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Influence de la cristallochimie des argiles sur le potentiel redox du fer structural

Abstract : 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.
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Jebril Hadi. Influence de la cristallochimie des argiles sur le potentiel redox du fer structural. Sciences de la Terre. Université de Grenoble, 2012. Français. ⟨NNT : 2012GRENU029⟩. ⟨tel-00864040⟩

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