Layer Charge Distribution Research Articles

Stepwise increasing TMC concentration in NF membranes fabrication for controlled PA layer thickness, compactness, and charge distribution: Maintaining high permeance, enhancing desalination, and mechanistic insights

Piperazine (PIP)-based polyamide (PA) nanofiltration (NF) membranes exhibit limited rejection of divalent cations due to their negatively charged surfaces and insufficient compactness, restricting their broader application in various water treatment processes. To overcome this challenge, a novel strategy—stepwise increasing TMC concentration (SITC) during the interfacial polymerization (IP)—was proposed to optimize the PA layer structure. Conventional PA layer demonstrates a vertical diffusion structure: a compact barrier region underneath a thick and loose region. The regulatory goal of SITC is to enhance compactness while minimizing the increase in hydraulic resistance. The regulatory mechanism suggested SITC could regulate the IP reaction in both over time (throughout the IP process) and across the reaction region. During the initial IP stage, SITC reduced the reaction rate and increased PIP/TMC ratio in the bottom reaction zone, thereby improving the compactness and potential of final bottom barrier region. Thus, SITC can effectively restrict the permeation of cations, significantly increasing MgCl2 rejection from 33.70 % to 92.12 %. In the subsequent diffusion-limited stage, SITC enhanced the restriction on PIP diffusion, decreasing the thickness of the final upper loose region and surface potential. Those advantageous characteristics allowed the membrane to maintain a high rejection for Na2SO4 (>98.44 %). Concurrently, the membrane maintained a high water permeance (13.00 L·m−2·h−1·bar−1), attributed to the thin (16.69 nm) and optimized compactness distribution of its PA layer. This study provided a novel mechanism insight into the functional customization of NF membranes by decoupling the synthesis-property-performance relationship.

Rheological properties of magnesium bentonite and sepiolite suspensions after dynamic ageing at high temperatures

Abstract The rheological properties of three Na-activated, trioctahedral Mg-bentonites (hectorite clay from the CMS Source Clay Project repository, saponite clay from Spain and stevensite clay from Rhassoul, Morocco) and a sepiolite clay from Greece were examined after dynamic ageing at temperatures up to 230°C. The 5% w/v suspensions were prepared by dispersing the clay mineral samples in distilled water. The suspensions underwent dynamic, thermal ageing for 16 h before determination of the viscosity, filtration loss, filter cake thickness and pH and the concentration of dissolved Na+ and Mg2+. Thermal ageing contributed to the dispersion of clay particles, with a direct effect on plastic and apparent viscosity, introducing pseudoplastic behaviour. With the exception of the stevensite clay at 230°C that displayed limited dissolution at 230°C and partial conversion to kerolite, the clays were stable at high temperatures. The Na-activation of all clays except for stevensite was not adversely affected by thermal ageing. Thermal ageing of stevensite at 230°C facilitated Na exchange and yielded suspension with high viscosity and low filtrate loss. Only the suspensions of hectorite and those of stevensite aged at 230°C met with American Petroleum Institute specifications. The thermal behaviour and rheological properties of the clays might be interpreted according to the intrinsic properties of the clay minerals, such as layer charge and charge distribution.

Molecular dynamics simulation of hydrogen diffusion in water-saturated clay minerals; implications for Underground Hydrogen Storage (UHS)

A suitable option for Underground Hydrogen Storage (UHS) is aquifers composed of porous and permeable media. However, the successful execution and long-term safety of the UHS process require further understanding of hydrogen transport behavior in the pathway of caprock to eliminate the possibility of hydrogen leakage. In this regard, we employ Molecular Dynamics simulation to predict the hydrogen diffusion coefficient in five different water-saturated clay minerals, major components of caprock, and to assess the effects of clay mineral characteristics in terms of pore size (1–8 nm), layer charge distribution (octahedral sheet and tetrahedral sheets), and counterion type (Na+ and Ca2+) on hydrogen diffusion. The results show that the diffusion of hydrogen in negatively charged water-saturated clay minerals significantly increases for pore size increments of less than 2 nm (critical pore size) based on layer charge distribution and interlayer cations. However, no considerable changes in the hydrogen diffusion coefficients are observed for pore sizes larger than 2 nm. While the hydrogen diffusion coefficient in pyrophyllite (no charge, no interlayer cation) experiences a monotonous rise as pore size increases. Our findings in this study assist in better experimental test design to examine the possibility of leakage in actual field implementation by focusing on the clay mineral characteristics.

