Thermodynamic description of U(VI) solubility and hydrolysis in dilute to concentrated NaCl solutions at T=25, 55 and 80 °C

The solubility and hydrolysis of U(VI) were investigated in 0.10-5.6 m NaCl solutions with 4 <= pH(m) <= 14.3 (pH(m)= -log [H+]) at T= 25, 55 and 80 degrees C. Batch experiments were conducted under Ar atmosphere in the absence of carbonate. Solubility was studied from undersaturation conditions using UO3 center dot 2H(2)O(cr) and Na2U2O2 center dot H2O(cr) solid phases, equilibrated in acidic (4 <= pH(m) <= 6) and alkaline (8.2 <= pH(m) <= 14.3) NaCl solutions, respectively. Solid phases were previously tempered in solution at T=80 degrees C to avoid changes in the crystallinity of the solid phase in the course of the solubility experiments. Starting materials and solid phases isolated at the end of the solubility experiments were characterized by powder XRD, SEM-EDS, TRLFS and quantitative chemical analysis. The enthalpy of dissolution of Na2U2O7 center dot H2O(cr) at 25-80 degrees C was measured independently by means of solution-drop calorimetry. Solid phase characterization indicates the transformation of UO3 center dot 2H(2)O(cr) into a sodium uranate-like phase with a molar ratio Na:U approximate to 0.4-0.5 in acidic solutions with [NaCl] >= 0.51 m at T=80 degrees C. In contrast, Na2U2O7 center dot H2O(cr) equilibrated in alkaline NaCl solutions remains unaltered within the investigated pH(m), NaCl concentration and temperature range. The solubility of Na2U2O7 center dot H2O(cr) in the alkaline pH.-range is noticeably enhanced at T= 55 and 80 degrees C relative to T= 25 degrees C. Combined results from solubility and calorimetric experiments indicate that this effect results from the increased acidity of water at elevated temperature, together with an enhanced hydrolysis of U(VI) and a minor contribution due to a decreased stability of Na2U2O7 center dot H2O(cr) under these experimental conditions. A thermodynamic model describing the solubility and hydrolysis equilibria of U(VI) in alkaline solutions at T=25-80 degrees C is developed, including log *K-s,K-0 degrees {Na2U2O7 center dot H2O(cr)}, log *beta(1,4)degrees, and related reaction enthalpies. The standard free energy and enthalpy of formation of Na2U2O7 center dot H2O(cr) calculated from these data are also provided. These data can be implemented in thermodynamic databases and allow accurate solubility and speciation calculations for U(VI) in dilute to concentrated alkaline NaCl solutions in the temperature range T=25 80 degrees C.