166 research outputs found
Etude expérimentale de l'hydratation des smectites par simulation des raies 00ℓ de diffraction des rayons X. Implications pour l'étude d'une perturbation thermique sur la minéralogie de l'argilite du site Meuse-Haute Marne
No full textVictor A. Drits, Russian Academy of Sciences, Moscou, Russie / Laurent Michot, LEM, Nancy, France / Javier Cuadros, Natural History Museum, Londres, U.K. / Alfred Delville, CRMD, Orléans, France / Emmanuel Jacquot, ANDRA, Châtenay-Malabry, France / Bruno Lanson, LGIT, Grenoble, FranceThe structural modifications affecting the reactive mineral constituents of the clay barriers (smectite) and possibly resulting from the thermal pulse related to nuclear waste storage are essentially limited to the amount and location of the layer charge deficit. These modifications likely impact the hydration properties of these minerals, and a specific methodology has thus been developed to describe, using simulation of X-ray diffraction profiles (00ℓ reflections), these hydration properties and specifically the heterogeneity resulting from the interstratification of different layer types, each exhibiting a specific hydration state. The detailed study of the hydration properties of a low-charge montmorillonite (octahedral charge) has shown that the affinity of the interlayer cation for water rules the hydration state and the thickness of hydrated smectite layers. If the layer charge is increased, the transition between the different hydration states is shifted, following a water desorption isotherm, towards lower relative humidities. In addition, the hydration of studied beidellites (tetrahedral charge) was shown to be more heterogeneous than that of montmorillonites. The developed methodology also allowed describing the structural modifications resulting from a chemical perturbation (chlorinated anionic background, pH). Finally, the link between the thickness of elementary layers and the amount of interlayer water molecules has been evidenced. A new structure model has also been determined for these interlayer species allowing an improved description of their positional distribution in bihydrated interlayers.Suite à la perturbation thermique induite par le stockage de déchets radioactifs, les modifications structurales risquant d'affecter les constituants minéraux réactifs (smectite) de la barrière argileuse concernent principalement la quantité et la localisation de la charge foliaire et modifient potentiellement les propriétés d'hydratation de ces minéraux. Une méthodologie a donc été développée afin de décrire, par simulation des diffractogrammes de rayons X (raies 00ℓ), ces propriétés d'hydratation et en particulier l'hétérogénéité résultant de l'interstratification de feuillets présentant différents états d'hydratation. L'étude des propriétés d'hydratation d'une montmorillonite (charge octaédrique) bassecharge a montré que l'affinité du cation interfoliaire pour l'eau gouverne l'état d'hydratation et l'épaisseur des feuillets hydratés. L'augmentation de la quantité de charge induit, en désorption d'eau, un décalage vers les plus basses humidités relatives des transitions entre les différents états d'hydratation. D'autre part, les beidellites étudiées (charge tétraédrique) présentent des états d'hydratation plus hétérogènes que les montmorillonites. L'approche développée a également permis de décrire les modifications structurales induites par une perturbation chimique (influence du fond chloré, pH). Enfin, nous avons pu mettre en évidence le lien structural entre l'épaisseur des feuillets élémentaires et la quantité d'eau interfoliaire, pour laquelle un nouveau modèle structural, permettant de mieux décrire leur distribution de position, a été établi pour les feuillets bi-hydratés
Etude expérimentale de l'hydratation des smectites par simulation des raies 00ℓ de diffraction des rayons X. Implications pour l'étude d'une perturbation thermique sur la minéralogie de l'argilite du site Meuse-Haute Marne
No full textVictor A. Drits, Russian Academy of Sciences, Moscou, Russie / Laurent Michot, LEM, Nancy, France / Javier Cuadros, Natural History Museum, Londres, U.K. / Alfred Delville, CRMD, Orléans, France / Emmanuel Jacquot, ANDRA, Châtenay-Malabry, France / Bruno Lanson, LGIT, Grenoble, FranceThe structural modifications affecting the reactive mineral constituents of the clay barriers (smectite) and possibly resulting from the thermal pulse related to nuclear waste storage are essentially limited to the amount and location of the layer charge deficit. These modifications likely impact the hydration properties of these minerals, and a specific methodology has thus been developed to describe, using simulation of X-ray diffraction profiles (00ℓ reflections), these hydration properties and specifically the heterogeneity resulting from the interstratification of different layer types, each exhibiting a specific hydration state. The detailed study of the hydration properties of a low-charge montmorillonite (octahedral charge) has shown that the affinity of the interlayer cation for water rules the hydration state and the thickness of hydrated smectite layers. If the layer charge is increased, the transition between the different hydration states is shifted, following a water desorption isotherm, towards lower relative humidities. In addition, the hydration of studied beidellites (tetrahedral charge) was shown to be more heterogeneous than that of montmorillonites. The developed methodology also allowed describing the structural modifications resulting from a chemical perturbation (chlorinated anionic background, pH). Finally, the link between the thickness of elementary layers and the amount of interlayer water molecules has