Aqueous complexes of lanthanides(III) and actinides(III) with the carbonate and sulphate ions. Thermodynamic study by time-resolved laser-induced fluorescence spectroscopy and electro-spray-ionisation mass spectrometry

The prediction of the environmental impact of a possible geological disposal of radioactive wastes is supported by the thermodynamic modelling of the radionuclides behaviour in the groundwater. In this framework, the analogy between lanthanides and actinides(III) is confirmed by a critical analysis of the literature and the comparison with experimental results obtained here. The limiting complex, Eu(CO3)33-, is identified by solubility measurements in Na2CO3 solutions. Then the formation constants of the complexes Eu(CO3)i3-2i (i=1-3) and Eu(SO4)i3-2i (i=1-2) are measured by TRLFS. The formation of aqueous LaSO4+ is studied by ESI-MS and is in good agreement with the expected speciation. The enthalpy and entropy of the reaction Cm(CO3)2- + CO32- ↔ Cm(CO3)33- are deduced from TRLFS measurements of the equilibrium constant between 10 and 70 C. The ionic strength effect is calculated using the SIT formula. (author)