Control of Structural Hydrophobicity and Cation Solvation on Interlayer Water Transport during Clay Dehydration.

Swelling clay hydration/dehydration is important to many environmental and industrial processes. Experimental studies usually probe equilibrium hydration states in an averaged manner and thus cannot capture the fast water transport and structural change in interlayers during hydration/dehydration. Using molecular simulations and thermogravimetric analyses, we observe a two-stage dehydration process. The first stage is controlled by evaporation at the edges: water molecules near hydrophobic sites and the first few water molecules of the hydration shell of cations move fast to particle edges for evaporation. The second stage is controlled by slow desorption of the last 1-2 water molecules from the cations and slow transport through the interlayers. The two-stage dehydration is strongly coupled with interlayer collapse and the coordination number changes of cations, all of which depend on layer charge distribution. This mechanistic interpretation of clay dehydration can be key to the coupled chemomechanical behavior in natural/engineered barriers.

Anisotropic Elastic Properties of Montmorillonite With Different Layer Charge Densities and Layer Charge Distributions Through Molecular Dynamic Simulation

The knowledge of the anisotropic elastic properties of clay minerals is of crucial importance for the exploration and development of shale oil and gas. Montmorillonite (MMT) is a common natural clay mineral with different layer charge densities and layer charge distributions due to different geological conditions. Therefore, it is important to understand the currently poorly known effect of layer charge density and layer charge distribution on the anisotropic elastic properties of MMTs. This work aims to obtain such knowledge by studying the anisotropic elastic properties of different MMTs under stratigraphic conditions through molecular dynamic simulations. We showed that the in-plane compressional coefficients C11, C22 and C12 decrease with the increasing layer charge density for MMTs with different layer charge distributions, and the MMTs with the layer charges distributed on the two tetrahedral (T) sheets were found to have the smallest C11, C22 and C12. We also showed that the out-of-plane compressional coefficients C33, C13 and C23 of the MMTs with the layer charges distributed in the two T sheets decrease, while those with the layer charges in the octahedral (O) sheet increase and those with layer charges distributed in both the O sheet and the T sheets do not vary much with the increasing layer charge density. The variations of the anisotropic compressional elastic coefficients with different layer charge densities and layer charge distributions were found to be a result of the impact of the density and distribution of layer charges on the molecular interactions within the MMT layer. We further demonstrated that the layer charge density and layer charge distribution do not influence significantly the shear coefficients C44, C55, and C66. The results revealed the mechanisms of how the density and distribution of layer charges affect the anisotropic elastic properties of MMTs and will contribute to the more successful exploration and development of unconventional resources in MMT bearing shale reservoirs.

Nucleation and dissociation of carbon dioxide hydrate in the inter- and intra-particle pores of dioctahedral smectite: Mechanistic insights from molecular dynamics simulations

The storage of CO2 in the form of gas hydrate is becoming common place in the field of carbon capture and storage. Natural gas hydrate deposits are typically rich in clay minerals, yet their potential effect on CO2 hydrate nucleation and dissociation is rarely elucidated. In the present work, the nucleation and dissociation of CO2 hydrate in the inter- and intra-particle pores of dioctahedral smectite particles were investigated using molecular dynamics simulations. The results show that the hydrate nucleation clearly occurred in the inter-particle pores, whereas clathrate-like structures were formed in the intra-particle pores or on the edge surface of smectite. The properties of smectite, specifically the layer charge distribution and the type of exchangeable cations, are the main factors affecting the nucleation and dissociation of CO2 hydrate by changing the structural ordering of water molecules. The diffusion of CO2 molecules into the intra-particle pores enhances the formation of clathrate-like structures with increasing water content, but it does not increase the structural ordering of water molecules. Increasing the temperature or adding electrolytes in the smectite particles have accelerated the CO2 hydrate dissociation. Overall, the nucleation and dissociation mechanisms of CO2 hydrate in the linked inter- and intra-particle pores of dioctahedral smectite particles not only substantially affects the carbon storage and methane extraction in hydrate reservoirs, but also have potentially important impacts on pore fluids transportation in marine sediments.

