Complexation of Cm(III) and Eu(III) with aqueous phosphates at elevated temperatures

Thermodynamic databases are essential for the safety assessments of radioactive waste repositories. They have to be reliable, comprehensive, and describe the key mechanisms controlling the mobility of contaminants in the environment. However, in many cases these prerequisites are not fulfilled. An important example is the complexation of actinides with aqueous phosphates, for which this work provides complexation constants for spectroscopically identified species at 25 °C and at elevated temperature. The complexation of Cm(III) was studied at sub micromolar concentrations by laser induced luminescence spectroscopy as a function of total phosphate concentration (0-0.06 M ΣPO4) in the temperature range 25-90°C, using NaClO4 as a background electrolyte at –log[H+] ranging from from 2.5 to 3.6. The formation of both CmH2PO42+ and Cm(H2PO4)2+ complexes was revealed, the latter being spectroscopically evidenced for the first time. Complexation constants were found to increase when raising the ionic strength from 0.5 to 3.0 M. Temperature-dependent (25 to 90 °C) complexation constants for the identified species were derived, and were recalculated to standard conditions with the van´t Hoff equation and the Specific Ion Interaction Theory. Endothermic and entropy driven reactions were established for both complexes. In addition, relativistic quantum chemical investigations were performed to study the complexation strength of Cm(III) with aqueous phosphates and to provide insight in potential changes of the coordination number with increasing temperature and to probe the character of the Cm water and Cm phosphate bonds.