ABSTRACT CODE Synthesis of solid alkaline lanthanide carbonates

Stability and mobility of actinides (An) species in ground-waters may govern their long time behaviour in potential radioactive waste repositories. For this purpose, a Callovo-Oxfordian geological formation is studied in France; it is in anoxic chemical conditions, where Pu, Am and Cm can be stable at the +3 oxidation state, and where their aqueous speciation is often dominated by carbonate complexes. The stoichiometry of the limiting carbonate complex of An(III) is still discussed, as typically seen in the thorough NEA reviews of published thermochemical data. Robouch evidenced Am(CO3)33- as the limiting complex [2], which was later confirmed by Giffaut [3] and Runde [4]. However, M(CO3)33- [1,2] and M(CO3)45- [1,6-8] stoichiometries have been reported for the limiting complexes of An(III) and their chemical lanthanide analogues (Ln(III)). Vercouter has recently proposed Eu(CO3)33-as the limiting complex in concentrated sodium carbonate solutions, by studying the solubility of NaEu(CO3)2(s), and this stoichiometry was a key point for sensitive analysis for his SLRT study of the other aqueous complexes[9]; while Faucherre evidenced Ln(CO3)45- in concentrated potassium carbonate solutions using the sam