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Radiochimica Acta

International Journal for chemical aspects of nuclear science and technology

Editor-in-Chief: Qaim, Syed M.


IMPACT FACTOR 2018: 1.339

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2193-3405
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Volume 107, Issue 8

Issues

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

Francesco Endrizzi
  • Corresponding author
  • Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany
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/ Xavier Gaona
  • Corresponding author
  • Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany
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/ Zhicheng Zhang
  • Lawrence Berkeley National Laboratory, Chemical Sciences Division, One Cyclotron Road, Berkeley, CA 94720, USA
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/ Chao Xu
  • Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, P.R. China
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/ Linfeng Rao
  • Lawrence Berkeley National Laboratory, Chemical Sciences Division, One Cyclotron Road, Berkeley, CA 94720, USA
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/ Carmen Garcia-Perez
  • Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany
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/ Marcus Altmaier
  • Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany
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Published Online: 2019-05-27 | DOI: https://doi.org/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 ≤ 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 log *Ks,0°{Na2U2O7H2O(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.

This article offers supplementary material which is provided at the end of the article.

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

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About the article

Received: 2018-09-04

Accepted: 2019-02-15

Published Online: 2019-05-27

Published in Print: 2019-07-26


Citation Information: Radiochimica Acta, Volume 107, Issue 8, Pages 663–678, ISSN (Online) 2193-3405, ISSN (Print) 0033-8230, DOI: https://doi.org/10.1515/ract-2018-3056.

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