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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

CiteScore 2018: 1.20

SCImago Journal Rank (SJR) 2018: 0.333
Source Normalized Impact per Paper (SNIP) 2018: 0.720

Online
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2193-3405
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Volume 104, Issue 6

Issues

Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part II: Thermodynamics and implications for source term estimations of nuclear waste disposal

David Fellhauer
  • Corresponding author
  • Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany
  • European Commission, Joint Research Centre, Institute for Transuranium Elements, Karlsruhe, Germany
  • Heidelberg University, Institute of Physical Chemistry, Heidelberg, Germany
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  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Marcus Altmaier
  • Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany
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  • De Gruyter OnlineGoogle Scholar
/ Xavier Gaona
  • Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Johannes Lützenkirchen
  • Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany
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/ Thomas Fanghänel
  • European Commission, Joint Research Centre, Institute for Transuranium Elements, Karlsruhe, Germany
  • Heidelberg University, Institute of Physical Chemistry, Heidelberg, Germany
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  • De Gruyter OnlineGoogle Scholar
Published Online: 2016-02-17 | DOI: https://doi.org/10.1515/ract-2015-2490

Abstract

The results of comprehensive solubility experiments with Np(V) in dilute to concentrated CaCl2 solutions which included the spectroscopic investigation of the predominant aqueous Np(V) species and the thorough analysis of the solubility controlling Np(V) solid phases [1](Fellhauer, D., Rothe, J., Altmaier, M., Neck, V., Runke, J., Wiss, T., Fanghänel, Th., Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part I: Experimental solubility (Radiochim. Acta, DOI 10.1515/ract-2015-2489), in the following referred to as “Part I”), showed that the concentration of Np(V) in these systems is limited by equilibrium reactions between previously unknown (qua)ternary solid compounds, CaNpO2(OH)2.6Cl0.4 · 2H2O(s) (I), Ca0.5NpO2(OH)2 · 1.3H2O(s) (II) and Ca0.5NpO2(OH)2(s) (III), and ternary aqueous complexes of Np(V) with the general formulae Cax[NpO2(OH)2]2x–1, Cay[NpO2(OH)5]2y–4 with y ≈ 2.4 ± 1.5 (EXAFS result) and innersphere Np(V) chloro complexes, Caz[NpO2Cl]2z+ with z = 0 and 1 (EXAFS result). A systematic thermodynamic evaluation of the experimental solubility data based on the specific ion interaction theory (SIT) and the Pitzer approach is performed. The stoichiometries of the ternary hydrolysis complexes are assessed as Ca[NpO2(OH)2]+ and Ca3[NpO2(OH)5]2+. The corresponding thermodynamic solubility and complex formation constants at I = 0 as well as the ion interaction parameters (SIT and Pitzer) were derived yielding a comprehensive geochemical model for the Np(V) solubility behavior in pH neutral and alkaline CaCl2 solutions over a large range of ionic strengths. This significantly improves source term estimations for scenarios with calcium dominated aquatic systems. The principally different behavior of Np(V) in NaCl and CaCl2 solutions is highlighted, and the impact of the new findings on geochemical modelling of systems relevant for nuclear waste disposal is discussed.

Keywords: Neptunium; solubility; hydrolysis; chloro complexation; Ca-Np(V)-OH solid phases; ternary Ca-Np(V)-OH complexes; Pitzer model; SIT

About the article

Accepted: 2016-01-13

Received: 2015-07-27

Published Online: 2016-02-17

Published in Print: 2016-06-28


Citation Information: Radiochimica Acta, Volume 104, Issue 6, Pages 381–397, ISSN (Online) 2193-3405, ISSN (Print) 0033-8230, DOI: https://doi.org/10.1515/ract-2015-2490.

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