Thermodynamic description of U(VI) solubility and hydrolysis in dilute to concentrated NaCl solutions at T = 25, 55 and 80 °C
Author:
Endrizzi Francesco1, Gaona Xavier1, Zhang Zhicheng2, Xu Chao3, Rao Linfeng2, Garcia-Perez Carmen1, Altmaier Marcus1
Affiliation:
1. Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal , P.O. Box 3640 , 76021 Karlsruhe , Germany 2. Lawrence Berkeley National Laboratory , Chemical Sciences Division , One Cyclotron Road , Berkeley, CA 94720 , USA 3. Institute of Nuclear and New Energy Technology, Tsinghua University , Beijing 100084 , P.R. China
Abstract
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
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{
Na
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U
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H
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cr
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,
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β
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4
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$\log^* {\rm K}_{\rm s,0}^{\circ} \ \{{\rm Na}_{2}{\rm U}_{2}{\rm O}_{7} \cdot {\rm H}_{2}{\rm O}({\rm cr})\}, \log^{*} \beta _{1,4}^{\circ} $
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.
Publisher
Walter de Gruyter GmbH
Subject
Physical and Theoretical Chemistry
Reference43 articles.
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