Effect of Temperature on the Complexation of Uranium(VI) with Fluoride in Aqueous Solutions

Complexation of U(VI) with fluoride at elevated temperatures in aqueous solutions was studied by spectrophotometry. Four successive complexes, UO(2)F(+), UO(2)F(2)(aq), UO(2)F(3)(-), and UO(2)F(4)(2-), were identified, and the stability constants at 25, 40, 55, and 70 degrees C were calculated. The stability of the complexes increased as the temperature was elevated. The enthalpies of complexation at 25 degrees C were determined by microcalorimetry. Thermodynamic parameters indicate that the complexation of U(VI) with fluoride in aqueous solutions at 25 to 70 degrees C is slightly endothermic and entropy-driven. The Specific Ion Interaction (SIT) approach was used to obtain the thermodynamic parameters of complexation at infinite dilution. Structural information on the U(VI)/fluoride complexes was obtained by extended X-ray absorption fine structure spectroscopy.