Wyoming Montmorillonite Research Articles

Modeling Realistic Clay Systems with ClayCode.

Clays are a broad class of ubiquitous layered materials. Their specific chemophysical properties are intimately connected to their molecular structure, featuring repeating patterns broken by substitutions. Molecular dynamics simulations can provide insight into the mechanisms leading to the emergent properties of these layered materials; however, up to now, idealized clay structures have been simulated to make the modeling process tractable. We present ClayCode, a software facilitating the modeling of clay systems closely resembling experimentally determined structures. By comparing a realistic model to a commonly used montmorillonite clay model, we demonstrate that idealized models feature noticeably different ionic adsorption patterns. We then present an application of ClayCode to the study the competitive barium and sodium adsorption on Wyoming montmorillonite, Georgia kaolinite, and Montana Illite, of interest in the context of nuclear waste disposal.

Determining the stoichiometric composition of Wyoming montmorillonite using improved transmission electron microscopy-energy dispersive X-ray (TEM-EDX) techniques

Abstract Transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDX) represents an effective tool for determining the stoichiometric composition of clay minerals, but the methodology is often hampered by analytical difficulties. Studies of beam-sensitive minerals, such as smectites, are associated with low count intensities and dynamic loss of cations (e.g. K+, Na+, and Al3+), which can lead to erroneous quantifications of composition. After exploring how to minimize cation migration by reducing the beam current density to <5 pA cm–2, the most reliable and consistent compositions were determined using 1 μm2 area measurements of particles acquired in normal TEM mode where the electron beam was parallel, the degree of specimen damage was at its minimum and good acquisition intensities (>10,000 cps) were acquired. Based on 528 TEM-EDX area analyses, the composition of Wyoming montmorillonites (SWy-1, SWy-2, and SWy-3) was studied in their natural and Ca-saturated states from thin (<50 nm thick) particle aggregates lying on lacey carbon films. Overall, the TEM-EDX results confirmed the heterogeneous charge distributions of montmorillonite at the particle and sample levels. The average composition per formula unit of SWy-1 to -3 was determined as: (Na0.12Ca0.04Mg0.03K0.02)(Si3.91Al0.09)(Al1.57Mg0.27Fe0.19)2.03 O10(OH)2 · nH2O, where the tetrahedral and octahedral layer charges are –0.09 and –0.19 per O10(OH)2, respectively, and the total layer charge ranges from –0.25 to –0.30 per O10(OH)2 (mean of –0.28). This study demonstrates how TEM-EDX can provide new insight into the natural heterogeneities of smectite chemistry as long as adequate calibration and specimen damage control procedures are implemented.

Interlayer Cation Polarizability Affects Supercritical Carbon Dioxide Adsorption by Swelling Clays.

Several strategies for mitigating the build-up of atmospheric carbon dioxide (CO2) bring wet supercritical CO2 (scCO2) in contact with phyllosilicates such as illites and smectites. While some work has examined the role of the charge-balancing cation and smectite framework features on CO2/smectite interactions, to our knowledge no one has examined how the polarizability of the charge-balancing cation influences these behaviors. In this paper, the scCO2 adsorption properties of Pb2+, Rb+, and NH4+ saturated smectite clays at variable relative humidity are studied by integrating in situ high-pressure X-ray diffraction (XRD), infrared spectroscopic titrations, and magic angle spinning nuclear magnetic resonance (MAS NMR) methods. The results are combined with previously published data for Na+, Cs+, and Ca2+ saturated versions of the same smectites to isolate the roles of the charge-balancing cations and perform two independent tests of the role of charge-balancing cation polarizability in determining the interlayer fluid properties and smectite expansion. Independent correlations developed for (i) San Bernardino hectorite (SHCa-1) and (ii) Wyoming montmorillonite (SWy-2) both show that cation polarizability is important in predicting the interlayer composition (mol% CO2 in the interlayer fluid and CO2/cation ratio in interlayer) and the expansion behavior for smectites in contact with wet and dry scCO2. In particular, this study shows that the charge-balancing cation polarizability is the most important cation-associated parameter in determining the expansion of the trioctahedral smectite, hectorite, when in contact with dry scCO2. While both independent tests show that cation polarizability is an important factor in smectite-scCO2 systems, the correlations for hectorite are different from those determined for montmorillonite. The root of these differences is likely associated with the roles of the smectite framework on adsorption, warranting follow-up studies with a larger number of unique smectite frameworks. Overall, the results show that the polarizability of the charge-balancing cation should be considered when preparing smectite clays (or industrial processes involving smectites) to capture CO2 and in predicting the behavior of caprocks over time.