been evidenced. A new structure model has also been determined for these interlayer species allowing an improved description of their positional distribution in bihydrated interlayers.Suite à la perturbation thermique induite par le stockage de déchets radioactifs, les modifications structurales risquant d'affecter les constituants minéraux réactifs (smectite) de la barrière argileuse concernent principalement la quantité et la localisation de la charge foliaire et modifient potentiellement les propriétés d'hydratation de ces minéraux. Une méthodologie a donc été développée afin de décrire, par simulation des diffractogrammes de rayons X (raies 00ℓ), ces propriétés d'hydratation et en particulier l'hétérogénéité résultant de l'interstratification de feuillets présentant différents états d'hydratation. L'étude des propriétés d'hydratation d'une montmorillonite (charge octaédrique) bassecharge a montré que l'affinité du cation interfoliaire pour l'eau gouverne l'état d'hydratation et l'épaisseur des feuillets hydratés. L'augmentation de la quantité de charge induit, en désorption d'eau, un décalage vers les plus basses humidités relatives des transitions entre les différents états d'hydratation. D'autre part, les beidellites étudiées (charge tétraédrique) présentent des états d'hydratation plus hétérogènes que les montmorillonites. L'approche développée a également permis de décrire les modifications structurales induites par une perturbation chimique (influence du fond chloré, pH). Enfin, nous avons pu mettre en évidence le lien structural entre l'épaisseur des feuillets élémentaires et la quantité d'eau interfoliaire, pour laquelle un nouveau modèle structural, permettant de mieux décrire leur distribution de position, a été établi pour les feuillets bi-hydratés
Investigation of the Interlayer Organization of Water and Ions In Smectite from the Combined Use of Diffraction Experiments And Molecular Simulations. a Review of Methodology, Applications, And Perspectives
No full textUn nouveau pas vers le contrôle des propriétés des MXènes
No full textHighlight sur l'article DOI: 10.1021/acs.chemmater.8b0397
Un nouveau pas vers le contrôle des propriétés des MXènes
No full textHighlight sur l'article DOI: 10.1021/acs.chemmater.8b03976Lien:https://inc.cnrs.fr/fr/cnrsinfo/un-nouveau-pas-vers-le-controle-des-proprietes-des-mxene
Thermodynamics of hydration of MX80-Na: an experimental study of the hydration energies
No full textHydration properties of swelling clay minerals may be very variable depending on the chemical composition of the clay, on the nature of the interlayer cations and on the interlayer charge (Berend et al., 1995; Vieillard et al., 2011). The Wyoming smectite has been largely studied, notably for assessing its hydration behavior as a function of the interlayer cations, in connection with its structural characteristics (Ferrage et al., 2005; Salles et al., 2007). In the present work, carried out as part of a collaborative Andra/BRGM/HydrASA research program for ThermoChimie project, we propose an original experimental study, based on adsorption and desorption isotherms performed on MX80 clay samples. The goal is to determine energetic contributions to the reactions of hydration, which have been revealed to be non-negligible with respect to the stability of the clay minerals (Gailhanou et al., submitted). In particular, the present work addresses the problems of the hysteresis loop between adsorption and desorption isotherms and of the irreversibility of hydration reactions. This is directly related to the application of classical thermodynamics to the hydration reactions of clay minerals. In a first stage, an experimental study is dedicated to better understand the origin of the hysteresis loop which is systematically observed for the adsorption-desorption isotherms at 25°C. The development of the hysteresis loop has been studied by considering several kinetically related parameters: stabilization periods, temperatures (from 25°C to 60°C) and hydration steps (Figure 1). No sensible change was observed in the hysteresis loop. Therefore, the amount of adsorbed water depends on the followed reaction pathway (adsorption or desorption). The variations in microstructures and in the distribution of hydration layers (0/1/2 water layers; Ferrage et al., 2005) as a function of relative humidity (RH) could provide a possible explanation for this phenomenon
Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 1. Results from X-ray Diffraction Profile Modeling
No full textInternational audienceThe dehydration of two Na-saturated synthetic saponites with contrasting layer charge was studied by modeling the X-ray diffraction (XRD) patterns recorded along a water vapor desorption isotherm. The interlayer configurations used to reproduce the XRD data over a large angular range include Na+ cations located in the interlayer midplane and H2O molecules normally distributed about one or two main positions for mono- and bihydrated layers, respectively. Although strongly reduced in comparison to natural smectites, hydration heterogeneity was systematically observed for these synthetic saponites, especially along the transition between two hydration states. Using improved models for the description of the interlayer organization, the influence of layer charge on the structure of interlayer water can be precisely assessed. In addition, the comparison with water contents obtained from water vapor gravimetry experiments allows discriminating the relative contributions of H2O molecules from 1W and 2W interlayers (crystalline water) and from the pore space network
Experimental evidence for calcium-chloride ion pairs in the interlayer of montmorillonite. A XRD profile modeling approach.
