Influence de la cristallochimie des smectites sur la structuration de l’eau et des cations interfoliaires.

Swelling clay minerals such as smectites are ubiquitous at the Earth surface and possess major hydration ability and contaminant uptake/retention capacity. As a consequence smectites exert a pivotal influence on elemental transfers in surficial environments. These properties are especially relevant also when smectites are used as sealant in engineered or geological barriers for waste disposal facilities. As interlayer H2O molecules account for more than 80% of smectite water in undersaturated conditions, characterization of H2O organization and dynamics in smectites interlayers is essential to determining the geometrical and dynamical properties of clay barriers for waste disposal and to predicting the mobility of contaminant whose principal vector is water. Within this general framework, the present works describe, in a first time, the structuration of interlayer water/cations in saturated conditions. Then, in a second time, review the influence of structural parameters such as the amount and location of layer charge deficit and the chemical composition (and more especially the presence of structural fluorine/hydroxyl) on smectite hydration properties. A set of samples covering the whole compositional range of swelling phyllosilicates has thus been synthesized and characterized chemically and structurally.Special attention was paid to determining the amount (water vapor sorption isotherms) and the distribution (X-ray diffraction) of interlayer water. Molecular modeling allowed unraveling the origin of the contrasting behaviors observed experimentally and to determine the influence of the different crystal-chemical parameters on smectite hydration. This step is essential for the prediction of smectite reactivity in the environment from a limited number of crystal-chemical parameters. Molecular modeling allowed unraveling the origin of the contrasting behaviors observed experimentally and to determine the influence of the different crystal-chemical parameters on smectite hydration. This step is essential for the prediction of smectite reactivity in the environment from a limited number of crystal-chemical parameters.Les minéraux argileux gonflants tels que les smectites sont omniprésents à la surface de la Terre et possèdent d’importantes capacités d'hydratation et d’absorption/rétention de contaminants.Ainsi, ces dernières exercent une influence clé sur les transferts d’éléments dans les environnements de surface. Ces propriétés sont particulièrement importantes quand les smectites sont utilisées comme barrières ouvragées ou géologiques pour les installations de contrôle des déchets. Les molécules d’eau dans l’interfoliaire représentent plus de 80% de l'eau en contact avec les smectites, dans des conditions non saturées. La caractérisation de l'organisation et de la dynamique de l’eau dans les smectites est donc essentielle pour prédire la mobilité des contaminants, dont le principal vecteur est l'eau. Dans ce cadre général, les présents travaux décrivent, dans un premier temps, la structuration interfoliaire eau/cations dans des conditions saturées, jusqu’alors méconnue. Puis, dans un second temps, nous avons examiné l'influence des paramètres structuraux (tels que la quantité et l'emplacement des charges, la composition chimique et plus particulièrement la présence de groupements fluors/hydroxyles) sur les propriétés d'hydratation des smectites. Une série d'échantillons couvrant toute la gamme de composition des smectites a donc été synthétisée et caractérisée structurellement. Une attention particulière a été accordée à la détermination du montant (isothermes vapeur d'eau de sorption) et la distribution (de diffraction des rayons X) de l'eau dans l’espace interfoliaire. La modélisation moléculaire a permis d’aller plus loin dans la compréhension de l'origine des comportements contrastés observés expérimentalement, et de déterminer l'influence des différents paramètres cristallochimiques sur l'hydratation de la smectite. Cette étape est essentielle pour la prédiction de la réactivité des smectites dans l'environnement sur la base d'un nombre limité de paramètres cristallochimiques

Influence of smectites crystal chemistry on the structure of water and interlayer cations