Carbendazim-clay complexes for its potential use as antimicrobial additives in mortars

Mortars containing biocidal chemicals are widely applied as they show resistance to microbiological growth, thus preventing the aging of heritage materials. However, factors such as ultraviolet radiation, the weather etc. could greatly affect the biocidal activity of the active principles in mortars. The aim of this study was to optimize the preparation of several complexes of the biocide carbendazim (CBZ) with clays: natural Wyoming montmorillonite SWy-1 and two commercial organoclays, Cloisite® 10 and Cloisite® 20, and to test their antimicrobial activity for their potential application in restoration mortars. The optimized material here proposed would minimize the amount of active principle needed avoiding its leaching and consequently reducing its environmental impact with a maximum heritage compatibility.Prepared organoclay-CBZ adsorption complexes were inoculated with microorganisms (algae and fungus) to assess the antimicrobial activity. In this sense, one of the CL20-CBZ adsorption complexes which showed the best algicidal and fungicidal activity was used to prepare the antimicrobial mortar, and subsequently compared to those with pure CBZ as an antimicrobial additive. Furthermore, the mortars specimens were subjected to a standardized leaching test to check the effectiveness of the antimicrobial activity. After the leaching tests, the antimicrobial action of mortar containing only CBZ was lower compared to CL20-CBZ which had a good antimicrobial effect. We can conclude from these results that the heritage lime mortars could be easily prepared and have good efficiency against microbial contamination.

Theoretical and experimental investigation on the intercalation of metformin into layered clay minerals

This work introduces a comparative study on the use of two layered clays, a natural Wyoming montmorillonite (commercialized as Cloisite®Na; Mt) and a synthetic hectorite (commercialized as Laponite® XLG; Lap), to encapsulate metformin (MF), which is the most widely used oral drug for the treatment of type II diabetes. The aim was the evaluation of the suitability of the synthetic Lap clay as substrate for the controlled release of MF in comparison to the montmorillonite clay previously studied, in view to dispose of a convenient and well-regulated clay-based component for pharmaceutical formulation production. The combination of molecular modeling and experimental characterization techniques, such as FTIR, XRD, CHN chemical analysis and EDX, allowed us to study the adsorption process and the final molecular arrangement of MF in the interlayer space of the two silicates. The intercalation of MF in both clays follows an ion-exchange mechanism that leads to a basal spacing increase of 0.36 nm and 0.40 nm for the Lap-MF and Mt-MF systems, respectively, and the intercalated amount of MF matches the cation exchange capacity of each clay. Calculations at molecular level showed thermodynamically favorable systems produced by cation exchange reaction, where the MF species form a monolayer in the interlayer of both clays. In view to the application of these materials as delivery systems of MF, the release of the drug from Lap-MF and Mt-MF intercalation compounds was evaluated in pH conditions that simulate those of the gastrointestinal tract. Interestingly, Lap-MF allows a complete release of the loaded drug, guaranteeing the possibility of reaching an efficient drug delivery system. However, the large MF release in acid medium in both systems makes necessary a further encapsulation of the intercalation compound in a protective polymer matrix to increase the delivery in the intestine. Promising results were obtained using pectin as encapsulating matrix, leading to a gradual release of MF in the simulated gastrointestinal tract conditions.

