Cation-cation interactions between NpO2+ and UO22+ were studied at different temperatures (283.15 K to 358.15 K) and different ionc strengths (3 � 4.5 mol dm-3). The complex between NpO2+ and UO22+ was weak and became stronger as the temperature was increased. The molar enthalpy of complexation was directly determined for the first time by microcalorimetry to be (4.2 � 1.6) kJ mol-1 at 298.15K, in good agreement with the trend in the stability constant at different temperatures. The small and positive enthalpy and entropy of complexation support the argument that the cation-cation complex between NpO2+ and UO22+ is of inner-sphere type, where the NpO2+ moiety enters the primary equatorial coordination sphere of UO22+ and coordinates to UO22+ with one axial oxygen of NpO2+. At each temperature, the stability constants of the cation-cation complex were found to increase as the ionic strength was increased. In addition, the effect of the cation-cation interaction between NpO2+ and UO22+ on the behavior of neptunium in solvent extraction and disproportionation is investigated.

Cation-cation interaction between Np(V) and U(VI) and its effect on the behavior of Np(V) in solvent extraction and disproportionation

245th National Meeting (2013)

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