International audienceA first exercice was led, considering only the americium(III) species in solution. Even with different data sets, all the exercises are in relatively good agreement, concluding to successive complexation of the radioelement by phosphate (pH range from 0 to 3), citrate (pH range from 3 to 8) and hydroxo-carbonate (pH range from 8 to 14). Nevertheless, the speciation of americium differs for each pH range from one work to an other. These results are still observed when the speciation calculation includes the formation of solids. Nevertheless, some workers did not integrate in their data set the formation constant of Am(PO4). This solid is known to be very insoluble in typical biological media. All the calculations where Am(PO4) was not in the TDB presented a speciation where americium is solvated (mostly under citrate forms). When the formation of Am(PO4) is considered, the actinide is quantitatively present under this solid species. This work is a representative example of the impact of the thermodynamic data used in speciation exercises. Some exercices, using uncompleted TDB had wrong results and led to the conclusion of the necessity in the use of expertised and extended TDB

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Abstract. The biogeochemical behaviour of americium in subsurface soils plays a dominant role on its potential migration, but is currently poorly known. This research investigates the relevant processes controlling americium biogeochemical speciation in the rhizosphere of an agricultural soil. Lixiviation tests were performed on columns packed with a calcareous soil contaminated with 241 Am, under steady-state unsaturated or saturated hydric flow conditions. The columns were percolated with various soil solutions, containing citrate and/or glucose to simulate root exudates. The bio-physico-chemical parameters of the outlet solutions were monitored continuously. The percolation of a solution in equilibrium with the soil or containing low exudate concentrations (10 -4 M) released less than 5 mBq/cm 3 of americium. Poor remobilisations were also observed when injecting 10 -2 M glucose solutions, despite effective glucose microbial degradation, production of secondary ligands, and important chemical changes. On the contrary, percolation of 10 -2 M citrate solution released 1000 times more americium, in accordance with previous studies. 10000 times greater releases were observed when glucose was added to citrate at 10 -2 M. The remobilisation of americium resulting from both dissolution of calcite and iron oxides, and complexation by citrate, as well as the applicability of simple parameterisation of partition coefficient are discussed.