Carbohydrate concentrations and enzyme activities as influenced by exchangeable cations, mineralogy and clay content

Abstract Enzymes can exist in bound or free form within the soil, but the impact of interactions between extracellular enzymes with the amount and type of clay minerals and exchangeable cations on the activities of enzymes and decomposition or retention of carbohydrates in soil is still poorly understood. Appropriate amounts of homoionic Na-, Ca- and Al-clay minerals from Georgia kaolinite, Illinois illite and Wyoming montmorillonite were mixed with pure sand to prepare artificial soils with different clay contents, exchange cations and clay types to examine the effects of exchangeable cations, mineralogy and clay content on the concentrations of hot water and dilute acid extractable carbohydrates and activities of acid and alkaline phosphatases and CM-cellulase. There was a significant effect of clay content on the concentrations of carbohydrates and activities of enzymes. The concentrations of carbohydrates increased when the clay contents of soils increased from 0 to 5 and 10%, respectively, showing that the clay contents influence the capacity of soils for stabilization of carbohydrates. But opposite trends were found in enzyme activities. The enzyme activities decreased significantly as the clay contents of the artificial soils increased. The concentrations of carbohydrates and activities of enzymes were significantly affected by exchangeable cations. In soils with 5 and 10% clay, the concentrations of carbohydrates were maximum in Al-soils and minimum in Ca-soils, in contrast, the activities of enzymes were maximum in Ca-soils and minimum in Al-soils. There was a significant effect of clay mineralogy on the concentrations of carbohydrates and activities of enzymes. The concentrations of carbohydrates were highest in soils with Wyoming montmorillonite clay mineral and lowest in soils with Georgia kaolinite clay mineral. But the activities of enzymes were lower in soils with Wyoming montmorillonite clay mineral than soils with Georgia kaolinite and Illinois illite clay minerals, indicating the influence of specific surface area (SSA) and cation exchange capacity (CEC) of clay minerals on carbohydrate retention and enzyme activities. The results of this study indicate that carbohydrates are stabilized in soils through the interaction with clay minerals and a small amount of clay (5%) significantly increases carbohydrate retention and reduces enzyme activities in soils. Exchangeable cations exert their influence on enzyme activities and hence carbohydrate dynamics by controlling the activities of enzymes through modifying the physicochemical characteristics of soils.

Adsorption and Selective Electrochemical Analysis of Epinephrine Using Clay-Modified Glassy Carbon Electrode

Epinephrine (EP), also known as adrenaline, is one of the important neurotransmitters in the central nervous system (CNS) of mammals. EP is involved in a number of mental disorders, and is used as a drug for heart surgeries and serious allergic reactions. Analysis of EP is, therefore, essential for nerve physiology and for the development of EP medications. Most of the methods developed for EP analysis have low sensitivity and are time consuming. Current methods focus on electrochemical detection because of low cost, simplicity, and high sensitivity. None of these methods, however, focused on the interfacial behavior and adsorption properties of EP, which is of great importance to biochemical and pharmacological research. The Standard Wyoming montmorillonite (SWy-2) clay was used for this project. SWy-2 clay demonstrates good electrochemical properties and high thermal stability. Strong interaction between EP and the clay is expected as SWy-2 is an Fe-bearing smectite clay mineral that contains about 2.3 mass percent lattice Fe. The clay suspension was pre-mixed with glycerol (5% by volume) to prevent formation of cracks in the films. Glycerol is known to be very stable, inert, and nonvolatile. Glycerol intercalates into clay resulting in extremely strong interactions. Hence, glycerol-clay modified glassy carbon electrode (GCME) would increase sensitivity, selectivity, adsorption, and electron transfer rate. This project explored the various factors that affect EP adsorption at electrodes, using the GCME. The GCME films were first characterized using an FTIR microscope system, which confirmed high quality films. The interfacial behavior of EP at the bare glassy carbon electrode (BE) and the GCME were then evaluated and compared. The oxidation of EP, as well as the accumulation, adsorption, and reduction of adrenochrome (EP’s oxidation product) were monitored through continuous repetitive cyclic voltammetric scanning. The potential was scanned between -0.6 V and 0.6 V (vs Ag/AgCl) at a scan rate of 50 mV/s, using 50.0 μM EP in 0.10 M PBS (pH 7.4). Three major peaks were monitored; an oxidation peak corresponding to the oxidation of EP to adrenochrome (peak “a” around 0.28 V), a reduction peak corresponding to the reduction of adrenochrome to leucichrome (peak “b” around -0.19 V), and another oxidation peak corresponding to the oxidation of leucochrome to EP (peak “c” around -0.14 V). At the BE, peak “a” decreased with subsequent cycles while peak “b” increased. Peak “c” was initially absent, appeared after the second cycle, and increased with subsequent cycles. This indicates production, adsorption, and accumulation of adrenochrome at the electrode surface. All peaks levelled off at the seventh cycle, when system equilibrium and maximum adsorptive accumulation (MAA) were achieved. At the GCME, on the other hand, peak “c” was present during the first scan. Also, peak “b” was initially higher and decreased with repetitive cycles. This indicates that the GCME effectively catalyzed the oxidation of EP and facilitated the accumulation and adsorption of adrenochrome. Progressive accumulation was also observed during repetitive scanning as peak “c” increased. System equilibrium and MAA were achieved at the sixth cycle. System conditions, such as glycerol-clay composition, concentration range, scan rate range, and pH were optimized. System equilibration and MAA were achieved only at the physiological pH of 7.4. A linear response of EP was obtained in the range of 0.2 μM to 75.0 μM, with detection limit of 0.10 μM, which is lower than most published reports that use conventional electrodes. The system obeyed the Langmuir isotherm, with an adsorption coefficient of 41.3 L/g. Further, the enhanced adsorption of adrenochrome at GCME, and hence increased reduction peak, was utilized for the selective determination of EP in the presence of serotonin (5-HT), excess ascorbic acid (AA), and excess uric acid (UA). All these substances coexist in biological fluids and have similar oxidation potentials. Various materials have been used to modify electrodes for selective detection or co-detection of EP, 5-HT, AA, and UA. However, these modified electrodes only make use of the oxidation potential while suppressing the adsorption process, which is usually challenging. This method is unique in that the GCME rather enhances preferential adsorption. This makes it easier to detect EP using the enhanced reduction peak, which unequivocally belongs to EP. Urine samples were also spiked with EP, 5-HT, AA, and UA, and then analyzed. The EP reduction peak remained unaltered (within an error of 5%) irrespective of the presence of urine or the spiked interfering species. This work offers different conditions under which adsorption can be enhanced, prevented, or minimized, depending on the intended application. The system offers high sensitivity and reproducibility, with long-term stability.

