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

• Published in: Radiochimica acta Vol. 107; no. 8; pp. 663 - 678
• Main Authors: Endrizzi, Francesco, Gaona, Xavier, Zhang, Zhicheng, Xu, Chao, Rao, Linfeng, Garcia-Perez, Carmen, Altmaier, Marcus
• Format: Journal Article
• Language: English
• Published: De Gruyter 01.08.2019
• Subjects: calorimetry; clarkeite; hydrolysis; metaschoepite; sodium uranate; solubility; temperature; thermodynamics; Uranium(VI)
• ISSN: 0033-8230; 2193-3405
• DOI: 10.1515/ract-2018-3056

The solubility and hydrolysis of U(VI) were investigated in 0.10–5.6 m NaCl solutions with 4 ≤ pH  ≤ 14.3 (pH  = −log [H ]) at  = 25, 55 and 80 °C. Batch experiments were conducted under Ar atmosphere in the absence of carbonate. Solubility was studied from undersaturation conditions using UO  · 2H O(cr) and Na  · H O(cr) solid phases, equilibrated in acidic (4 ≤ pH  ≤ 6) and alkaline (8.2 ≤ pH  ≤ 14.3) NaCl solutions, respectively. Solid phases were previously tempered in solution at  = 80 °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 Na  · H O(cr) at 25–80 °C was measured independently by means of solution-drop calorimetry. Solid phase characterization indicates the transformation of UO  · 2H O(cr) into a sodium uranate-like phase with a molar ratio Na:U ≈ 0.4–0.5 in acidic solutions with [NaCl] ≥ 0.51 m at  = 80 °C. In contrast, Na  · H O(cr) equilibrated in alkaline NaCl solutions remains unaltered within the investigated pH , NaCl concentration and temperature range. The solubility of Na  · H O(cr) in the alkaline pH -range is noticeably enhanced at  = 55 and 80 °C relative to  = 25 °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 Na  · H O(cr) under these experimental conditions. A thermodynamic model describing the solubility and hydrolysis equilibria of U(VI) in alkaline solutions at  = 25–80 °C is developed, including and related reaction enthalpies. The standard free energy and enthalpy of formation of Na  · H O(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  = 25–80 °C.