Effects of layer-charge distribution on swelling behavior of mixed-layer illite-montmorillonite clays: A molecular dynamics simulation study

One of the intriguing properties of clay minerals is swelling, which is considered an essential characteristic for various geological processes, environmental science, and engineering. Almost all previous experimental and simulation studies focused on the understanding of the swelling behavior of pure clays. The fact is that about 30 percent of all clays are a pure type, and virtually 70 percent of clays are the mixed-layer one that swelling mechanism is still elusive. In this research, molecular dynamics simulations were used to exclusively investigate the effect of layer charge distribution on the crystalline swelling and hydration behavior of the most common types of mixed clays, so-called illite-montmorillonite mixed-layer clay (I-Mt MLC) with Na+ and K+ counterions in the presence of different water concentrations. Based on the results, we found that the layer charge density and charge location of both illite and Mt significantly influenced the I-Mt clays' swelling behavior. Regardless of Mt charge density, the presence of highly charged illite led to powerful interactions of counterions with the surface sheet, resulting in less swelling behavior of the I-Mt system. Also, the various polarization power of clay surface due to the difference in charge distribution of octahedral sheets had a significant influence on water molecules and interlayer cations interaction with clay surface, which affected the swelling behavior of I-Mt clays. Overall, our findings demonstrated the new insight into I-Mt clays' swelling behavior, which is a function of charge distribution of both illite and Mt.

Swelling capacity of mixed talc-like/stevensite layers in white/green clay infillings (“deweylite”/ “garnierite”) from serpentine veins of faulted peridotites, New Caledonia

AbstractWhite (Mg-rich) and green (Ni-rich) clay infillings (“deweylite”/ “garnierite”) found in serpentine veins of faulted peridotite formations from New Caledonia consist of an intimate mixture of fine-grained and poorly ordered 1:1 and 2:1 layer silicates, commonly referred to as non-expandable serpentine-like (SL) and talc-like (TL) minerals. New data on the swelling and shrinking capacity of these layer silicates were gathered from X-ray diffraction (XRD) after saturation of the clay fractions with different cations (Ca2+, Li+, K+), ethylene glycol (EG) solvation, and heat treatments. Simultaneously, layer charge distribution and vacancy density, respectively, were investigated by FTIR spectroscopy on NH4-saturated clay fractions and XRD on Li-saturated clay fractions before and after heating (Hofmann Klemen treatment). Five clay infillings, with dominant 2:1 layer silicates and variable Ni contents, were selected for this study, from a large set of veinlets, according to their swelling capacity. The crystal chemistry of these samples was characterized by FTIR spectroscopy and bulk chemical analyses.The swelling ability of the clay infillings is attributed to the 2:1 layer silicates. It does not seem to be affected by the relative fraction of Mg and Ni in their octahedral sheets. In XRD patterns, the swelling ability is reflected by slight shifts of the basal reflection of the 2:1 layer silicates toward low angles for bulk samples and by splitting of the peak into two contributions for clay fractions saturated with Ca (or Li) and solvated with EG. The split increases with the swelling capacity of the sample. It originates mainly from octahedral-layer charge generated by vacant sites. Such results lead us to consider the 2:1 layer silicates of the infillings as an intimate mixture of non-expandable (TL) and expandable (stevensite) phases. In agreement with previous studies that suggested a contribution of hydrothermal processes in the alteration of serpentine species into 2:1 layer silicates, we propose that the proportion of expandable phases in the clay infillings (or vacancy sites in the octahedral sheets of the 2:1 layer silicates) could be used as an efficient means for assessing the temperature of their formation. Clay infillings mostly made of stevensite would have formed at ambient temperatures, whereas those consisting mainly of non-expandable TL would have formed at higher temperatures.