Clay-Nanoarchitectures as Photocatalysts by In Situ Assembly of ZnO Nanoparticles and Clay Minerals.

ZnO-clay nanoarchitectures have been prepared by In Situ generation of ZnO nanoparticles from zinc acetylacetonate in isopropanol under reflux in the presence of organoclays. Two layered clays, a commercial Wyoming montmorillonite and a smectite from Gafsa (Tunisia), and a fibrous clay, sepiolite from Vallecas-Vicalvaro (Spain), modified with cetyltrimethylammonium ions were the organoclay substrates for assembling the formed nanoparticles. After a convenient thermal treatment, the organic matter is eliminated and the ZnO/clay nanoarchitectures consolidated. XRD, FE-SEM and TEM, among other characterization techniques confirmed the presence of the ZnO in wurtzite phase assembled to the clays in the final porous ZnO/clay nanoarchitectures. The activity as photocatalysts of the resulting materials was evaluated using ibuprofen as a model drug in view to explore the usefulness of these ZnO/clay nanoarchitectures in the removal of emergent pollutants in water.

Micelle-like microaggregate morphology in framework of gelled montmorillonite

Clay particles build (3-D) structured network within the aqueous suspension. Such a network prevents aggregates from settling under gravity force. It also immobilize (retain) water within a structure producing stable gel. To better understand this phenomenon, a microstructural investigation was conducted on Wyoming montmorillonite clay suspension which was gelled by aluminium chlorohydrate. Gel morphology was studied with aid of the synchrotron-powered transmission x-ray microscope (TXM) and cryogenic scanning electron microscope (Cryo-SEM).New type of globular micro-morphology and the particle space arrangement was observed. For the first time, globular micro-aggregate morphology was found where flexible smectite flakes were curled to build globular aggregates. These aggregates were assembled onto multilayer, micelle-like globular superstructure. This new smectite gel micro- morphology may be similar to earlier described pseudoglobular microstructure model observed in eluvial and hydrothermal clay deposits.

