (clay with high plasticity) using USCS. This study revealed that at 16% CPA and compacted within the range of 9.6-15.3% water content, black cotton soil–CPA blend compacted with not lower than Standard Proctor compaction energy fulfil design parameters requirements for its application as liner/cover in municipal waste repository facility. • Pozzolanic behaviour of CPA for the treatment of expansive soil was established. • Black cotton soil-CPA blend was found to be suitable as compacted barrier material in MSW landfill. • Landfill barrier parameters were fulfilled at 16% CPA by mass of dry soil when compacted at 9.6-15.3% water content.

Biological factors affecting the evolution of safety barrier materials in the Yeniseisky deep geological repository

This article presents information on the microbial impact on the stability of cement, clay and steel engineering safety barriers in a deep geological repository (DGR) for radioactive waste in the gneiss crystal massif at the Yeniseisky test site (Krasnoyarsk region, Russia). Based on microbial diversity data collected from groundwater and bentonite clay core samples in the proposed DGR construction area, the most likely microbial processes are steel corrosion, cement degradation and changes in clay barrier properties. This study establishes the synergistic effect of chemical and microbial processes in engineered barrier materials. An important aspect of engineering barrier safety is clay materials, which are simultaneously a source of microflora, organic matter and elements necessary for microbial development. It should be noted that the microflora of clay materials includes predominantly aerobic organotrophic bacteria introduced during the technical preparation of clays. The rate of microbial processes may increase at elevated temperatures or in contact with hydrogen and iron-containing products of steel corrosion. An initial assessment of the risk of intensification of microbial processes in protective barriers has been carried out; biocidal additives suppressing microbial activity in clay material and Portland cement have been proposed.

Functionalized Clays for Radionuclide Sequestration: A Review

Performance assessment of nuclear waste disposal options requires implementation of effective buffer materials. Buffer materials must meet longevity requirements and scavenge challenging radioisotopes, e.g., iodine-129 (I-129) and technetium-99 (Tc-99), both long-lived, highly mobile, anionic species in oxic environments. In many proposed nuclear waste repositories, heat generating radioactive waste will be surrounded by clay buffer material, which swells to fill the gap between the waste package and the host geology, restricting radionuclide transport to diffusive processes for several hundred years. The clay can be functionalized to further the limit release of Tc-99, as pertechnetate (TcO4–), and I-129, as iodate (IO3–) and iodide (I–). Here, we review clay buffer functionalization and the potential to enhance sequestration of anionic radionuclides that have an increased risk of leaching out of a disposal facility. Functionalization with quaternary amines, nanosilver, bismuth-based materials, silane, and aluminum pillars have been demonstrated to improve the removal of targeted contaminants from solution. This review defines key research questions that remain: (i) can functionalized clay immobilize TcO4–, IO3–, and I– to the same extent as the reactive components themselves? (ii) Does functionalization affect the ability of the clay barrier to perform its safety functions of protecting the waste canister and limiting radionuclide transport? In addition to radionuclide retention, a multimodal analytical approach is required to assess functionalized clay performance, and recommended techniques are reviewed here. The key parameters that can influence radionuclide sequestration by functionalized clay are also evaluated. Theoretical approaches, including both density functional theory calculations and molecular dynamics simulations, can inform the energetics and kinetics of radionuclide interactions with functionalized clays. This combined experimental and theoretical approach can link atomistic and microscopic processes to predict the macroscopic physical properties of the functionalized clays.

Thermal effects of the hydraulic conductivity and threshold gradient of sand–clay liners in municipal solid waste landfills

High leachate heads and dry cracking of clay barriers are common problems in landfills. The use of sand-clay barriers with a non-darcy percolation patterns can not only prevent leakage of large amounts of leachate but can also potentially reduce the development of dry fractures due to loss of water. Thus, it has been suggested that sand-clay materials could be used as barriers at the bottoms of landfills or as a vertical barriers. However, biochemical degradation of waste in a landfill generates much heat, and the temperature can reach 60 °C. A high-temperature field can inevitably affect the permeability of sand-clay barriers. The change in sand-clay barrier permeability under the influence of temperature has received increasing attention. Therefore, in this study, a temperature-controlled permeameter with flexible walls was used to conduct experiments on permeability of sand-clay mixtures at four temperatures. The results indicated that the hydraulic conductivity gradually increased and the threshold gradient gradually decreased with increasing temperature. The hydraulic conductivity and threshold gradient exhibited exponential relationship with the temperature, respectively. Experiments were designed to measure changes in the permeate viscosity at different temperatures. The previously established hydraulic conductivity equation (that considered only the influence of temperature on the permeate viscosity) greatly differed from the measured values. In contrast, the newly established hydraulic conductivity equation (that considered the influence of temperature on both the permeate viscosity and intrinsic permeability) agreed well with the measured values. Statistics based on a large amount of data (collected previously) showed that the intrinsic clay permeability exhibited a power law relationship, with increasing temperature to a threshold gradient at 25 °C (Eq. (9)). Above the threshold, the intrinsic permeability experimental data increasingly deviated from Eq. (9). To consider the effect of the temperature on both the intrinsic permeability and permeate viscosity, an equation for calculation of the threshold gradient was proposed, and it could be found that the calculated values suitably agreed with the measured values. The results of this study provide an important theoretical basis and data to support the development of high-performance barriers with long service life for landfills.

