Coordination of the limiting U(IV) carbonate species in aqueous solution – a comparative EXAFS and XRD investigation

The carbonate anion is one of the most important ligand in natural waters. The solubility of heavy metals like uranium is strongly affected by carbonate through the forming anionic complexes [1]. Uranium is well soluble in oxidation state VI. The oxidation state V is rather instable due to the fast disproportionation reaction. In contrast, the solubility of uranium IV is rather low. The situation changes completely in presence of carbonate. Because the disproportionation reaction of U(V) is strongly enforced by cation-cation interactions, strong ligands like carbonate are able to suppress this reaction. In consequence, U(V) can be stabilized over months in carbonate solution. Although U(IV) strongly tent to form polynuclear species and colloids, carbonate enhances its solubility by forming mononuclear U(IV) carbonato complexes. The coordination of U(VI) and U(V) has been described. The coordination of the limiting U(IV) carbonate complex is actual under debate. The aim of this study is to determine the coordination of the solution species and to preserve it, if possible, in a crystal structure. Because EXAFS provides only a radial distribution function of the next interatomic distances but no information on their spatial arrangement, the comparison with the crystal structure provide necessary information on the ligand arrangement. We used EXAFS again to follow the crystallization process and to proof that no rearrangement of carbonate occurs during the crystallization process.