Micelle-Like Microaggregate Morphology in Framework of Gelled Montmorillonite

Particle space arrangement within three-dimensional (3D) structured networks in clay suspension may prevent clay particles and aggregates from settling under gravity force and encapsulate water within such a network which results in stabilize gel formation. To better understand this phenomenon, a microstructural investigation was conducted on Wyoming montmorillonite clay suspension gelled by aluminium chlorohydrate in water. Gel morphology was studied with the aid of a synchrotron-powered transmission x-ray microscope (TXM) and cryogenic scanning electron microscope (Cryo-SEM). A new type of globular micro-morphology and the particle space arrangement was observed. For the first time, globular micro-aggregate morphology was found where flexible smectite flakes were curled and build globular aggregates. These aggregates were observed to assembly multilayer, micella like globular superstructure. This new smectite gel micro-morphology may be similar to earlier described pseudoglobular microstructural model observed in eluvial and hydrothermal clay deposits.

Preparation, characterization and application in controlled release of Ibuprofen-loaded Guar Gum/Montmorillonite Bionanocomposites

Neutral guar gum-montmorillonite and cationic guar gum-montmorillonite nanocomposites loaded with ibuprofen were prepared, and their ability to control in vitro release of the drug was presented. These materials exhibited a reduced initial burst release effect and sustained release of up to several hours in a pH 7.4 simulated intestinal fluid. Blank experiments suggested a nanocomposite-mediated action. Improved properties were demonstrated for two different clay minerals: Sodium Wyoming (SWy-2) montmorillonite from the Source Clay Repository and a Portuguese montmorillonite from the Benavila bentonite deposit. The prepared materials were based on abundant, low-cost, natural minerals and plant-extracted biopolymers, and were synthesized using a facile method, thus exhibiting a low environmental impact.

Mesoscale simulation of clay aggregate formation and mechanical properties

This paper proposes a novel methodology for understanding the meso-scale aggregation of clay platelets in water. We use Molecular Dynamics simulations using the CLAYFF force fields to represent the interactions between two layers of Wyoming montmorillonite (Na-smectite) in bulk water. The analyses are used to establish the potential of mean force at different spacings between the layers for edge-to-edge and face-to-face interactions. This is accomplished by finding the change in free energy as a function of the separation distance between the platelets using thermodynamic perturbation theory with a simple overlap sampling method. These nanoscale results are then used to calibrate the Gay–Berne (GB) potential that represents each platelet as a single-site ellipsoidal body. A coarse-graining upscaling approach then uses the GB potentials and molecular dynamics to represent the meso-scale aggregation of clay platelets (at submicron length scale). Results from meso-scale simulations obtain the equilibrium/jamming configurations for mono-disperse clay platelets. The results show aggregation for a range of clay platelets dimensions and pressures with mean stack size ranging from 3 to 8 platelets. The particle assemblies become more ordered and exhibit more pronounced elastic anisotropy at higher confining pressures. The results are in good agreement with previously measured nano-indentation moduli over a wide range of clay packing densities.

Effect of phosphate on U(VI) sorption to montmorillonite: Ternary complexation and precipitation barriers

Phosphate addition is a potential treatment method to lower the solubility of U(VI) in soil and groundwater systems by causing U(VI) phosphate precipitation as well as enhancing adsorption. Previous work has shown that iron oxide surfaces may facilitate the nucleation of U(VI) phosphate minerals and, that under weakly acidic conditions, phosphate also enhances U(VI) adsorption to such phases. Like iron oxides, clays are important reactive phases in the subsurface but little is known about the interaction of U(VI) and phosphate with these minerals. The effect of aqueous phosphate on U(VI) binding to Wyoming montmorillonite (SWy-2) in air-equilibrated systems was investigated. Equilibrium U(VI) uptake to montmorillonite was determined at pH 4, 6 and 8 at discrete initial phosphate concentrations between 0 and 100μM. The observed behavior of U(VI) indicates a transition from adsorption to precipitation with increasing total uranium and phosphate concentrations at all pH values. At the highest phosphate concentration examined at each pH value, a barrier to U(VI) phosphate nucleation is observed. At lower concentrations, phosphate has no effect on macroscopic U(VI) adsorption. To assess the mechanisms of U(VI)-phosphate interactions on smectite surfaces, U(VI) speciation was investigated under selected conditions using laser-induced fluorescence spectroscopy (LIFS) and extended X-ray absorption fine-structure (EXAFS) spectroscopy. Samples above the precipitation threshold display EXAFS and LIFS spectral signatures consistent with the autunite family of U(VI) phosphate minerals. However, at lower U(VI) concentrations, changes in LIFS spectra upon phosphate addition suggest that U(VI)-phosphate ternary surface complexes form on the montmorillonite surface at pH 4 and 6 despite the lack of a macroscopic effect on adsorption. The speciation of solid-associated U(VI) below the precipitation threshold at pH 8 is dominated by U(VI)-carbonate surface complexes. This work reveals that ternary complexation may occur without a macroscopic signature, which is attributed to phosphate not appreciably binding to smectite in the absence of U(VI), with U(VI) surface complexes serving as the sole reactive surface sites for phosphate. This study shows that phosphate does not enhance U(VI) adsorption to smectite clay minerals, unlike oxide phases, and that a barrier to homogeneous nucleation of U(VI) phosphates was not affected by the presence of the smectite surface.