Effect of layer charge and charge distribution on the formation of chitosan - smectite bionanocomposites

Chitosan-clay bionanocomposites were prepared from chitosan biopolymer solutions and four different smectites (SAz-1 and SWy-2 montmorillonites, SBId-1 beidellite and NAu-1 nontronite), and the effect of layer charge and charge distribution on the formation of bionanocomposites was examined. The intercalation of the chitosan in the homoionic Na+-smectite samples showed that all samples produced chitosan-smectite nanocomposites even at the lowest chitosan-clay ratios. The adsorption isotherms of chitosan by the smectites were classified as L- type isotherms. The equilibrium data were fitted to Langmuir and Freundlich isotherms and the equilibrium adsorption was best described by the Langmuir isotherm model for SWy-2 and SBId-1 while the Freundlich model achieved a better description for SAz-1 and NAu-1 for the higher chitosan concentrations. The adsorption of chitosan exceeded the CEC in the montmorillonites (SWy-2 and SAz-1) whereas in SBId-1 and NAu-1 with tetrahedral charge it either reached or it was lower than the CEC showing that layer charge and charge localization may hinder chitosan adsorption.

Surface plasmon resonance study of adhesion kinetics of smectites on the Au/water interface: Clay and organo-clay film formation

Surface plasmon resonance was employed for the in-situ investigation of film formation from aqueous dispersions of clay minerals on gold/water interface. High charge (SAz-1) and intermediate charge smectite (STx-1b) were studied for their ability to form stable clay films on gold surfaces and their adhesion kinetics at different concentrations was investigated. Layer charge and charge distribution seems to affect not only the kinetics of film formation but also the overall amount of clay mineral on the gold substrate. Subsequently to stable clay mineral film formation, the in-situ preparation of hybrid organo-clay films with the introduction of alkylammonium chains and their adsorption kinetics was examined. Alkylammonium adsorption via ion exchange in the interlayer of the clay minerals is controlled by the density of the clay mineral film in full accordance with layer charge and charge localization. Hence, a simple self-assembly procedure for the in-situ preparation of hybrid organo-clay films for novel applications is proposed.

Colloidal properties of different smectite clays: Significance for the bentonite barrier erosion and radionuclide transport in radioactive waste repositories

Compacted bentonite is envisaged as engineered barrier in high-level radioactive waste (HLRW) repositories and, to assess its functionality at a long term, many clay properties must be investigated. Bentonite erosion poses a potential problem, because a significant mass loss would jeopardize the performance of the barrier; in addition clay colloids generated from the eroded material might play a role on radionuclide transport.In this study, the intrinsic characteristics of colloids obtained from different smectitic clays (mass, size and surface charge) were analysed, focusing on their relation with the main physicochemical properties of the bulk clays: the smectite content, the composition of interlayer cations, the total layer charge and its distribution between the tetrahedral and octahedral layer.Results demonstrated that the predominance of Na as interlayer cation highly favours colloid detachment and the formation of colloids with the smallest size and the highest electrophoretic mobilities; the contrary occurs when Ca prevails (less detachment, larger colloids and lower mobilities). Besides, apart from the Na/Ca content, also the layer charge and charge distribution were shown to affect the intrinsic colloidal properties. For example, the increase of tetrahedral charge (τ) provokes stronger interactions between the TOT smectite layers, because cations (especially divalent) can be better bound to the clay surface. A τ increase causes the formation of larger aggregates with less probability of detachment. However, if the main exchange ion is Na, which strongly favours clay dispersion, high τ gives rise to an increased mass loss as, in fact, the released particles are bigger.The results were discussed in a generalised form, concluding on the potential erodibility of different smectites and the implications of this erosion on radionuclide migration in a HLRW repository.