Les minéraux argileux gonflants tels que les smectites sont omniprésents à la surface de la Terre et possèdent d'importantes capacités d'hydratation et d'absorption/rétention de contaminants. Ainsi, ces dernières exercent une influence clé sur les transferts d'éléments dans les environnements de surface. Ces propriétés sont particulièrement importantes quand les smectites sont utilisées comme barrières ouvragées ou géologiques pour les installations de contrôle des déchets. Les molécules d'eau dans l'interfoliaire représentent plus de 80% de l'eau en contact avec les smectites, dans des conditions non saturées. La caractérisation de l'organisation et de la dynamique de l'eau dans les smectites est donc essentielle pour prédire la mobilité des contaminants, dont le principal vecteur est l'eau. Dans ce cadre général, les présents travaux décrivent, dans un premier temps, la structuration interfoliaire eau/cations dans des conditions saturées, jusqu'alors méconnue. Puis, dans un second temps, nous avons examiné l'influence des paramètres structuraux (tels que la quantité et l'emplacement des charges, la composition chimique et plus particulièrement la présence de groupements fluors/hydroxyles) sur les propriétés d'hydratation des smectites. Une série d'échantillons couvrant toute la gamme de composition des smectites a donc été synthétisée et caractérisée structurellement. Une attention particulière a été accordée à la détermination du montant (isothermes vapeur d'eau de sorption) et la distribution (de diffraction des rayons X) de l'eau dans l'espace interfoliaire. La modélisation moléculaire a permis d'aller plus loin dans la compréhension de l'origine des comportements contrastés observés expérimentalement, et de déterminer l'influence des différents paramètres cristallochimiques sur l'hydratation de la smectite. Cette étape est essentielle pour la prédiction de la réactivité des smectites dans l'environnement sur la base d'un nombre limité de paramètres cristallochimiques.Swelling clay minerals such as smectites are ubiquitous at the Earth surface and possess major hydration ability and contaminant uptake/retention capacity. As a consequence smectites exert a pivotal influence on elemental transfers in surficial environments. These properties are especially relevant also when smectites are used as sealant in engineered or geological barriers for waste disposal facilities. As interlayer H2O molecules account for more than 80% of smectite water in undersaturated conditions, characterization of H2O organization and dynamics in smectites interlayers is essential to determining the geometrical and dynamical properties of clay barriers for waste disposal and to predicting the mobility of contaminant whose principal vector is water. Within this general framework, the present works describe, in a first time, the structuration of interlayer water/cations in saturated conditions. Then, in a second time, review the influence of structural parameters such as the amount and location of layer charge deficit and the chemical composition (and more especially the presence of structural fluorine/hydroxyl) on smectite hydration properties. A set of samples covering the whole compositional range of swelling phyllosilicates has thus been synthesized and characterized chemically and structurally. Special attention was paid to determining the amount (water vapor sorption isotherms) and the distribution (X-ray diffraction) of interlayer water. Molecular modeling allowed unraveling the origin of the contrasting behaviors observed experimentally and to determine the influence of the different crystal-chemical parameters on smectite hydration. This step is essential for the prediction of smectite reactivity in the environment from a limited number of crystal-chemical parameters. Molecular modeling allowed unraveling the origin of the contrasting behaviors observed experimentally and to determine the influence of the different crystal-chemical parameters on smectite hydration. This step is essential for the prediction of smectite reactivity in the environment from a limited number of crystal-chemical parameters

