Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series

International audienceThe aim of this study is to synthesize and finely characterize montmorillonite samples, dioctahedral smectites without tetrahedral charges (structural formulae Nax(Al(2 − x)Mgx)Si4O10(OH)2), to allow their use as reference samples in clay science. The montmorillonites synthesis under hydrothermal conditions at different pressures and with various layer charge deficit has been attempted. The temperature was fixed at 320 °C, the pressure parameter values were 20 MPa, 80 MPa, 120 MPa and 200 MPa. The Mg content varied from 0.25 to 0.60 per half unit cell. The reaction products have been characterized with multi-technique analyses (ICP-AES, EMP, CEC, XRD, FTIR, NMR and TGA). Montmorillonite phase was only produced at 120 and 200 MPa. At 20 and 80 MPa, the results suggest that a 0.33 and 0.16-tetrahedral charge deficit exist in the formed samples. Moreover, the octahedral occupancies are higher than two (2.15 and 2.07 at 20 and 80 MPa respectively). In these experimental conditions, the synthetic smectites are mixtures between montmorillonite, beidellite and saponite. At 120 MPa and for a Mg content of 0.25 or higher than 0.33, the synthetic products were also mixtures of smectites. Tetrahedral charge deficits of 0.11, 0.11 and 0.15 were found for Mg contents of 0.25, 0.50 and 0.60 respectively. The octahedral occupancy was also higher than 2.00. A montmorillonite phase with only octahedral charges and an octahedral occupancy near 2.00 was synthesized for a Mg content of 0.33 and at pressures equal to or higher than 120 MPa. This low charge reference smectite shows a very low amount of accessory minerals and an octahedral charge deficit only created by the presence of magnesium in the structure. This montmorillonite can be compared structurally to the most studied natural one: the montmorillonite SWy-2 from Wyoming

Thermodynamic assessment of the variation of the surface areas of two synthetic swelling clays during adsorption of water

International audienceTwo synthetic smectites (montmorillonite and beidellite) are studied by a water adsorption technique in order to assess their specific surface areas under atmospheric conditions. A route recently proposed for extracting the thermodynamic data from experimental adsorption isotherms is used. The variation of the specific surface area during water adsorption is obtained, which can be linked to the enlargement of the interlayer space determined using X-ray diffraction. This variation is compared to an idealized specific surface area obtained from TEM and X-ray measurements in agreement with crystallographic models. All these results are also compared with those obtained previously for a natural montmorillonite. A simple view of swelling is proposed

Experimental study of smectite interaction with metal iron at low temperature: 1. Smectite destabilization.

Interaction between metal Fe and a variety of natural and synthetic smectite samples with contrasting crystal chemistry was studied by scanning electron microscopy and X-ray diffraction from experiments conducted at 80°C. These experiments demonstrate an important reactivity contrast as a function of smectite crystal chemistry. An XRD method involving the use of an internal standard allowed quantification of the relative proportion of smectite destabilized as a function of initial pH conditions as well as of smectite structural parameters. In mildly acidic to neutral pH conditions, a significant proportion of metal Fe is corroded to form magnetite without smectite destabilization. Under basic pH conditions, smectite and metal Fe are partly destabilized to form magnetite and newly-formed 1:1 phyllosilicate phases (odinite and crondstedtite). More specifically, systematic destabilization of both metal Fe and smectite is observed for dioctahedral smectites while trioctahedral smectites are essentially unaffected under similar experimental conditions. In addition, smectite reactivity is enhanced with increasing Fe3+ content and with the presence of Na+ cations in smectite interlayers. A conceptual model for smectite destabilization is proposed. This model involves first the release of protons from smectite structure, MeFe3+OH groups being deprotonated preferentially and metal Fe acting as proton acceptor. Corrosion of metal Fe results from its interaction with these protons. The Fe2+ cations resulting from this corrosion process sorb on the edges of smectite particles to induce the reduction of structural Fe3+ and migrate into smectite interlayers to compensate for the increased layer-charge deficit. Interlayer Fe2+ cations subsequently migrate to the octahedral sheet of smectite because of the extremely large layer-charge deficit. At low temperature, this migration is favored by the reaction time and by the absence of protons within the ditrigonal cavity. Smectite destabilization results from the inability of the tetrahedral sheets to accommodate the larger dimensions of the newly formed trioctahedral domains resulting from the migration of Fe2+ cations