Molecular Dynamics Simulation of Alkylammonium-Intercalated Vermiculites

AbstractIn order to understand the microscopic properties of alkylammonium-intercalated vermiculites, molecular dynamics simulations employing the clayff-CVFF force field were performed to obtain the interlayer structures and dynamics. The layering behavior of alkyl chains was uncovered. With the model used in the present study (1.2 e per unit cell), the alkyl chains formed monolayers with carbon-chain lengths of C6, bilayers from C7 to C10, and pseudo-trimolecular layers from C15 to C18. Intermediate states also existed between bilayer and pseudo-trimolecular layer states from C11 to C14. The ammonium groups had two locations: most ammonium groups were located over the six-member rings (~90%), and the rest above the substitution sites (~10%). The ammonium groups interacted with the vermiculite surface through H bonds between ammonium H atoms and surface O atoms. The ammonium groups were fixed firmly on surfaces and, therefore, had very low mobility. The alkyl chains were slightly more mobile. The similarities and differences between alkylammonium-intercalated vermiculites and smectites were revealed. The layering behaviors of alkyl chains were similar in alkylammonium-intercalated vermiculites and smectites: the alkyl chain behavior was controlled by both the amount of layer charge and the carbon chain length. The distributions of ammonium groups, however, were different, caused by the layer-charge distribution in vermiculites being different from that in smectites. The atomic-level results derived in the present study will be useful for future research into and the applications of organo-vermiculites.

Impact of Intrinsic Structural Properties on the Hydration of 2:1 Layer Silicates.

Several 2:1 layer silicates comprising di- and trioctahedral smectites of different layer charge between 0.2 and 0.4 per formula unit and a trioctahedral vermiculite were studied by an in situ method that allowed Fourier transform infrared spectroscopy (FTIR) spectra and water vapor sorption isotherms to be obtained simultaneously. The particle size and shape of the selected materials were determined using X-ray diffraction and gas adsorption analyses, which provided an estimate of the particle size with resulting edge site proportion. The aim of this study was to elucidate the hydration mechanism in 2:1 layer silicates during desorption and adsorption of water vapor. Domains in the desorption and adsorption of water vapor of the smectite samples with a slightly increasing slope were explained by a heterogeneous layer charge distribution, which enables the coexistence of different hydration states even under controlled conditions. Whereas hysteresis was observed over the entire isothermal range of the smectites, the isotherm of the vermiculite sample only showed hysteresis in the transition from the monohydrated state (1W) to the bihydrated state (2W). We also revealed that hysteresis is a function of the layer charge distribution, the achieved water content, and the particle size with resulting edge site contribution. Increasing the edge site proportions led to an increased hysteresis. The findings from the experimental FTIR/gravimetric analysis showed that the transition from 2W to 1W and backward is visible using infrared spectroscopy. The shifting of δ(H-O-H) was influenced by the layer charge and octahedral substitutions. As a final point, we use water as a sensor molecule to describe the OH groups of the octahedral sheet and show that the observed shifts result from a change in the tilting angle. Our experimental results were supported by ab initio thermodynamic simulations that revealed the different shifting behavior of δ(H-O-H) and δ(M x+-OH-N y+) related to the differences in surface charge density and octahedral compositions.

