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Abstract. Uranium behaviour in soils is controlled by actions and interactions between physico-chemical and biological processes that also determine its bioavailability. In soil solution, uranium (+VI) aqueous speciation undergoes tremendous changes mainly depending on pH, carbonates, phosphates and organic matter. In a first approach, plants (Phaseolus vulgaris) were grown using hydroponics to allow an easy control of the composition of the exposure media. The latter, here an artificial soil solution, was designed to control the uranium species in solution. A geochemical speciation code was used to perform the solution speciation calculations. On this theoretical basis, three domains were defined for short-duration well-defined laboratory experiments in simplified conditions: pH 4.9, 5.8 and 7 where predicted dominant species are uranyl ions, hydroxyl complexes and carbonates respectively. For these domains, the influence of plant growth stage on transfer was determined. The Free Ion Model (or its derivated the Biotic Ligand Model) was tested to determine if U uptake was governed by the free uranyl species or if other metal complexes could be assimilated. The effect of different variables on root assimilation efficiency and phytotoxicity was explored: presence of ligands such as phosphates or carbonates and competitive ions such as Ca 2+ at the 3 pHs.

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