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Neptunium is one of the few radioactive elements that are of great concern in the disposal of nuclear wastes in the geological repository, due to its hazards and the long half-life of the isotope, 237 Np (t1/2 = 2.14 × 106 years). To understand and predict the migration behavior of neptunium in the geological media, it is of importance to study its hydrolysis at elevated temperatures, because the temperature in the waste package and the vicinity of the repository could be high. Moreover, the chemical analogy between neptunium(V) and plutonium(V) adds even greater value to this investigation, because the latter could exist at tracer levels in neutral and slightly oxidizing waters but is difficult to study due to its rather labile redox behavior............

The complexation of uranium(VI) and samarium(III) with oxydiacetate (ODA) in 1.05 mol kg-1 NaClO4 is studied at variable temperatures (25-70 C). Three U(VI)/ODA complexes (UO2L, UO2L22-, and UO2HL2-) and three Sm(III)/ODA complexes (SmLj(3-2j)+ with j = 1, 2, 3) are identified in this temperature range. The formation constants and the molar enthalpies of complexation are determined by potentiometry and calorimetry. The complexation of uranium(VI) and samarium(III) with oxydiacetate becomes more endothermic at higher temperatures. However, the complexes become stronger due to increasingly more positive entropy of complexation at higher temperatures that exceeds the increase in the enthalpy of complexation. The values of the heat capacity of complexation (Cp in J K-1 mol-1) are 95 ± 6, 297 ± 14, and 162 ± 19 for UO2L, UO2L22-, and UO2HL2-, and 142 ± 6, 198 ± 14, and 157 ± 19 for SmL+, SmL2-, and SmL33-, respectively. The thermodynamic parameters, in conjunction with the structural information from spectroscopy, help to identify the coordination modes in the uranium oxydiacetate complexes. The effect of temperature on the thermodynamics of the complexation is discussed in terms of the electrostatic model and the change in the solvent structure.