Réactivité fer métal/smectites en milieu hydraté à 80°C

M. Michel Mudry Professeur (LME Polytech' Université d'Orléans)Président de juryM. Gilles Berger Chargé de recherches CNRS ( LMTG Toulouse)RapporteurM. Bruce Velde Directeur de recherches (ENS Paris) RapporteurM. Alain Plançon Professeur (ISTO Université d'Orléans) Directeur de thèseM. Alain Baronnet Professeur (CRMC2 Université d'Aix Marseille 3)ExaminateurM. Michel Jullien Chercheur (CEA Cadarache) ExaminateurThe storage of high activity nuclear wastes generally considers a metal container (iron), surrounded by an engineered argillaceous barrier (smectites). X-ray diffraction, infrared spectrometry and transmission electron microscopy used for chemical analysis and determination of the valence state of iron, make it possible to determine the evolution of the iron metal + smectite in presence of water at 80°C. The rate of detabilization of smectites varies from 0 to 100% depending on its characteristics: chemical composition, interlayer space occupancy, amount of octahedral iron. This smectite-iron metal reaction generates magnetite as well as a gel phase. Inside this gel, a new, very iron rich, 1 :1 phyllosilicate crystallises, with particles where the Fe(II)/Fe(III) ratio is 20/80 (close to odinite) or 50/50 (close to cronstedtite).Les dispositifs de stockage envisagés pour les déchets nucléaires de haute activité font intervenir une barrière ouvragée argileuse (smectites) ainsi que des conteneurs métalliques (fer). La diffraction des rayons X, la spectrométrie infrarouge et la microscopie électronique à transmission (pour l'analyse chimique et l'état d'oxydation du fer) permettent de déterminer l'évolution du système fer métal/smectites dans un milieu hydraté à 80°C. Le taux de déstabilisation des smectites en présence de fer métal varie de 0 à 100% en fonction des paramètres structuraux de la smectite (composition chimique, espace interfoliaire, quantité de fer octaédrique). La déstabilisation de la smectite et du fer métal génère de la magnétite ainsi qu'une phase gel à l'intérieur de laquelle cristallise une nouvelle phase argileuse très riche en fer. Cette phase montre deux degrés d'oxydation, avec un rapport fer(II)/fer(III) 20/80 (phase proche des odinites) ou 50/50 (phase proche des cronstedtites)

Réactivité fer métal-smectites en milieu hydraté à 80C

Les dispositifs de stockage envisagés pour les déchets nucléaires de haute activité font intervenir une barrière ouvragée argileuse (smectites) ainsi que des conteneurs métalliques (fer). La diffraction des rayons X, la spectrométrie infrarouge et la microscopie électronique à transmission (pour l'analyse chimique et l'état d'oxydation du fer) permettent de déterminer l'évolution du système fer métal/smectites dans un milieu hydraté à 80ʿC. Le taux de déstabilisation des smectites en présence de fer métal varie de 0 à 100% en fonction des paramètres structuraux de la smectite (composition chimique, espace interfoliaire, quantité de fer octaédrique). La déstabilisation de la smectite et du fer métal génère de la magnétite ainsi qu'une phase gel à l'intérieur de laquelle cristallise une nouvelle phase argileuse très riche en fer. Cette phase montre deux degrés d'oxydation, avec un rapport fer(II)/fer(III) 20/80 (phase proche des odinites) ou 50/50 (phase proche des cronstedtites).ORLEANS-BU Sciences (452342104) / SudocORLEANS-ISTO (452342307) / SudocSudocFranceF

Meta-clay interactions at low temperature : reactivity of nontronite temperature : reactivity of nontronite with different metals.

Interactions between metals and smectites have been studied with the aim to predict the reactivity of clayey engineered barriers with the metallic container used for the radioactive waste confinement. Recent results (Lantenois, 2003) has showed that the reactivity of the reaction between metallic iron and Garfield nontronite depends of the pH. They were done at 80C during 45 days. We present in this work, results obtained with titanium, chromium, manganese, cobalt, nickel, copper and zinc metals. Clay sample is the Garfield nontronite (Na0.05Ca0.23K0.01) (Si3.5Al0.49) (Fe3+1.86 Al0.18 Mg0.02)O10OH2. The reactions have been realised at pH 5 and 9. Experiments have been performed with a 1/2/50 ratio for the clay / metal powder / MilliQ water. pH of the solutions have been fixed at 5 and 9 with NaOH or HCl add. The mixture have been placed in a Teflon reactor and heating at 80C during 45 days. At the end of the reaction, solutions have been filtered and analyzed by ICP-AES and solid phases were characterized by XRD and infrared spectroscopy. Results : XRD pattern analysis show that nontronite reacted with the seven studied metals. Oxides or hydroxides has been identified. Two main modifications of Garfield nontronite have been observed. - Modification of smectite interlayer space : after the reaction, (chromium, cobalt, nickel, copper) cations are in the interlayer space. They can be hydrated or hydroxyled. In the case of hydroxyled cations, interlayer brucitic sheets can be identified because (00ℓ) reflections are identical to chlorites ones. - Destabilisation of the smectite phase : The reaction with zinc and tin cations lead to the formation of new silicate phases. For example, willemite (Zn2SiO4) and varlamoffite (Sn,Fe)(O,OH)2 phases have been obtained after the reaction with zinc and tin metals respectively

Structural transformation of Garfield nontronite during interactions with metals.