Investigating the effect of hematite additive on the performance of clay barriers for radioactive waste disposals

Investigating the effect of hematite additive on the performance of clay barriers for radioactive waste disposals

Effect of red mud additive on the performance of clay barriers for radioactive waste disposals: an experimental study

Effect of red mud additive on the performance of clay barriers for radioactive waste disposals: an experimental study

Clay and carbon materials-based engineered barriers for technetium immobilization

Immobilization of long-lived 99Tc is one of the most complicated tasks faced during disposal of radioactive waste. Bentonite clays, which are common components used for construction of engineered barriers are not capable of preventing migration of highly soluble oxidized technetium forms. Modification of bentonite clay materials with natural and industrial coals of various genesis, added in amount of 5 mass %, was carried out as part of this research. The anthracite from Omsukchan coal cluster, coke of Kuznetsk basin and activated carbons of AG-3 and CAU marks have demonstrated high ability to immobilize technetium under oxidizing conditions with values of partition coefficient exceeding 103 ml/g. The resulting composite materials are characterized by low water permeability (Kp < 1–5·10−11 m/s at a dry density of 1.1–1.2 g/cm3), which is mandatory for engineered barriers. Dispersed materials are easy-flowing (ff > 4), which allows the construction of clay barriers by dry, cavity-free backfilling. Quantitative immobilization of technetium occurs during the first day of interaction, with strongly-bonded forms accounting for over 90% of all technetium in the barrier material.

Geotechnical behavior of fly ash-coal ash and bentonite clay composite as a landfill barrier material with special emphasis on desiccation cracks

Compacted clay barriers have been used to retard leakage of contaminated fluids from landfillsites. These liners may shrink, crack, desiccate and lose their integrity if the liner material doesn't have permeability (10−9 m/s), unconfined compressive strength (UCS = 200 kPa) and no desiccation cracks at all as per the CPCB-2001 and USEPA 1989 guidelines. These three parameters form the basis of utilizing any material as a liner material however bentonite clay (BC) due to excessive desiccation cracks upon drying and wetting events can't be used individually. The usual practice to overcome failure, of barrier material the usual practice is to amend BC with sand though being costly and unsustainable. Recently, Fly ash (FA) and Coal ash (CA) byproducts from thermal power plant, have been explored as a substitute of sand in landfill liner material for sustainable practices. The purpose of this study is to investigate and and compare the various composition of B-FA and B-CA in terms of adequate strength, permeability, and shrinkage/desiccation in such a way to meet the CPCB-2001 and USEPA 1989 guidelines. The variation of specific gravity, liquid limit, plastic limit, and DFSI with varying compositions of bentonite was also studied. The present study demonstrates that with the increment of BC permeability was found to decrease and 80CA20B, 80FA20B, 80FA15B5CA combinations were meeting CPCB-2001 permeability criterion. To replicate the field condition the overburdening pressure was varied from 100 to 800 kPa and the permeability was found to be within the prescribed range for the three compositions. In total, 72 UCS, including triplicates for 7, 14 and 28 days, were conducted to see if any pozzolanic reaction was ensuingand its effect on the strength. The maximum UCS was found to be 302 kPa for 80CA20B composition while 277 kPa for 80FA20B. Strength Improvement Factor (SIF) and Mobilized peak strain factor (MPSF) were found to be maximum for the 80CA20B composition for 28 days as 1.54 and 0.44 respectively. The composition 80CA20B demonstrated better desiccation crack resistance behaviour than 80FA20B and 80FA15B5CA. The volumetric shrinkage for 80CA20B, 80FA20B and 80FA15B5CA were 4.6, 10.49 and 4.2%. Further, there were no desiccation cracks observed if the volumetric shrinkage is around 4%. The water retention behavior of all the three compositions was almost same. The microstructure analysis for morphology, compositional characterization, and thermal analysis of the various constituents was also carried out.

