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Impact de la matière organique sur le comportement des terres rares en solution: étude expérimentale et modélisation

Abstract : The objectives of this thesis were mainly to understand the impact of organic matter on the rare earth elements behavior in solution and in particular: (i) to understand why the development of negative cerium anomaly in oxidizing organic water does not occur; (ii) to test models capacity to predict rare earth elements speciation in organic-rich water; (iii) to evaluate competitive reactions between humic acids and carbonate towards rare earth elements complexation.
The first part was devoted to two experimental studies on the impact of organic complexation on the redox behavior of cerium and adsorption of rare earth elements on manganese and iron oxide surface. Rare earth elements complexation by organic matter inhibits at the same time, the oxidation of Ce(III) in Ce(IV) and the development of a tetrad effect displayed by rare earth elements pattern. This inhibition is due to the development of an organic screen between rare earth elements and oxidizing/adsorbant surfaces which represent manganese and iron oxides. The second part presents an evaluation of two models, Model VI and SHM, in their capacity of rare earth elements with natural organic matter in natural water. The comparison between calculated results and ultrafiltration experiments made it possible to validate Model VI in its capacity to predict the speciation of rare earth elements in organic water. On the other hand, the divergence between the results obtained with SHM and experimental results indicates that SHM, in the current state of the program, is not ready to precisely describe the speciation of rare earth elements in natural water. In a second time, this study made it possible to provide a reliable and coherent set of Model VI specific rare earth elements - humic acids complexation constants. The third part of this work was devoted to competitive reactions between humic acids and carbonates, toward rare earth elements complexation. According to the pH and relationship between humic acid and carbonate, rare earth elements will be distributed between these two ligands. The competition between both ligands also induces formation of a positive cerium anomaly onto humic acids and negative in the solution. A new mechanism able to develop a negative Ce anomaly in natural water, without oxidizing surface or bacterium, was thus highlighted.
Organic matter is responsible for strong complexation of rare earth elements in natural water. A greater attention must thus be paid to this complexed form to study the transport of rare earth elements in water. This study also calls into question the use of the Ce anomaly as redox proxy of the conditions in organic-rich waters.
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Olivier Pourret. Impact de la matière organique sur le comportement des terres rares en solution: étude expérimentale et modélisation. Géochimie. Université Rennes 1, 2006. Français. ⟨tel-00135155⟩

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