Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part I: experimental results
<section class="abstract"><h2 class="abstractTitle text-title my-1" id="d661e2">Abstract</h2><p>The aqueous chemistry of Np(V) in alkaline 0.01 to 5.5 M CaCl<sub>2</sub> solutions at <em>T</em> = 23 ± 2 ℃ was thoroughly st...
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| Hauptverfasser: | , , , , , , |
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| Dokumenttyp: | Article (Journal) |
| Sprache: | Englisch |
| Veröffentlicht: |
17.02.2016
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| In: |
Radiochimica acta
Year: 2016, Jahrgang: 104, Heft: 6, Pages: 355-379 |
| ISSN: | 2193-3405 |
| DOI: | 10.1515/ract-2015-2489 |
| Online-Zugang: | Verlag, lizenzpflichtig, Volltext: https://doi.org/10.1515/ract-2015-2489 Verlag, lizenzpflichtig, Volltext: https://www.degruyterbrill.com/view/journals/ract/104/6/article-p355.xml 
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| Verfasserangaben: | David Fellhauer, Jörg Rothe, Marcus Altmaier, Volker Neck, Jörg Runke, Thierry Wiss und Thomas Fanghänel |
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| 520 | |a <section class="abstract"><h2 class="abstractTitle text-title my-1" id="d661e2">Abstract</h2><p>The aqueous chemistry of Np(V) in alkaline 0.01 to 5.5 M CaCl<sub>2</sub> solutions at <em>T</em> = 23 ± 2 ℃ was thoroughly studied by long-term batch solubility experiments from both over- and undersaturation. Applying a comprehensive set of experimental and spectroscopic techniques including Vis/NIR and Np L3-edge EXAFS, the solubility controlling Np(V) solid phases and the predominant aqueous Np(V) species were identified. The results demonstrate that the solubility behavior of Np(V) in alkaline CaCl<sub>2</sub> solutions differs completely from the one reported for alkaline NaCl and NaClO<sub>4</sub> solutions: as solubility limiting solid phases, three so far not considered Ca-Np(V)-OH compounds were identified and their solubility and stability in CaCl<sub>2</sub> solutions analyzed: CaNpO<sub>2</sub>(OH)<sub>2.6</sub>Cl<sub>0.4</sub> · 2H<sub>2</sub>O(s) <strong>(I)</strong> (metastable), Ca<sub>0.5</sub>NpO<sub>2</sub>(OH)<sub>2</sub> · 1.3H<sub>2</sub>O(s) <strong>(II)</strong> (long-term metastable) and Ca<sub>0.5</sub>NpO<sub>2</sub>(OH)<sub>2</sub>(s) <strong>(III)</strong> (stable). The considerably higher stability of these (qua)ternary phases which readily form in alkaline CaCl<sub>2</sub> solutions from oversaturation (addition of NpO<sub>2</sub><sup>+</sup>) and undersaturation (addition of binary NpO<sub>2</sub>OH(am)) limit the Np(V) equilibrium concentrations to values that are up to 3 log-units lower compared to those for NpO<sub>2</sub>OH(am) in NaCl and NaClO<sub>4</sub> systems. Based on systematic evaluation of the pH dependence (solubility curve slopes), Vis/NIR and EXAFS spectroscopic information, unhydrolysed NpO<sub>2</sub><sup>+</sup> and innersphere chloro complexes, NpO<sub>2</sub>Cl(aq) and Ca<sub><em>z</em></sub>[NpO<sub>2</sub>Cl]<sup>2<em>z</em>+</sup> with <em>z</em> = 1 (EXAFS, Vis/NIR), were identified as prevailing Np(V) species in less alkaline solutions with pH<sub>m</sub> < 10.5 for [CaCl<sub>2</sub>] ≤ 2.0 M and [CaCl<sub>2</sub>] > 2.0 M, respectively. The steep increase of the Np(V) solubility for pH<sub>m</sub> > 11 with slopes of approximately + 2.5 (for solid phase <strong>(I)</strong>) and + 3 (for solids <strong>(II)</strong> and <strong>(III)</strong>) as well as the Np L3-edge EXAFS results for the aqueous Np(V) species in 4.5 M CaCl<sub>2</sub>/pH<sub>m</sub> ≈ 12 confirm the presence of ternary Ca-Np(V)-OH complexes Ca<sub><em>x</em></sub>[NpO<sub>2</sub>(OH)<sub>2</sub>]<sup>2<em>x-</em>1</sup> (<em>x</em> ≈ 1 is estimated within the thermodynamic evaluation in a subsequent paper (Fellhauer, D., Altmaier, M., Gaona, X., Lützenkirchen, J., Fanghänel, Th., Np(V) solubility, speciation and solid phase formation in alkaline CaCl<sub>2</sub> solutions. Part II: Thermodynamics and implications for source term estimations of nuclear waste disposal (Radiochim. Acta, DOI 10.1515/ract-2015-2490), in the following referred to as “Part II” [1]) and Ca<sub><em>y</em></sub>[NpO<sub>2</sub>(OH)<sub>5</sub>]<sup>2<em>y-</em>4</sup> with <em>y</em> ≈ 2.4 ± 1.5 (EXAFS result) as predominant Np(V) hydrolysis species in solution for [CaCl<sub>2</sub>] ≥ 0.25 and pH<sub>m</sub> > 10.5. The thermodynamic evaluation and implications of the new findings with respect to source term estimations for nuclear waste disposal scenarios are discussed in “Part II” [1]).</p></section> | ||
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