The role of diffusion induced electro-osmosis in the coupling between hydraulic and ionic fluxes through semipermeable clay soils

Most of the experimental research conducted to date has provided evidence on the semipermeable membrane behaviour of smectite-rich clay soils, the extent of which is typically quantified through the reflection coefficient, when the permeant (electrolyte) solution contains a single monovalent or divalent salt. Under such conditions, the osmotic flow of solution is controlled to a great extent by the different accessibility of ions and water molecules to the soil porosity, which is referred to as the chemico-osmotic effect. However, theoretical simulations of coupled solute and solvent transport suggest that, when two or more cations that diffuse in water at different rates are present simultaneously in the permeant solution, the electro-osmotic effect, which stems from the condition of null electric current density through the porous medium, can be enhanced compared to the case of a single salt to such an extent that it becomes comparable to or even greater than the chemico-osmotic effect. An original closed-form analytical solution to the problem of calculating the diffusion potential, which in turn controls the magnitude of the electro-osmotic effect, is here illustrated, and the relative importance of the aforementioned contributions to multi-electrolyte systems is examined through the interpretation of laboratory test results from the literature pertaining to a bentonite amended clay soil in equilibrium with aqueous mixtures of potassium chloride (KCl) and hydrochloric acid (HCl). The proposed mechanistic model is shown to be able to quantitatively capture the impact of both chemico-osmosis and electro-osmosis on the measured reflection coefficient of smectite clays, thereby breaking new ground in the experimental and theoretical research on the osmotic properties of engineered clay barriers in contact with mixed aqueous electrolyte solutions.

Semi-empirical modelling of hydraulic conductivity of clayey soils exposed to deionized and saline environments

Clay liners are widely used as porous membrane barriers to control solute transport and to prevent the leakage of leachate both in horizontal and vertical flow scenarios, such as the isolated base and ramps of sanitary landfills. Despite the primary importance of saturated hydraulic conductivity in a reliable simulation of fluid flow through clay barriers, there is no model to predict hydraulic conductivity of clayey soils permeated with saline aqueous solutions because most of the current models were developed for pure water. Therefore, the main motivation behind this study is to derive semi-empirical models for simulating the hydraulic conductivity of clayey soils in the presence of arbitrary solute concentrations in addition to deionized water. In order to achieve this goal, a relatively comprehensive dataset of 842 measured hydraulic conductivities was retrieved from the experimental literature, where almost 44% of them are related to certain solute concentrations. Afterwards, two modelling approaches were introduced; the first one is a modified form of Mbonimpa et al.'s (2002) model, in which the constants are adjusted to take into consideration the variations in liquid limit due to a change in solute concentration. A modification term was added to the model for the sake of accuracy. In the second approach, a new form of solute concentration-dependent hydraulic conductivity function was proposed, where special attention was given to void ratio and adaptive liquid limit as effective parameters. The results revealed that hydraulic conductivity predictions could be erroneous if the influence of solute concentrations in permeating fluid is ignored. An error analysis was conducted to examine the models' applicability and deviations. A blind independent set of data, including 132 data points, was also used to verify models. On the other hand, both newly proposed models could predict the hydraulic conductivity for a variety of soils, salt species, and concentrations well. Therefore, the proposed modelling approaches are somehow unique by considering the salinity of the pore fluid in addition to deionized water. More importantly, both models are comprised of easy-to-measure parameters with clear physics-based implications.

Analysis of shear mock-up tests in a canister-clay system for spent nuclear fuel isolation

The aim of this paper is to simulate a canister-buffer mock-up test using the finite element computer code CODE_BRIGHT. The response of a copper canister put inside clay barriers that undergo hydration and swelling till complete saturation was investigated. Although the copper is very stiff and resistant and will be emplaced in a clay volume, shear deformations occurring in the clay barrier may have an impact on the canister. A Mock-up test was carried out in a steel cell at a scale of about 1:10 for assessing the consequences of a shear zone for a copper rod that was surrounded by compacted bentonite as a clay barrier. The challenge problem is to perform the simulation of the mock-up tests under the different shear rates that was carried out by Borgesson (1968). The results might contribute to a better understanding of the real shearing response of the system and validate the constitutive models used for the simulations.For this purpose, the mock-up test was simulated with CODE_BRIGHT in hydro-mechanical couplings in three dimensions. For the model validation, the results of the simulations were compared with the test measurements. The constitutive model used for the copper simulation was a viscoplastic model developed by Bodner and Partom. For the bentonite simulation, a viscoplastic model for unsaturated soils based on the Desai and Perzyna theory, using an elliptical yield surface corresponding to the modified Cam-Clay model, was used.