Molecular dynamics simulations of interlayer structure and mobility in hydrated Li-, Na- and K-montmorillonite clays

Molecular dynamics (MD) simulations were carried out to study the hydrate structure and diffusion of water and cations in the clay interlayer region. The goal of this work was to examine the mechanisms by which the clay counterions influence the swelling and dispersion of shale. Three series of MD simulations were performed to study Wyoming montmorillonites containing either Na+, K+ or Li+ counterions and 100, 200 or 300 mg of adsorbed water per gram of clay. Here, the TIP4P interaction model for liquid water is used in the MD simulations. The TIP4P results are compared with simulations using the MCY interaction model, particularly for the K-clay hydrates. For the one- and two-layer K-hydrates, the counterion speciation and water structure observed are in agreement with the MCY model. However, we observe that the two-layer hydrate persists up to high water contents and that – contrary to the MCY model – the hypothetical three-layer hydrate is unstable, in agreement with experimental observations. If one notes that the one- and two-layer hydrates are only saturated at 56 and 96 water molecules, respectively, then the observed equilibrium layer spacings are in excellent agreement with experiment. Integration of the ion density profiles shows that the surface complexes represent 4.4 out of 6 ions for the K-hydrate and only 2 out of 6 ions for Na-hydrate. This means that K+ ions adsorb more strongly and screen the negatively charged surface more effectively than Na+ ions. Self-diffusion coefficients were calculated from mean-square displacement analysis of ions and water molecules. For the Na+ and Li+ cases, satisfactory agreement with experimental quasi-elastic neutron scattering diffusion coefficients was found. No experimental values are available for the K+ case, and therefore predictions are made in this report. Diffusion coefficients for water in K-smectite increase from 10−10 to 10−9 m2 s−1 with water contents increasing from 100 to 300 mg g−1 of clay. We find that water diffusion rates in K-smectites are roughly twice as small as in Na- and Li-clays with the same water content. It appears that the microscopic diffusion coefficients calculated for interlayer diffusion of Na+ and K+ ions are two orders of magnitude smaller than the macroscopic values calculated from experimental work on hindered motion of water and ions through a clay membrane. This implies that the experimental clay film has a tortuous structure, with clay platelets aligned orthogonally to the main direction of mass transport, and a tortuosity factor of . The results show that the MD simulation technique is a valuable tool to study transport properties of water and ions in hydrated smectite clays.

Assessment of a predictive model to describe the migration of major inorganic cations in a Bt soil horizon

The aim of this study was to test the ability of a previous published model describing the sorption properties of complex solids (Bt soil horizon, sediment) under static conditions (batch mode) to describe sorption data obtained under dynamic conditions. This model assumes that the sorption properties of the multicomponent solid can be described by those of smectites present in the mineralogical assemblage. In our case, the reference smectite is a Wyoming montmorillonite. To test the model, experimental breakthrough curves of some major cations were obtained using a Bt soil horizon in different physico-chemical conditions. The fairly good agreement between the different experimental data sets and predicted breakthrough curves demonstrates that our proposed model can be used to accurately predict ion exchange reactions occurring under dynamic conditions between Na+, Ca2+, and H+ cations in a complex mineralogical assemblage. In addition, this model is also able to accurately predict previously published experimental data obtained with another B soil horizons and using Na+, Ca2+, and Mg2+ as cations. Other models reported from the literature, based either on sorption properties of pure smectites or of complex assemblages, are not able to accurately interpret experimental data proposed in this study motivating our purpose to propose another model. Therefore, our predicted model represents an alternative to models based on the generalized composite approach, which describes the reactivity of a complex material using generic sorption sites for which reactivity is not explicitly related to the properties of the individual phases of the complex material.