Scientific bulletin

Swelling clay minerals such as smectites are ubiquitous at the Earth surface and possess major hydration ability and contaminant uptake/retention capacity. As a consequence smectites exert a pivotal influence on elemental transfers in surficial environments. These properties are especially relevant also when smectites are used as sealant in engineered or geological barriers for waste disposal facilities. As interlayer H2O molecules account for more than 80% of smectite water in undersaturated conditions, characterization of H2O organization and dynamics in smectites interlayers is essential to determining the geometrical and dynamical properties of clay barriers for waste disposal and to predicting the mobility of contaminant whose principal vector is water. Within this general framework, the present works describe, in a first time, the structuration of interlayer water/cations in saturated conditions. Then, in a second time, review the influence of structural parameters such as the amount and location of layer charge deficit and the chemical composition (and more especially the presence of structural fluorine/hydroxyl) on smectite hydration properties. A set of samples covering the whole compositional range of swelling phyllosilicates has thus been synthesized and characterized chemically and structurally. Special attention was paid to determining the amount (water vapor sorption isotherms) and the distribution (X-ray diffraction) of interlayer water. Molecular modeling allowed unraveling the origin of the contrasting behaviors observed experimentally and to determine the influence of the different crystal-chemical parameters on smectite hydration. This step is essential for the prediction of smectite reactivity in the environment from a limited number of crystal-chemical parameters. Molecular modeling allowed unraveling the origin of the contrasting behaviors observed experimentally and to determine the influence of the different crystal-chemical parameters on smectite hydration. This step is essential for the prediction of smectite reactivity in the environment from a limited number of crystal-chemical parameters.Les minéraux argileux gonflants tels que les smectites sont omniprésents à la surface de la Terre et possèdent d'importantes capacités d'hydratation et d'absorption/rétention de contaminants. Ainsi, ces dernières exercent une influence clé sur les transferts d'éléments dans les environnements de surface. Ces propriétés sont particulièrement importantes quand les smectites sont utilisées comme barrières ouvragées ou géologiques pour les installations de contrôle des déchets. Les molécules d'eau dans l'interfoliaire représentent plus de 80% de l'eau en contact avec les smectites, dans des conditions non saturées. La caractérisation de l'organisation et de la dynamique de l'eau dans les smectites est donc essentielle pour prédire la mobilité des contaminants, dont le principal vecteur est l'eau. Dans ce cadre général, les présents travaux décrivent, dans un premier temps, la structuration interfoliaire eau/cations dans des conditions saturées, jusqu'alors méconnue. Puis, dans un second temps, nous avons examiné l'influence des paramètres structuraux (tels que la quantité et l'emplacement des charges, la composition chimique et plus particulièrement la présence de groupements fluors/hydroxyles) sur les propriétés d'hydratation des smectites. Une série d'échantillons couvrant toute la gamme de composition des smectites a donc été synthétisée et caractérisée structurellement. Une attention particulière a été accordée à la détermination du montant (isothermes vapeur d'eau de sorption) et la distribution (de diffraction des rayons X) de l'eau dans l'espace interfoliaire. La modélisation moléculaire a permis d'aller plus loin dans la compréhension de l'origine des comportements contrastés observés expérimentalement, et de déterminer l'influence des différents paramètres cristallochimiques sur l'hydratation de la smectite. Cette étape est essentielle pour la prédiction de la réactivité des smectites dans l'environnement sur la base d'un nombre limité de paramètres cristallochimiques

Interlayer structure model of tri-hydrated low-charge smectite by X-ray diffraction and Monte Carlo modeling in the Grand Canonical ensemble

International audienceThe present study aims primarily at refining a structure model for interlayer cations and H2O molecules in tri-hydrated (3W) smectite (d(001) = 18-19 angstrom). The <2 mu m fraction of the SWy-2 source clay (low-charge montmorillonite) was saturated by Mg2+, Ca2+, Ba2+, or Na cations, before collection of X-ray diffraction (XRD) patterns at 98% relative humidity. Experimental d(001) values derived for the essentially homogeneous 3W hydrates provided volume constraints for Grand Canonical Monte Carlo (GCMC) simulations. Computed atomic density distribution of interlayer species were used in turn to calculate XRD intensities of 00l reflections. The agreement between calculated and experimental 00l intensities allowed validating the GCMC results of both interlayer H2O content and distribution of interlayer species (cations and H2O molecules). Computed atomic density profiles do not correspond to the usual model of three discrete planes of H2O molecules but rather exhibit two sharp planes of H2O molecules wetting the clay surfaces (at similar to 2.7 angstrom from the clay layer surface). Additional H2O molecules belong to cation hydration shells or define a poorly organized ensemble filling internal voids. This alternative model suggests that the stability of the 3W hydrate results from the dual interaction of some H2O molecules with interlayer cation, through their second hydration shell; and with the 2:1 clay surface. Computed atomic density profiles were approximated to propose an interlayer structure model for 3W smectite. This simplified model includes two sets of two planes (symmetrical relative to the interlayer mid-plane) for H2O molecules and one set for interlayer cations. This model allows reproducing experimental XRD patterns for the different samples investigated and thus represents a valid set of parameters for routine quantitative analysis of XRD profiles in an effort to determine smectite reactivity close to water-saturated conditions. Implications of such studies are crucial to provide experimental constraints on the behavior of the main vector of element transfer under conditions common in surficial environments and prevailing in waste repositories. In addition, the present study provides an experimental validation of structure models derived from the widely used ClayFF model, and thus allows its use to predict the fate of water in clayey systems close to water-saturated conditions

Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume

The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle I (EM I) source of basalts from, for example, Pitcairn or Walvis Ridge has been variously attributed to recycled pelagic sediments, lower continental crust, or recycled subcontinental lithosphere. Our study helps resolve this debate by showing that Pitcairn lavas contain sulfides whose sulfur isotopic compositions are affected by mass-independent fractionation (S-MIF down to Δ33S = −0.8), something which is thought to have occurred on Earth only before 2.45 Ga, constraining the youngest possible age of the EM I source component. With this independent age constraint and a Monte Carlo refinement modeling of lead isotopes, we place the likely Pitcairn source age at 2.5 Ga to 2.6 Ga. The Pb, Sr, Nd, and Hf isotopic mixing arrays show that the Archean EM I material was poor in trace elements, resembling Archean sediment. After subduction, this Archean sediment apparently remained stored in the deep Earth for billions of years before returning to the surface as Pitcairn´s characteristic EM I signature. The presence of negative S-MIF in the deep mantle may also help resolve the problem of an apparent deficit of negative Δ33S anomalies so far found in surface reservoirs

Molecular Dynamics Simulations of Water Structure and Diffusion in a 1 nm Diameter Silica Nanopore as a Function of Surface Charge and Alkali Metal Counterion Identity

International audienceWater confined in nanopores—particularly in pores narrower than 2 nm—displays distinct physicochemical properties that remain incompletely examined despite their importance in nanofluidics, molecular biology, geology, and materials sciences. Here, we use molecular dynamics simulations to investigate the coordination structure and mobility of water and alkali metals (Li, Na, K, Cs) inside a 1 nm diameter cylindrical silica nanopore as a function of surface charge density, a model system particularly relevant to the alteration kinetics of silicate glasses and minerals in geologic formations. We find that the presence of a negative surface charge and adsorbed counterions within the pore strongly impacts water structure and dynamics. In particular, it significantly orients water O–H bonds toward the surface and slows water diffusion by almost 1 order of magnitude. Ion crowding in the charged nanopore enhances the tendency of counterions to coordinate closely with the silica surface, which moderates the impact of ions on water dynamics. Co-ions are strongly excluded from the nanopore at all surface charges, suggesting that the 1 nm diameter cylindrical silica nanopores likely exhibit nearly ideal semipermeable membrane transport properties

Smectite fluorination and its impact on interlayer water content and structure: A way to fine tune the hydrophilicity of clay surfaces?

International audienceIn addition to isomorphic cation substitutions, smectite layers may present anionic substitutions with fluorine replacing the structural hydroxyl groups and inducing a reduced number of H2O molecules hydrating interlayer cations. The resulting additional versatility of smectite layers could be used to fine-tune the hydrophilicity of pure or intercalated smectite pending a detailed understanding of interlayer water organization. The present article thus reports on the hydration of (fluoro-)hectorite samples exhibiting similar charge density (structural formulae: [Na0.8 nH2O]inter[Mg5.2Li0.8]oct[Si8.0]tetO20(OH,F)4). Water sorption isotherms and PIGE/PIXE analyses allowed constraining the water content over the probed range of relative humidity and the bulk chemistry, respectively. Modeling of X-ray diffraction (XRD) profiles obtained along water vapor desorption isotherms allowed gaining additional insights in the distribution of interlayer H2O molecules. Compared to hydroxylated smectites of similar charge, fluorinated hectorites contain 30% less interlayer H2O molecules although transitions between discrete hydration states are observed for similar values of relative humidity. These molecules (3-4 and 6-8 H2O molecules per cation in the mono- and bi-hydrated states, respectively) predominantly belong to the hydration sphere of interlayer cations, although additional H2O molecules are present in the bihydrated state. As a consequence, the positional disorder of interlayer H2O molecules is much reduced in fluorinated samples, thus increasing the minimum distance from an interlayer H2O molecule to the smectite layer, most likely owing to the hydrophobicity of fluorinated layers. Finally, the distribution of layers with a given hydration state is more heterogeneous within smectite crystals for fluorinated smectites, compared to hydroxylated ones, possibly as the result of the improved crystallinit