Potassium Fixation by Oxidized and Reduced Forms of Phyllosilicates

Core Ideas Understanding how soils fix and release K is crucial for soil fertility. K fixation and release are controlled by layer silicate collapse and expansion. K fixation is affected by layer charge, charge distribution, and Fe redox. The greater the negative layer charge, the greater the amounts of K fixation. Tetrahedrally derived charge favors more K fixation than octahedrally derived charge. Potassium fixation traps K+ ions in the interlayer region of phyllosilicates. This study determined if increased negative interlayer charge caused by structural Fe reduction leads to increased K+ fixation. The five reference clays used were illite (IMt‐1), kaolinite (KGa‐1b), montmorillonite (STx‐1b), nontronite (NAu‐2), and vermiculite (VTx‐1). Soil clays were fractionated from the upper 15 cm of a Belvue loam and a Cherokee silt. Potassium fixation capacities were measured on clay samples of unreduced and reduced forms of each clay. Iron (II) and total Fe contents were determined, and K+ fixation was measured by K saturating the clays, followed by five washes of MgCl2 solution. Iron reduction significantly increased the amounts of K+ fixed by NAu‐2 and VTx‐1. An increase in Fe(II) content caused increases in layer charge and K+ fixation. Although NAu‐2 exhibited a greater increase in Fe(II) content on reduction than VTx‐1, the increase in K+ fixation on reduction was greater for VTx‐1 because of the tetrahedral location of Fe in VTx‐1. For IMt‐1, KGa‐1b, and STx‐1b, Fe reduction did not significantly affect the K+ fixation capacities because of their low Fe contents. The Belvue loam released K+ in both unreduced and reduced forms. The Cherokee silt did not appreciably release or fix K+ in either form. Although much K is removed in the first wash, small amounts of K were removed in subsequent washes, especially for reduced samples of NAu‐2 and VTx‐1. The washing procedure caused reduced Fe to reoxidize, which resulted in K that was previously fixed to be released.

Investigation of the effect of finite-sized ions on the nanowire field-effect transistor in electrolyte concentration using a modified Poisson–Boltzmann model

ABSTRACTOne-dimensional (1D) nanowire field-effect transistors (FETs) have recently played a major role in sensing applications. Due to charging of the surface functional chemical groups with protonation and deprotonation, the transport properties of these nanowire transistors affect the aqueous environment, altering the electrical double layer (EDL) potential drops and charge distributions in the electrolyte concentration. In this work, we have implemented the simple modified Poisson–Boltzmann (MPB) theory in a 1D silicon nanowire FET, and the effect of the various finite sizes of ions in z:z symmetric electrolyte concentration was investigated. For a given ionic concentration and surface charge, the EDL potential drop, accumulation of charges and the charge distributions of NaCl and CsCl ions were studied. From the MPB model results with the nanowire FET, it was observed that the potential drop of the EDL depends on the size of the ions in the electrolyte. The study of various electrostatic investigations of finite-sized ions was successfully done by implementing the MPB model on a silicon nanowire FET. It can be used in both chemical and biological sensors.

Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays.

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K+ and has high affinity for Cs+. Although exchange of interlayer K+ for Cs+ is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs+-K+ exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs+ for the smaller K+ significantly lowers the migration barrier of neighboring K+, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs+ exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.

Efficient electrochemomechanical energy conversion in nanochannels grafted with polyelectrolyte layers with pH-dependent charge density

Nanochannels, functionalized by grafting with a layer of charged polyelectrolyte (PE), have been employed for a large number of applications such as flow control, ion sensing, ion manipulation, current rectification and nanoionic diode fabrication. Recently, we established that such PE-grafted nanochannels, often denoted as “soft” nanochannels, can be employed for highly efficient, streaming-current-induced electrochemomechanical energy conversion in the presence of a background pressure-driven transport. In this paper, we extend our calculation for the practically realizable situation when the PE layer demonstrates a pH-dependent charge density. Consideration of such pH dependence necessitates consideration of hydrogen and hydroxyl ions in the electric double layer charge distribution, cubic distribution of the monomer profile, and a PE layer-induced drag force that accounts for this given distribution of the monomer profile. Our results express a hitherto unknown dependence of the streaming electric field (or the streaming potential) and the efficiency of the resultant energy conversion on parameters such as the pH of the surrounding electrolyte and the $$\hbox {pK}_{\mathrm{a}}$$ of the ionizable group that ionizes to produce the PE charge—we demonstrate that increase in the pH and the PE layer thickness and decrease in the $$\hbox {pK}_{\mathrm{a}}$$ and the ion concentration substantially enhance the energy conversion efficiency.

