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

Francesco Endrizzi; Xavier Gaona; Zhicheng Zhang; Chao Xu; Linfeng Rao; Carmen Garcia-Perez; Marcus Altmaier

doi:10.1515/ract-2018-3056

Abstract

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 °C. Batch experiments were conducted under Ar atmosphere in the absence of carbonate. Solubility was studied from undersaturation conditions using UO₃ · 2H₂O(cr) and Na₂U₂O₇ · 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 °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 · H₂O(cr) at 25–80 °C was measured independently by means of solution-drop calorimetry. Solid phase characterization indicates the transformation of UO₃ · 2H₂O(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 T = 80 °C. In contrast, Na₂U₂O₇ · H₂O(cr) equilibrated in alkaline NaCl solutions remains unaltered within the investigated pH_m, NaCl concentration and temperature range. The solubility of Na₂U₂O₇ · H₂O(cr) in the alkaline pH_m-range is noticeably enhanced at T = 55 and 80 °C relative to T = 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 Na₂U₂O₇ · 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 °C is developed, including log *Ks,0°{Na2U2O7⋅H2O(cr)}, log *β1,4∘ and related reaction enthalpies. The standard free energy and enthalpy of formation of Na₂U₂O₇ · 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 °C.

Keywords: Uranium(VI); solubility; hydrolysis; temperature; metaschoepite; sodium uranate; clarkeite; calorimetry; thermodynamics

Acknowledgments

The authors would like to thank F. Geyer, C. Walschburger, M. Böttle, S. Heck, S. Moisei-Rabung, T. Kisely and E. Soballa (KIT–INE) for their lab assistance and ICP–MS, ICP–OES, TG–DTA, TOC and SEM–EDS analyses. This work was partially funded by the German Federal Ministry for Education and Research (BMBF). KIT–INE is working in ThermAc under the contract 02NUK039A. The calorimetric experiments were supported by the Director, Office of Science, Office of Basic Energy Science of the US Department of Energy, under Contract No. DE-AC02-05CH11231 at Lawrence Berkeley National Laboratory.

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Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/ract-2018-3056).

Received: 2018-09-04

Accepted: 2019-02-15

Published Online: 2019-05-27

Published in Print: 2019-07-26

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