Th. univ.--Chimie--Paris 6, 1993. / Bibliogr. p. 192-206. 1993 d'après la déclaration de dépôt légal.

Thèse--Sc. nat.--Paris XI, 1982. / Bibliogr. p. 163-172.

Reproduction de : Thèse de doctorat : Chimie : Le Mans : 2004. / Titre provenant de l'écran-titre. Bibliogr. en fin de chapitres.

Inaug.-Diss.--Geologie--Ruprecht-Karls-Universität Heidelberg, 1992. / Bibliogr. p. 152-157.

Thèse de doctorat : Physique, chimie et biologie de l'environnement : Strasbourg 1 : 2008. / Texte en anglais. Titre provenant de l'écran-titre. Bibliogr. p. 180-192.

Bentonite clays have long been used as additives in animal feed, aiming to improve pellet quality and prevent caking. Certain bentonites are also capable of deactivating aflatoxin B_(1) (AfB_(1)) in feed by adsorption, therefore, detoxifying the feed. However, a 10–fold difference in adsorption capacity has been observed among selected bentonites. The major mineralogical and chemical properties of smectites in determining their adsorption capacities for AfB_(1) are still poorly understood. Improved knowledge of the key controlling factors of aflatoxin adsorption to bentonite clays is needed to guide the selection, modification, and application of the clays as aflatoxin binders. The objective of this study was to test a hypothesis that a smectite's selectivity and adsorption capacity for aflatoxin was mainly determined by the size matching requirement on interlayer surface domains and the aflatoxin molecules. Three approaches were used to vary the size of nanometer-scaled nonpolar domains in the interlayer of smectites: 1) exchanging interlayer cations, 2) selecting natural bentonites with different cation exchange capacities (CEC), and 3) reducing charge density of a high CEC smectite. Six bentonites were fractionated, with their major mineralogical and chemical properties determined. Clay suspensions saturated with different cations were tested for aflatoxin adsorption. Some aflatoxin-smectite complexes were prepared and analyzed with FTIR and XRD. AfB_(1) adsorption isotherms were fitted with Langmuir, modified Langmuir with adsorption dependent affinity, and exponential Langmuir models. Divalent exchange cations with low hydration energy in general resulted in a much higher adsorption capacity and affinity for all six natural bentonite clays. Cations with smaller hydration radii tended to further enhance the adsorption process for aflatoxin on smectites. Charge density of smectite had shown significant effects on the adsorption capacity, affinity, and the isotherm shape. Aflatoxin adsorption isotherms on the six natural smectites and the CEC-reduced 5OK samples by Hofmann and Klemen effects suggested that there is an optimal CEC range between 80~110 cmol(+)/kg for the best aflatoxin binding smectites. When the smectite has a CEC within this range, the mineral has the highest affinity and adsorption capacity for AfB_(1). The aflatoxin adsorption results after cation exchange treatment, selection of different CEC smectites, and the CEC reduction on 5OK confirmed the importance of size and polarity matching on the nanometer scale in smectites’ adsorption for AfB_(1). All clay samples tested in this study were capable of adsorbing aflatoxin into interlayers, and the charge density seemed to have no effect on bonding strength.

Clay minerals detected in ancient Martian terrains help constrain Mars’ climate and aqueous alteration history. Since Mars’ primordial origin, atmospheric redox conditions have evolved from reducing to oxidizing and clay minerals may record the effects of that transition. Ferrous trioctahedral smectites of varying iron content were synthesized and subjected to oxidation by O2 and by UV irradiation to address these as potential oxidizing agents. UV irradiation (112.77 hours with 450 W Hg lamp) of smectites equivalent to approximately four years of flux on the Martian surface caused incomplete oxidation (Fe3+/ΣFe = 16-18%). O2 experiments (two hour, twelve hour, two day, and five day) produced more oxidation in smectites with higher Fe content at the same exposure times. Photooxidation caused octahedral sheet contraction; however, chemical oxidation allowed more contraction to occur in the high Fe smectites. The mid and high Fe smectites had observable changes in their visible-near infrared (VNIR) reflectance spectra with the formation of a nontronite (Fe3+, Mg)2-OH feature at 2.3 µm, even with partial oxidation. With both oxidation experiments, the reflectance spectra lost its initial MMM-OH features (AlAl(Fe2+,Mg) and Fe2+MgMg-OH) and produced a single nontronite-like MM-OH feature. UV irradiation produced a secondary nontronite phase, possibly on the surface of the higher Fe content smectites; however there was no evidence for iron ejection. Ferrous smectites are capable of undergoing UV photooxidation under aqueous conditions and this process could have occurred during early Martian history. Distinguishing between UV and O2 oxidation in smectites cannot be completed exclusively with Martian spectra; however, the lack of secondary oxides may hint at alteration history based on the nature of mineral assemblages detected on Mars. / Geology

