Complexation of U(VI) with malonate at variable temperatures

The complexation between uranium(VI) and malonate in 1.05 mol kg(-1) NaClO4 was studied at variable temperatures (25, 35, 45, 55 and 70degreesC). The formation constants of three successive complexes, UO2(OOCCH2COO), UO2(OOCCH2COO)(2)(2-) and UO2(OOCCH2COO)(3)(4-), and the molar enthalpies of complexation were determined by potentiometry and calorimetry. The heat capacity of the complexation, DeltaCdegrees(p,m(MLj)), is calculated to be 96 +/- 12, 195 +/- 15 and 267 +/- 22 J K-1 mol(-1) for j = 1, 2 and 3, respectively. Extended X-ray Absorption Fine Structure Spectroscopy helped to characterize the coordination modes in the complexes in solution. UV/Vis absorption and luminescence spectra at different temperatures provided qualitative information on the temperature effect. The effect of temperature on the complexation between uranium(VI) and malonate is discussed in terms of the electrostatic model and compared with the complexation between uranium(VI) and acetate.