Combined Salt and Temperature Impact on Montmorillonite Hydration

AbstractBentonite is to be used as a sealing material for long-term storage of radioactive waste. During permafrost periods the buffer may freeze, causing the following: montmorillonite dehydration, ice formation, and pressure build-up that may fracture the surrounding rock. No previous study has been done on freezing of bentonite in saline water. Using small and wide angle X-ray scattering, the present study aimed to increase understanding of the combined impact of salt and temperature on the hydration (swelling) of Wyoming montmorillonite. The basal spacing of the Na-montmorillonite was very dependent on the water content, while this was not the case for the Ca-montmorillonite (after reaching 19 Å). The basal spacing of the free-swelling Na-montmorillonite (34–280 Å) was estimated successfully using simple calculations. During freezing of Na-montmorillonite in NaCl solution, both ice and hydrohalite formed (at -50 and -100ºC). At starting concentrations ≥ 1.5 M the basal spacing was not affected by freezing. During freezing of Ca-montmorillonite in CaCl2 solution, ice formed; antarcticite formed only sporadically. The basal spacing of the Ca-montmorillonite at high NaCl concentrations (>1 M) was greater at -50 and -100ºC (18 Å) than at 20ºC (16 Å). The opposite was observed at low concentrations. This change was attributed to small amounts of salts introduced into the montmorillonite interlayer, hence changing the interlayer water properties. The montmorillonite hydration was also temperature dependent; decreasing temperature increased the hydration (as long as no ice was formed) and increasing the temperature decreased the hydration. This was attributed to the temperature impact on the entropy of the hydration reaction. This observation was also reproduced in an experiment up to 90ºC. A small amount of salt in the groundwater was noted to reduce significantly the potential problem of ice formation in bentonite sealings.

Synthesis, characterization and catalytic evaluation of zirconia-pillared bentonite for 1,3-dioxalane synthesis

The aim of this work is to study the solids obtained by pillaring Tunisian bentonite with zirconium polycations from the hydrolysis of zirconium tetrachloride. For comparative purposes, the pillaring of Wyoming montmorillonite, a reference clay, is also studied. Variations in the dialysis number, concentration of Zr solution, pH, temperature in the synthesis procedure as well as the pillaring method, affect the structure and properties of the pillared clay obtained. It has been verified that the structure of the pillared clays strongly depends on the method of preparation and the pH Zr solution in some cases, well ordered pillared clays (with basal spacings between 15 and 18.2 A) have been obtained, while in several others noncrystalline solids yielded. Delamination of the clays seems to take place both when the intercalation solutions obtained from zirconium tetrachloride have pH > 2.8 and when they are subjected to a reflux treatment. Best results were obtained at pH = 2.8 [Zr4+] = 0.1 M, at room temperature, and when using five dialysis, the direct method. The specific total surface area and micropore volume of the solids have reached the values up to 200 m2 g−1 and 0.06 cm3 g−1, respectively. Potentiometric titration and mass titration data suggest that pillared materials produce a shift of the point zero charge toward the point zero charge of the zirconium oxide. The amount of ZrO2 fixed have varied between 6 and 30% of the weight of the pillared samples. The activity of the zirconium pillared clay for 1,3-dioxolane synthesis has been investigated. The method of intercalation has been found to be the most suitable for this reaction.

Microbial reduction of Fe(III) in smectite minerals by thermophilic methanogen Methanothermobacter thermautotrophicus