Le fer joue un rôle majeur dans un grand nombre de processus biogéochimiques impliquant des transferts d'électrons, c'est-à-dire des réactions redox. Les argiles forment une famille de minéraux ubiquitaires, dont certains membres contiennent du fer au sein de leur structure. La structure des argiles confère à ce fer structural (Festr) des propriétés redox spécifiques. Celles-ci font l'objet d'un nombre croissant d'études, portant sur divers domaines de recherche, allant des études agronomiques sur l'accessibilité des nutriments dans les sols ou sur la maturation de la matière organique, aux études sur la rétention des métaux lourds ou des radioéléments dans des barrières argileuses. L'étude des propriétés redox du Festr demeure toutefois un challenge scientifique. La structure cristallographique des argiles conditionne les propriétés redox du Festr, mais les transformations redox du Festr ont également en retour un impact sur cette structure. Les propriétés structurales de l'argile et la manière dont elles évoluent au cours des processus redox sont ainsi influencées par l'histoire redox de l'argile, c'est-à-dire l'étendue et le nombre de cycles redox qu'elle a pu subir. Etant donnés le nombre et la complexité des mécanismes mis en jeu, et étant donné la variété des structures argileuses, les travaux de cette thèse se sont focalisés sur le groupe particulier des smectites dioctaédriques, plus précisément sur les membres ferrugineux que sont les nontronites, et sur l'impact de la réduction de leur Festr sur leurs propriétés structurales. Le mécanisme de réduction étudié est lui-même restreint à la réduction par de puissants agents réducteurs chimiques de type dithionite. En parallèle à un travail de revue critique de l'abondante littérature disponible sur cet aspect spécifique, des travaux expérimentaux ont été conduits et plusieurs approches théoriques ont été discutées et développées pour conduire à un modèle structural des propriétés redox du Festr et de leur relations avec la structure smectitique. De nouvelles mesures de charge négatives du feuillet argileux en fonction du niveau de réduction, réalisées au cours de cette thèse, indiquent clairement que, contrairement au comportement précédemment supposé, la charge structurale du feuillet n'augmente pas de façon monotone avec la réduction du Festr , mais est marquée par une chute significative à partir d'un niveau de réduction donné. Le domaine de réduction d'une nontronite est ainsi divisé en deux domaines : le premier est marqué par une augmentation de la charge négative et des changements structuraux réversibles ; le second est caractérisé par une chute de la charge négative et des transformations structurales moins ou non réversibles. Les efforts de modélisation réalisés durant cette thèse se sont restreints à la première phase de réduction, c'est-à-dire la phase réversible. Un premier modèle empirique permet de modéliser l'évolution de la CEC de nontronites à partir de leur formule structurale (qui donne la composition et la charge) et d'un paramètre empirique qui est commun à une série de nontronites pour lesquelles des données sont disponibles. Dans un second temps, un algorithme mimant la progression de la réduction du fer structural a été mis au point, permettant de lier l'évolution de la charge négative d'un type particulier de nontronites (pauvre en fer tétraédrique) à ce paramètre empirique, au mécanisme de réduction lui-même, ainsi qu'aux propriétés structurales de l'échantillon et à la manière dont ces propriétés évoluent au cours de la réduction. L'ensemble de ces informations a ensuite été traduit dans un modèle thermodynamique de réduction des argiles, permettant d'identifier les données nécessaires (disponibles et à acquérir) à l'établissement d'un modèle contraint par les informations structurales. / Iron plays a key role in many biogeochemical processes implying electron transfers (i.e. redox reactions).Clays form a family of ubiquitous minerals; some contain iron in their structure. Clay structure provides specific properties to this structural iron (Festr). A constantly growing set of studies focuses on Festr redox properties, interesting various research fields, such as agronomic study on soil's nutriments accessibility or organic matter growth, or even studies on heavy metals and radionuclides retention in clay barrier. Study on Festr redox properties remains however challenging. Clay' crystallographic structure rules Festr redox properties, but in return, Festr redox transformation has impact on this structure. The redox history of the clay (i.e. extent and number of prior redox cycles) influences both Festr redox properties and the way they evolve upon redox processes. Given the number and complexity of implied mechanisms, and given the variety of clay structures, this PhD thesis focuses on the particular group of dioctahedral smectites, more precisely on the iron rich end-members named nontronites and on the impact of Festr reduction on their structural properties. The studied reduction mechanism is itself restricted to reduction by powerful chemical reduces such as dithionite. Along with a critical review of available abundant literature on this specific aspect, experimental works have been conducted and several theoretical approaches have been discussed and developed to construct a structural model of Festr redox properties and their relations with the smectitic structure. New measures of clay negative layer charge as a function of reduction level done during this PhD clearly indicate, contrary to previously assumed behavior, negative layer charge do not monotonously increases upon Festr reduction, but shows a dramatic drop starting from a given reduction level. The reduction domain of a nontronite is thus spitted into two domains: the first is characterized by a negative layer charge increase and reversible structural changes; the second is characterized by a drop of the negative layer charge and less or even not reversible structural transformations. The models developed, during this PhD, are restricted to the first reduction sequence, i.e. the reversible one. A first empiric model allows modeling nontronite CEC changes from their unit-cell formula (which provides composition and charge) and using an empirical parameter which is common to a series of low tetrahedral-Fe-bearing nontronites for which experimental data are available. In a second approach, an algorithm simulating how Festr reduction progresses has been set up, allowing to link negative layer change of particular type of nontronites (tetrahedral-Fe-poor) to this empirical parameter, to the reduction mechanism itself, and to the structural properties of the sample and the way they evolve upon reduction. Following this, the whole of this set of information has been implemented in a thermodynamic model of Fetr reduction, making it possible to identify the required data (available or to acquire) for the establishment of a model constrained by structural information.