Landfill gas emission through compacted clay considering effects of crack pathway and intensity

Compacted clay barrier plays an important role in reducing landfill gas transport due to its low gas permeability. There is limited understanding of desiccation cracks and to what extent they can cause preferential pathways of landfill gas through compacted clay barriers. This study investigated the intensity and pathway of desiccation cracks as well as its effects on gas emission through compacted clay. The compacted clay with and without scratched compaction interface was subjected to drying to simulate desiccation cracks. The clay was then extruded from large containers into one dimensional columns to allow observation of crack propagation using an X-ray computerized tomography scanner. After that, gas emission rate was measured from each column under different gas pressures (i.e., 1, 5, 10 and 20 kPa). Furthermore, a simplified method is proposed to predict gas emission rate with consideration of intensity and characteristics of cracks. Test results demonstrated that desiccation cracks were initiated mainly at the center of each container (i.e., within 40% of container dimension). Gas emission rate can be increased at least 10 times with the presence of desiccation cracks (i.e., at gas pressure of 5 kPa). As compared to the depth and continuous pathway of cracks which significantly increased gas emission rate, the discontinuous crack pathway can reduce the gas emission rate by up to 3 times. The findings towards crack characteristics and gas emission observed in this study are crucial for safety design and long-term operation of compacted clay barriers in landfill covers.

Centrifuge model studies on desiccation cracking behaviour of fiber-reinforced expansive clay

In this study, randomly distributed fiber reinforcement on the desiccation cracking behavior of expansive clay was investigated. Modeling considerations for the desiccation cracking behavior of unreinforced and fiber-reinforced clay was established using dimensional analysis. A custom-designed setup consisting of a specimen container, heating assembly, and digital image acquisition system was designed, calibrated, and used in the present study. A series of desiccation cracking tests were performed on unreinforced and fiber-reinforced expansive clay in a balanced beam geotechnical centrifuge. The test setup performance was evaluated by conducting tests at varying gravity levels and performing modeling of models. Digital image analysis and particle image velocimetry techniques were used to obtain qualitative information about the cracking of clay under the influence of fiber reinforcement and the varying thickness of the clay layers. The specimens reinforced with fibers exhibited improved cracking resistance than unreinforced clay specimens. The results indicate that the desiccation cracking of clays can be successfully modeled in a geotechnical centrifuge, highlighting the fact that this study is the first-ever such study. This knowledge can be used to study the behavior of critical geotechnical structures, especially clay barriers of landfill cover subjected to desiccation cracking.

Characteristics and Rates of Microbial Processes in Clays of Different Mineral and Elemental Composition in Relation to Safety Prediction for ESB Clay Materials

Microbial activity in clay barrier materials during radioactive waste disposal can lead to a violation of important physicochemical properties of the barrier system as a whole, thereby reducing the safety of the storage facility. This work evaluates the activity of the microbial complex of five bentonite and two kaolin clays. All clay materials were found to contain microorganisms, mostly with organotrophic aerobic and anaerobic metabolism capable of forming hydrogen sulfide and transforming basic and impurity clay minerals. The activity of microorganisms can increase with the ingress of degradation products of aluminophosphate matrices and cement barriers, as well as radiolysis products. For all clay the rates of microbial processes were shown to exhibit a direct correlation with the content of organic matter, kaolinite, and potassium feldspar, and an inverse correlation with montmorillonite content. A systematic methodological approach is proposed for clay materials and their susceptibility to microbial processes. The approach makes it possible, based on the content of organic matter, biophilic elements in the samples, parameters of the respiratory activity of the microbial complex, the formation of hydrogen sulfide, and other parameters, to assess in advance the possibility of using them in barriers.

A multi-ion diffusion model through clay barriers under combined action of chemical and diffusion potential

A multi-ion diffusion model through clay barriers under combined action of chemical and diffusion potential

EURADWASTE’22 Paper – Host rocks and THMC processes in DGR

Deep geological disposal aims to contain and isolate radioactive waste from the biosphere. Repository systems are made of multiple barriers working together, typically comprising the natural geological barrier provided by the repository host rock and its surroundings and an engineered barrier system. Due to their excellent properties for the confinement of contaminants, including low permeability, high sorption capacity, and swelling/self-sealing capacity, clayey materials are considered as engineered and/or natural barriers in most repository designs under development in Europe. During the lifetime of the repository, clay barriers will be exposed to perturbations, among which those are resulting from gas and heat production within the system. It is important to verify that these perturbations will not be detrimental to the good functioning of these barriers. In this paper, it is shown how the two EURAD R&amp;D work packages, GAS and HITEC use a combination of experimental and modelling approaches to increase the understanding and predictability of the impact on clay barriers of the fundamental processes and their couplings related to gas and heat transport respectively, providing building blocks to support the evaluation of the robustness of the repository concepts.