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The luminescence spectra of U 4+ in aqueous solutions were observed in the UV-VIS region at ambient and liquid nitrogen temperatures. The excitation spectrum indicates that the luminescence is arising from the deexcitation of a 5 f electron at the 1 S 0 level and no other emissions of U 4+ in aqueous solutions were detected for other f -f transitions. All the luminescence peaks were assigned to the transitions from 1 S 0 to lower 5 f levels. To estimate the luminescence lifetime, luminescence decay curves were measured using time-resolved laser-induced fluorescence spectroscopy. At room temperature, the decay curve indicated that the lifetime was shorter than 20 ns. On the other hand, the frozen sample of U 4+ in aqueous solution at liquid nitrogen temperature showed the same emission spectrum as at room temperature and its lifetime was 149 ns in H 2 O system and 198 ns in D 2 O system. The longer lifetime at liquid nitrogen temperature made it possible to measure the spectrum of U 4+ at the concentration as low as 10 −6 M. The difference in the anion species (ClO 4 − , Cl − , SO 4 2− ) affected the structure of the emission spectrum to some extent.

Uranium(VI) complex / Europium(III) complex / Carboxylic acid / Stability constant / Enthalpy / EntropySummary. The thermodynamic quantities (∆G, ∆H and ∆S) of U(VI) complexation with oxalate, malonate, succinate, glutarate, adipate, phthalate, tartronate and malate, and of Eu(III) complexation with succinate, glutarate and adipate were determined by potentiometric and calorimetric titration techniques. The obtained thermodynamic quantities indicated that these complexation reactions are mainly driven by the entropy changes while the enthalpy changes are nearly zero or preventing the complexation. The thermodynamic quantities of 1 : 1 U(VI) complexes with 5 dicarboxylates having different carbon chain lengths ( − OOC−(CH 2 ) n −COO − ) showed an interesting tendency, that is, −∆H decreased and T ∆S increased systematically with increasing the length of the carbon chain from n = 2 to 6. The comparison of T ∆S values of 1 : 1 carboxylate formation with U(VI) and Eu(III) showed that T ∆S of U(VI) were always lager than those of Eu(III). The entropies of 1 : 1 complex formation of U(VI) with hydroxycarboxylates were always smaller than those with carboxylates of the same carbon chain length, while the enthalpies of uranyl hydroxycarboxylates were always larger. This tendency was also observed in the protonation and the complex formation of Eu(III).