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
Abstract
The solubility and hydrolysis of U(VI) were investigated in 0.10–5.6 m NaCl solutions with 4 ≤ pHm ≤ 14.3 (pHm = −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 ≤ pHm ≤ 6) and alkaline (8.2 ≤ pHm ≤ 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 pHm, NaCl concentration and temperature range. The solubility of Na2U2O7 · H2O(cr) in the alkaline pHm-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
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).
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Artikel in diesem Heft
- Frontmatter
- Thermodynamic description of U(VI) solubility and hydrolysis in dilute to concentrated NaCl solutions at T = 25, 55 and 80 °C
- Polonium-210 in honey samples from southern Poland
- Molybdenum and lanthanum as alternate burn-up monitors – development of chromatographic and mass spectrometric methods for determination of atom percent fission
- Removal of Cs-137 and Sr-90 from reactor actual liquid waste samples using a new synthesized bionanocomposite-based carboxymethylcellulose
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- Radiation – induced preparation of polyaniline/poly vinyl alcohol nanocomposites and their properties
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Artikel in diesem Heft
- Frontmatter
- Thermodynamic description of U(VI) solubility and hydrolysis in dilute to concentrated NaCl solutions at T = 25, 55 and 80 °C
- Polonium-210 in honey samples from southern Poland
- Molybdenum and lanthanum as alternate burn-up monitors – development of chromatographic and mass spectrometric methods for determination of atom percent fission
- Removal of Cs-137 and Sr-90 from reactor actual liquid waste samples using a new synthesized bionanocomposite-based carboxymethylcellulose
- Radiation stability of phosphine oxide functionalized pillar[5]arenes
- Radiation – induced preparation of polyaniline/poly vinyl alcohol nanocomposites and their properties
- Irradiated rubber composite with nano and micro fillers for mining rock application
- Gamma-ray shielding parameters of Li2B4O7 glasses: undoped and doped with magnetite, siderite and Zinc-Borate minerals cases