Influence of mineralogical and morphological properties on the cation exchange behavior of dioctahedral smectites

Structure and morphology determine the smectites properties. Thereby, edge sites play a key role for the reactivity and colloidal behavior of the smectite. Even so, the edge surface area represents only 1–3% of the total surface area its contribution is up to 10% of the cation exchange capacity (CEC). Na-saturated dioctahedral smectites <0.2 μm were characterized in detail with respect to their structure (layer charge, octahedral cation distribution, iron content, and layer charge distribution within tetrahedral and octahedral sheets). Furthermore, reduced charge montmorillonite (RCM) of these materials were studied. Particle dimension was estimated from argon adsorption, calculation of specific surface area and direct observation of single layers by atomic force microscopy (AFM), which also revealed morphology and particle size distribution of single layer. CEC was measured both for Na-saturated samples and RCM using the Cu-trien method over a pH range of 4–9. The Wyoming smectite with a mean layer diameter of 277 nm and a low layer charge of 0.26 eq/f.u. differs strongly from the others smectites with a mean layer diameter of about 100 nm and layer charges between 0.30 and 0.37 eq/f.u. Furthermore, the layer stacking is much higher for the Wyoming smectite in powder and RCM. Accordingly the edge properties differ between the four smectites. Based on calculation of edge site density and CEC measurements, we showed that the RCM can be used to study the edge site reactivity of the montmorillonites. The increase of CEC above their pHPZC,edge is equal for Na-saturated montmorillonite and RCM. The present study highlights the diversity in structure and morphology and resulting edge properties of four smectites.

Reaction of bentonites with pyrite concentrate after wetting and drying cycles at 80°C: relevance to radioactive waste (Radwaste) storage

AbstractThe mineral stability of two bentonites was studied in the presence of pyrite concentrate to simulate the possible reactions between the bentonite barrier used in a high-level nuclear waste (HLW) repository and host rock containing up to 5 wt.% of admixed pyrite. Smectite was the only bentonite mineral affected by pyrite treatment under the experimental conditions used. Bentonites reacted differently with pyrite due to the different nature of the smectites. The distinct crystal chemistry of the smectites controlled by the composition of the parent rocks influenced the smectite surface properties (cation exchange capacity and layer charge distribution) which resulted in a different response of the bentonites to pyrite treatment. A partial transformation of the original smectites into H-smectites represented the initial stage of smectite destabilization on acid attack. Rising non-equivalent isomorphous substitution in the octahedral sheets of the smectites enhanced smectite reactivity and thus the reaction between bentonite and pyrite, causing lower stability of the bentonite containing high-charge smectite.

Changes in layer charge of clay minerals upon acid treatment as obtained from their interactions with methylene blue

Adsorption of methylene blue (MB) on acid-treated clay minerals was used to follow the changes in layer charge (LC). Decreased values of cation exchange capacity (CEC) upon HCl attack confirmed reduction of LC in all studied systems. Second derivative absorption spectra (SDS) were used for better resolution of the individual bands in the spectra and for easier distinguishing between absorption of different dye species, such as the monomers absorbing near 675 nm, dimers (600 nm), H-aggregates (570 nm), and J-aggregates (775 nm). The changes in the SDS of smectites showed increasing rate of their dissolution in HCl with rising substitution for octahedral Al. The MB spectra of SWy-2 revealed heterogeneity of layer charge distribution in this montmorillonite. Formation of MB monomers confirmed that smectite/illite was more stable in HCl than illite despite of its higher content of swelling layers. The MB spectra were found to better reflect the changes in the LC of acid-treated clay minerals with non-swelling layers than the CEC values. The spectra of well-ordered kaolinite did not change at all within acid-treatment, while a slight increase of monomers band was observed for disordered kaolinite after 36 h. The obtained results confirmed that MB adsorption is a very sensitive tool for characterization of layer charge of different clay minerals modified upon acid treatment.