Orientation measurements of clay minerals by polarized attenuated total reflection infrared spectroscopy

International audienceThe orientation and organization of molecular guests within the interlayer of clay minerals control the reactivity and performance of tailored organo-clay materials. Such a detailed investigation of hybrid structure on the molecular scale is usually provided by computational methods with limited experimental validation. In this study, polarized attenuated total reflection infrared spectroscopy was used to extract quantitative orientation measurements of montmorillonite particles. The validity of the evanescent electric field amplitude calculations necessary to derive the order parameter was critically evaluated to propose a methodology for determining the orientation of the normal to the clay layer relative to a reference axis, enabling comparison with the results obtained from X-ray scattering experiments and molecular dynamic simulations. Subsequently, the orientation of the interlayer water dipole and surface hydroxyls with respect to the normal of the clay layer was experimentally determined, showing good agreement with molecular simulations. This methodology may provide quantitative insights into the molecular-level description of interfacial processes between organic molecules and clay minerals

Influence of layer charge on hydration properties of synthetic octahedrally-charged Na-saturated trioctahedral swelling phyllosilicates

International audienceSmectite hydration impacts dynamical properties of interlayer cations and thus the transfer and fate of H2O, contaminants, and nutriments in surficial environments where this ubiquitous clay mineral is often one of the main mineral components. The influence of key crystal-chemical parameters, such as the amount of charge or the presence of fluorine, rather than hydroxyl groups, in smectite anionic framework, on hydration, organization of interlayer species, and related properties has been described for tetrahedrally substituted trioctahedral smectites (saponites). Despite the ubiquitous character of octahedrally substituted smectites, that make most of the world bentonite deposits

Connecting molecular simulations and laboratory experiments for the study of time-resolved cation-exchange process in the interlayer of swelling clay minerals

International audienceIn the context of element migration in clay-rich media, self-diffusion coefficients of interlayer cations in swelling clay minerals obtained from molecular simulations are rarely used by macroscopic models predicting cation-exchange processes. Based on experiments and simulations, this study aims at (i) making a connection between molecular and sample scale processes to predict the dynamics of cation-exchange reactions between the interlayer space of millimetre disks of vermiculite and aqueous reservoirs, and (ii) assessing the role played by both self-diffusion and selectivity coefficients on this process.Time-resolved cation exchange experiments were performed using Ca-saturated vermiculite disks immersed in aqueous reservoirs with different NaCl or SrCl2 salinities. The results were reproduced via a finite-volume model constrained by (i) cation self-diffusion coefficients calculated by molecular dynamics simulations and (ii) interlayer selectivity coefficients drawn from “batch” cation-exchange isotherms.Results showed that considering the averaged values for both the cation-exchange selectivity coefficients and self-diffusion coefficients of the slowest interlayer cation led to good agreement between the experiments and simulations, validating the modelling strategy for the connection between the molecular and laboratory time scales. A sensitivity test regarding the influence of the two input parameters on the overall results was then performed. This study underlined a constrained upscaling strategy to better assess the role played by different intrinsic parameters of the clay/water systems (molecular self-diffusion coefficients in the interlayer space vs. selectivity coefficient) on the diffusion of cations during cation-exchange reaction in clay-rich media