Phyllosilicates are widespread on Noachian to Early Hesperian terrains on Mars and can help constrain the planet’s geologic and environmental history, particularly its aqueous and redox history, which in turn can provide clues about past habitability. A range of ferrous and mixed-valence smectites were synthesized and then exposed to varying O2 fluxes under dry conditions to determine if Fe/Mg trioctahedral smectites can be oxidized without an aqueous medium to form dioctahedral smectites like those observed on the surface of Mars. The appearance of a secondary peak in some unaltered samples indicates a separate phase formed during synthesis, which remained throughout the oxidation process. Partial oxidation was achieved by all samples, but only those with the highest starting Fe3+ content reached almost complete oxidation. Rapid initial oxidation was observed for all samples, but seemed to subside before oxidation was complete, indicating the process becomes unfavorable after a certain point. All three O2 fluxes used in this study were successful in partially oxidizing the smectite samples but no correlation was observed between O2 flux and actual amount of oxidation, i.e., a higher O2 flux does not necessarily result in higher production of ferric iron. The process of oxidation did cause octahedral sheet contraction in some cases; however, in some samples octahedral sheet expansion was observed with oxidation. No additional phases were formed upon oxidation and no Fe ejection was observed. Overall, the amount of oxidation observed for all samples indicates that O2 alteration of trioctahedral smectites into dioctahedral smectites can proceed under dry conditions, meaning oxidation could have continued on Mars after surface water dried up. / Geology

L’étude de l’adsorption et de la mobilité des ions dans les argiles est importante pour mieux appréhender de nombreux processus géochimiques et environnementaux, de même que pour prédire le comportement des radionucléides dans les conditions du stockage géologique. A cause de leurs tailles très petites (< 2μm), il n’est pas toujours évident d’étudier les argiles à l’aide des méthodes et techniques expérimentales existantes. L’une des alternatives à ce problème consiste alors à utiliser la modélisation moléculaire pour les étudier. En plus de leurs tailles fines, les argiles présentent également des structures complexes, qui peuvent survenir en raison de la multiplicité de possibilités de distributions et d’arrangements des substitutions isomorphiques dans leurs couches. Il a été clairement démontré qu’il existe une corrélation entre la distribution des substitutions dans les couches des argiles et les propriétés de ces dernières. En revanche, ceci reste à démontrer en ce qui concerne l’arrangement de ces substitutions dans les couches de l’argile. Dans ce travail, la modélisation moléculaire est utilisée pour déterminer et comparer les propriétés d’hydratation, ainsi que la structure et la mobilité des ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ni²⁺, UO₂²⁺) et de l’eau dans l’espace interfoliaire de trois modèles de montmorillonite, différents entre eux par l’arrangement des substitutions isomorphiques dans les couches de l’argile.L’adsorption et la diffusion des cations cités plus haut et de l’eau sont également étudiées à la surface de la montmorillonite et les résultats sont comparés à ceux obtenus dans l’espace interfoliaire à 298 K et à 363 K. Les résultats obtenus dans ce travail s’accordent bien avec les observations expérimentales, et font ressortir une corrélation plus ou moins importante entre le modèle d’argile utilisé et le type de propriété calculée. Cette corrélation dépend également de la nature du cation présent dans la structure de la montmorillonite et de sa teneur en eau. / The study of adsorption and ion mobility in clay minerals is important for a better understanding of many geochemical and environmental processes, as well as to predict the behavior of radionuclides in geological storage conditions. Because of their very small size (<2μm), it is not always easy to study clays by using the existing experimental methods and techniques. One alternative to this issue is to use computational molecular modeling to carry out clay studies. In addition to their tiny size, clays minerals also have complex structures, which can appear due to various possibilities in the distribution and arrangement of isomorphic substitutions in their layers. It has been clearly demonstrated that there is a strong correlation between the distribution of substitutions in the clay layers and their properties. However, this remains to be shown regarding the arrangement of the substitutions in the layers of the clay. In this work, computational molecular modeling techniques are used to determine and compare the hydration properties, as well as the structure and mobility of ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ni²⁺,UO₂²⁺) and water in the interlayer space of the three models of montmorillonite, that differ from each other by the arrangement of isomorphic substitutions in the clay layers.The adsorption and diffusion of the previously listed cations and water are also studied on the surface of montmorillonite clay and the results are compared to those obtained in the interlayer space both at 298 K and at 363 K. The data generated in this work agree well with experimental observations, and show a more or less significant correlation between the clay model used and the type of property calculated.