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Caractérisation structurale de la birnessite : Influence du protocole de synthèse

Abstract : Birnessite is a lamellar manganese oxide whose layers are built up of edge sharing MnO6 octahedra. The presence of heterovalent Mn cations and/or of vacant sites in these layers leads to a charge deficit compensated for by the presence of hydrated cations in the interlayer space. Because of their high specific area and of their strong oxidative character, these ubiquitous manganese oxides play a fundamental role in the fate of organic and metallic pollutants in the environment, but our imperfect knowledge of their structure limits the understanding and the modeling of this impact. This study aimed at classifying all different kinds of birnessite obtained using existing synthesis protocols according to two relevant criteria: layer symmetry and layer stacking mode, and at determining the structure of several essential varieties using X-ray and electron diffraction. Layers of hydrothermal birnessite contain vacant sites and, as a result, possess an hexagonal symmetry. Their stacking mode is 3R. In high-temperature birnessites, adjacent layers have an opposite orientation, which results in a two-layer polytype. The symmetry of these layers, linked to the origin of the layer charge deficit, depends on the temperature of synthesis. At 800°C the presence of vacant sites results in an hexagonal symmetry (2H polytype). At 1000°C, the layer charge deficit originates from the presence of Mn3+ cations in the layer lowering the layer symmetry (2O polytype). A variety of chemical and structural heterogeneities was also described in these samples, along with the occurrence of a new type of structural disorder. Finally we proved both the fundamental link between the origin of the layer charge and the layer symmetry, and the influence of physico-chemical parameters during synthesis (temperature, average manganese oxidation degree, nature of the interlayer cation) on the structure of the obtained compound. The chemical and thermal stabilities of these birnessites are also compared.
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Submitted on : Thursday, March 11, 2004 - 9:41:54 AM
Last modification on : Thursday, November 19, 2020 - 3:54:27 PM
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  • HAL Id : tel-00005304, version 1



Anne-Claire Gaillot. Caractérisation structurale de la birnessite : Influence du protocole de synthèse. Géologie appliquée. Université Joseph-Fourier - Grenoble I, 2002. Français. ⟨tel-00005304⟩



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