Clay minerals and thermophilic methanogens can co-exist in hot anoxic environments, including the continental subsurface, geysers, terrestrial hot springs, and deep-sea hydrothermal vent systems. However, it is unclear whether thermophilic methanogens are able to reduce structural Fe(III) in clay minerals. In this study, the ability of a thermophilic methanogen Methanothermobacter thermautotrophicus to reduce structural Fe(III) in iron-rich and iron-poor smectites, (nontronite NAu-2 and Wyoming montmorillonite SWy-2) and the relationship between iron reduction and methanogenesis were investigated. M. thermautotrophicus reduced Fe(III) in nontronite NAu-2 and montmorillonite SWy-2 with H2/CO2 as substrate. The extent of bioreduction was 27% for nontronite and 13–15% for montmorillonite. Anthraquinone-2,6-disulfonate (AQDS) did not enhance the extent of bioreduction, but accelerated the rate. When methanogenesis was inhibited via addition of 2-bromoethane sulfonate (BES), the extent of bioreduction decreased to 16% for NAu-2 and 9% for SWy-2. These data suggest that Fe(III) bioreduction and methanogenesis were mutually beneficial. The likely mechanism was that Fe(III) bioreduction lowered the reduction potential of the system so that methanogenesis became favorable, and methanogenesis in turn stimulated the growth of the methanogen, which enhanced Fe(III) bioreduction. NAu-2 was partly dissolved and high charge smectite and biogenic silica formed as a result of bioreduction.

Kinetics of partial dehydroxylation in dioctahedral 2:1 layer clay minerals

A multi-cycle heating and cooling thermogravimetric (TG) method was used to study the kinetic behavior of partially dehydroxylated illite, aluminoceladonite, and dioctahedral smectite samples. The method consists of consecutive heating cycles separated by intervals of cooling to room temperature, with the maximum cycle temperatures (MCTs) set incrementally higher in each consecutive cycle. In the studied samples, dehydroxylation of each portion of the initial OH groups follows the kinetics of a homogeneous zero-order reaction in each heating cycle. The activation energies ( E a ) were calculated in terms of this model for separate heating cycles of each sample with regression coefficients R 2 ≥ 0.999. A zero-order reaction determined at each heating cycle indicates that at each stage of partial dehydroxylation, there is no formation of an intermediate phase and the reaction is probably the direct transformation of the original layers into completely dehydroxylated layers. The Wyoming montmorillonite and illite consisting of cis -vacant ( cv ) layers had the highest E a values (53–55 kcal/mol). In the samples consisting of trans -vacant ( tv ) layers and having almost the same octahedral cation composition the maximum E a values varied from 45 to 30 kcal/mol and the E a of each sample in this group are similar over a wide range of the D T . For the samples consisting of a mixture of cv and tv illite structures, a bimodal distribution of the E a values exists with increasing MCT and D T . The maximum E a values for dehydroxylation of the tv and cv illite structures are different. The activation energies from the tv aluminoceladonite and Otay tv montmorillonite samples have similar maximum E a values (39.4 to 41.8 kcal/mol), but the variation in E a with D T has a skewed bell-like distribution that is probably related to a heterogeneous octahedral cation composition of the 2:1 layers. The E a values calculated for the mineral structures in this study are compared with those published and the main factors controlling the E a variation at different stages of the partial dehydroxylation are discussed.

Preparation and characterization of spermine-exchanged montmorillonite and interaction with the herbicide fluometuron

We studied the interaction between the naturally occurring polycation spermine (SPERM) and Na-exchanged SWy-2 Wyoming montmorillonite (Na-SWy-2 Mt) and between spermine-exchanged montmorillonite (Mt-SPERM) and the herbicide fluometuron. Mt-SPERM with spermine contents ranging between 0 and 1.2 times the cation exchange capacity (CEC) of Na-SWy-2 were studied by XRD, FTIR and TGA analysis. Spermine was intercalated stoichiometrically into Na-SWy-2 up to the CEC of the clay mineral, resulting in basal spacing of 1.29nm. TGA analysis indicated that intercalation of spermine into Na-SWy-2 increased the thermal stability of the organic cations and affected the dehydroxylation temperature of the clay mineral. FTIR measurements of Mt-SPERM samples showed the involvement of NH and CH groups of spermine in the interaction with the clay mineral. The intensity of the spermine NH and CH deformation vibrations increased linearly with the amount of spermine incorporated into Na-SWy-2 up to an amount close to the CEC of the montmorillonite. Structural modeling suggested that the arrangement of spermine cations formed a horizontal monolayer within the interlayer space of SWy-2 with strong interaction between the NH3+ and NH2+-groups of spermine and the montmorillonite surface oxygen atoms. Mt-SPERM showed pronounced affinity for the herbicide fluometuron.