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Radiochimica Acta

International Journal for chemical aspects of nuclear science and technology

Ed. by Qaim, Syed M.

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Solubility of tetravalent actinides in alkaline CaCl2 solutions and formation of Ca4[An(OH)8]4+ complexes: A study of Np(IV) and Pu(IV) under reducing conditions and the systematic trend in the An(IV) series

D. Fellhauer* / Volker Neck1 / Marcus Altmaier2 / Johannes Lützenkirchen3 / Thomas Fanghänel4

1 Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, Karlsruhe, Deutschland

2 Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, Karlsruhe, Deutschland

3 Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, Karlsruhe

4 European Commission, Institute for Transuranium Elements, Karlsruhe

* Correspondence address: Karlsruhe Institute of Technology, Institut für Nukleare Entsorgung, PO Box 3640, 76021 Karlsruhe,

Citation Information: Radiochimica Acta International journal for chemical aspects of nuclear science and technology. Volume 98, Issue 9-11, Pages 541–548, ISSN (Print) 0033-8230, DOI: 10.1524/ract.2010.1751, November 2010

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The solubility of Np(IV) and Pu(IV) hydrous oxides was studied at 22±2 °C under reducing conditions in alkaline CaCl2 solutions. Redox conditions were adjusted either with 2 mM Na2S2O4 or additions of iron powder. In 2.0 and 4.5 M CaCl2 and pHc=11–12, the neptunium and plutonium concentrations increase with a slope of +4 (log[An] vs. pHc) as expected for the formation of the complex Ca4[An(OH)8]4+ recently identified for Th(IV). At CaCl2 concentrations ≤1.0 M this effect is negligible for both Np(IV) and Pu(IV). The conditional equilibrium constants log*Ks,(4,1,8) for the reaction

An(OH)4(am) + 4 H2O + 4 Ca2+ ⇔ Ca4[An(OH)8]4+ + 4 H+

are evaluated with the SIT and Pitzer model using the parameters derived from analogous data for Th(IV) in 0.5–4.5 M CaCl2. The log*K°s,(4,1,8) values and the complex formation constants logβ°(4,1,8) follow systematic trends in the series Th(IV), Np(IV) and Pu(IV) which allows the estimation of the corresponding data for U(IV).

Keywords: Neptunium; Plutonium; Solubility; Ternary Ca-An(IV)-OH complexes; SIT

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