Smectite-metals interactions have been studied with the aim to predict the reactivity of clayey engineered barriers with the metallic container used for the radioactive waste confinement. Recent results (Lantenois, 2003) have shown that metallic iron reacted in contact with Garfield nontronite to form magnetite. This result has been obtained under acidic pH conditions at 80°C during 45 days. During this reaction, clay sample was not transformed. We present in this work, results obtained with zinc, cobalt, nickel and magnesium metal after interaction with the Garfield nontronite (Na0.05Ca0.23K0.01)(Fe3+1.86Al0.18Mg0.02)(Si3.5Al0.49)O10(OH)2. Experiments have been performed with a 1/2/50 ratio for the clay / metal powder / MilliQ water. pH of the solutions have been fixed at 5 with HCl add. The mixture have been placed in a Teflon reactor and heated at 80°C during 45 days. After the reaction, solids and solutions have been separated by filtering. Solutions have been analyzed by ICP-AES and solid phases have been characterized by XRD and infrared spectroscopy. Results: - XRD pattern analysis show that nontronite reacted with the metals. Oxides or hydroxides have been identified. A modification of smectite have always been observed: after the reaction, the position of 00ℓ reflections is modified. The 001 reflection shift from 15,2 Å to 14,4 Å and news 00ℓ reflections appear at 7,3 Å, 4,85 Å and 3,68 Å. This new phase is not hydrated and has no swelling property with ethylene-glycol. - FTIR patterns show that OH stretching bands of Gardfield nontronite are always present after the reaction. For all metals, OH bands appear between 3625 and 3660 cm-1, they are probably characteristic of the presence of brucitic M(OH)2 layer. These positions are not characteristic of the brucite phase. The localisation of brucite layer in the interlayer space of Garfield can explain this band. The results obtained during the interaction between Garfield nontronite and metallic iron are totally different with other metals such as magnesium, cobalt, nickel or zinc. Interactions with these metals seem to transform Garfield nontronite to chlorite-like minerals. Reference Lantenois (2003) : iron metal/smectites reactivity in aqueous solution at 80°C. PhD thesis, Université d'Orléans

Synthesis and characterisation of dioctahedral smectites.

The synthetic clays are used in a lot of studies to analyse clay properties. They permit to have samples without impurities and with a specific chemical composition. In synthetic smectite group, trioctahedral smectites as saponite were generally studied. Because the majority of natural smectites are dioctahedral ones, it can be interesting to product dioctahedral synthetic smectites. This is the objective of this work. The synthesized smectites should be mono-phased with well-known characteristics and chemical composition. In this study we have synthesized beidellites with different tetrahedral charges and different structural Fe3+ content and montmorillonites with different octahedral charges. Synthesis protocol : Gels used for starting materials were prepared according to the usual method from the following compounds : tetraethyl orthosilicate (TEOS), Mg(NO3)2.6H2O, Al(NO3)3.9H2O, Fe(NO3)3.9H2O, HNO3, Na2CO3, NH4OH and ethanol. After dissolution of Al, Mg nitrate and Na carbonate in nitric acid, the TEOS and ethanol were added. A precipitate was obtained after neutralization of the solution; this precipitate was dried and heated to 400°C to obtain Al, Mg, Si, Na oxides gel. The samples were synthesized in an internally heated pressure vessel. Starting gel was sealed with water solution in a gold capsule. The mixture was treated at 350°C under a variable pressure between 200 and 1500 Bars (depending on the sample) during 15 days. Results : First, we present results for synthetic beidellites with different tetrahedral charges. In the reaction products, no additional phases was observed by XRD. Secondly, beidellites with iron content were synthesized with a maximum FeV I/AlV I ratio equal to 1. In this syntheses, a small quantity of iron oxide was present in final product. Infrared analysis permit to assign the principal part of iron to the clay phase. Iron rich beidellites can not be obtained in these conditions. If we increase the FeV I/AlV I ratio above 1, the reacted product is not iron rich beidellite. We obtain a mixture between iron oxides, opal and sometimes smectite phase. Lastly, for montmorillonites synthesized at low pressure, only smectite was identified by XRD but this product is probably a mixture between beidellite and saponite. At more important pressure, the results seem to be different because only one smectite phase was identified by XRD

Effect of heteroatom doping on the surface acidity and hydrophilicity of Al, Ti, Zr-doped mesoporous SBA-15 obtained by repeated synthesis

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Hydrothermal synthesis of beidellites : Characterization and study of the cis- and trans-vacant character

Low-charge beidellites were synthesized by a hydrothermal treatment applied to an amorphous gel phase in basic solution. The hydrothermal conditions for the syntheses were chosen from the stability field of beidellite previously investigated in the literature. The synthetic samples were characterized chemically and structurally using X-ray diffraction, infrared spectroscopy, cation exchange capacity measurement, and chemical and thermal analyses. We compared the synthetic sample with a natural beidellite sample (SbId) from Idaho, USA, looking at chemical composition and particle size. The main difference is the octahedral site occupancy (cis- or trans-vacant layer structure). The natural SbId sample has trans-vacant layers and the synthetic sample has a preferentially cis-vacant character. This character can be modulated, using specific synthesis conditions. The cis- or trans-vacant layer structure of various synthetic beidellites was investigated at low temperature (<350°C and pressure (<25 MPa). Depending on the pressure and/or synthesis temperature, the proportion of cis-vacant layers ranges from 20 to 100% and increases with the layer-charge deficit