No full textMontmorillonite was equilibrated with high normality Cl - solutions to assess the possible presence of MeCl + ion pairs in smectite interlayers which is suggested by chemical modeling of cation exchange experimental studies. Structural modifications induced by the presence of such ion pairs, and more especially those related to smectite hydration properties, were characterized from the modeling of experimental X-ray diffraction (XRD) profiles. As compared to those obtained from samples prepared at low ionic strength, XRD patterns from samples equilibrated in high ionic strength CaCl2 solutions exhibited a small positional shift of 0
Cation diffusion in the interlayer space of swelling clay minerals - A combined macroscopic and microscopic study
No full textInternational audienceThis study investigates the diffusion process of calcium cations confined in the interlayer space of 5 mm disks of vermiculite swelling clay minerals during the Na-for-Ca exchange process. Diffusion experiments were performed at four NaCl salinities (3 x 10(-3) 3, 5 x 10(-2), 0.1 and 1 M) of the exchanger solution. A macroscopic analysis of the diffusion process based on the aqueous calcium concentrations released in the solution and on Ca-profiles obtained in the solid was performed using a pore diffusion model that has been classically used in the literature. The results obtained at the macroscopic scale showed that the apparent diffusion coefficients describing both aqueous and profiles data for Ca depend on the diffusion time and salinity of the aqueous reservoir. Such variations suggested that interlayer diffusion was driven by (1) the gradient of the sorbed species in the interlayer, which depends on the diffusion time due to the ion exchange equilibrium; and (2) the discontinuity, due to Donnan equilibrium, existing at the limit between the "internal disk border" and the "external disk border" in contact with the aqueous reservoir. Then, a set of molecular and Brownian dynamics simulations was used to (1) assess such interpretations and (2) quantitatively predict aqueous and profile data obtained at the macroscopic scale. For an aqueous reservoir with high salinity (1 M NaCl), a good agreement was obtained between the macroscopic data and the predictions obtained from Brownian dynamics simulations, confirming the role played by the gradient of the interlayer species that is suggested at the macroscopic scale and which is at the basis of the "surface diffusion models" published in literature. In addition, for aqueous reservoirs with lower salinity (5 x 10(-2) M), the results obtained by Brownian dynamics simulations and normalized to the exchange rate measured at infinite time showed that the diffusion properties of the species in the aqueous reservoir cannot be neglected to correctly interpret macroscopic data. This behavior confirms the role played by the ionic flux that exists at the "disk border", which can limit the global diffusion process in a low salinity reservoir, even if it is well stirred. Moreover, by assuming a tortuosity equal to 1 for monocrystals, the self- diffusion coefficient issued from molecular dynamics simulations is in good agreement with the apparent diffusion coefficient describing macroscopic data when the gradient of sorbed concentrations within the solid is null; this latter condition is obtained in our case at infinite time (20 days) when the initial Ca- saturated disks are fully exchanged with Na cations. Finally, the use of monocrystals allows us to have only interlayer porosity and then to obtain a self- diffusion coefficient for Ca from Molecular Dynamic simulations, which is in good agreement with Ca-surface mobility, which was defined by some authors to predict the "surface diffusion process" at the macroscopic scale
Influence of pH on the interlayer cationic composition and hydration state of Ca-montmorillonite: analytical chemistry, chemical modelling and XRD profile modelling study.
No full textThe hydration state of a <2 μm fraction of Ca-saturated SWy-2 montmorillonite was characterised after rapid equilibration (3 hours) under pH-controlled conditions (0.1-12.6 pH range). The solution composition was monitored together with the interlayer composition and X-ray diffraction (XRD) patterns were recorded on oriented preparations. Experimental XRD patterns were then fitted using a trial-and-error procedure to quantify the relative proportions of layers with different hydration states. The montmorillonite is mostly bi-hydrated in basic and near-neutral conditions whereas it is mostly mono-hydrated at low pH. The transition from the bi-hydrated to the mono-hydrated state occurs through very heterogeneous structures. However, the proportion of the different layer types determined from XRD profile modelling and that derived from chemical modelling using Phreeqc2 code strictly coincide. This correlation shows that the hydration modification is induced by a H3O +-for-Ca2+ exchange at low pH, the two species being distributed in different interlayers. This layer-by-layer exchange process occurs randomly in the layer stack. Under alkaline conditions, results from XRD profile modelling and from near infrared diffuse reflectance spectroscopy (NIR-DRS) clearly demonstrate that there is no CaOH +-for-Ca2+ exchange at high pH. The apparent increase in Ca sorption in smectite interlayers with increasing pH is thus probably related to the precipitation of CalciumSilicate-Hydrate (CSH) phases, which also accounts for the decrease in Si concentration under high-pH conditions. This precipitation is thermodynamically favoured
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