Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part I

• Verfasst von: Fellhauer, David [VerfasserIn]
• Verfasst von: Rothe, Jörg [VerfasserIn]
• Verfasst von: Altmaier, Marcus [VerfasserIn]
• Verfasst von: Neck, Volker [VerfasserIn]
• Verfasst von: Runke, Jörg [VerfasserIn]
• Verfasst von: Wiss, Thierry [VerfasserIn]
• Verfasst von: Fanghänel, Thomas [VerfasserIn]
• Titel: Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part I
• Titelzusatz: experimental results
• Verf.angabe: David Fellhauer, Jörg Rothe, Marcus Altmaier, Volker Neck, Jörg Runke, Thierry Wiss und Thomas Fanghänel
• E-Jahr: 2016
• Jahr: 17.02.2016
• Umfang: 25 S.
• Fussnoten: Gesehen am 15.05.2020
• Titel Quelle: Enthalten in: Radiochimica acta
• Ort Quelle: Berlin : De Gruyter, 1962
• Jahr Quelle: 2016
• Band/Heft Quelle: 104(2016), 6, Seite 355-379
• ISSN Quelle: 2193-3405
• DOI: doi:10.1515/ract-2015-2489
• Datenträger: Online-Ressource
• Sprache: eng
• K10plus-PPN: 1698319428

Abstract

<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 CaCl₂ 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 CaCl₂ solutions differs completely from the one reported for alkaline NaCl and NaClO₄ solutions: as solubility limiting solid phases, three so far not considered Ca-Np(V)-OH compounds were identified and their solubility and stability in CaCl₂ solutions analyzed: CaNpO₂(OH)_2.6Cl_0.4 · 2H₂O(s) <strong>(I)</strong> (metastable), Ca_0.5NpO₂(OH)₂ · 1.3H₂O(s) <strong>(II)</strong> (long-term metastable) and Ca_0.5NpO₂(OH)₂(s) <strong>(III)</strong> (stable). The considerably higher stability of these (qua)ternary phases which readily form in alkaline CaCl₂ solutions from oversaturation (addition of NpO₂⁺) and undersaturation (addition of binary NpO₂OH(am)) limit the Np(V) equilibrium concentrations to values that are up to 3 log-units lower compared to those for NpO₂OH(am) in NaCl and NaClO₄ systems. Based on systematic evaluation of the pH dependence (solubility curve slopes), Vis/NIR and EXAFS spectroscopic information, unhydrolysed NpO₂⁺ and innersphere chloro complexes, NpO₂Cl(aq) and Ca_<em>z</em>[NpO₂Cl]^2<em>z</em>+ with <em>z</em> = 1 (EXAFS, Vis/NIR), were identified as prevailing Np(V) species in less alkaline solutions with pH_m < 10.5 for [CaCl₂] ≤ 2.0 M and [CaCl₂] > 2.0 M, respectively. The steep increase of the Np(V) solubility for pH_m > 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 CaCl₂/pH_m ≈ 12 confirm the presence of ternary Ca-Np(V)-OH complexes Ca_<em>x</em>[NpO₂(OH)₂]^{2<em>x-</em>1} (<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 CaCl₂ 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_<em>y</em>[NpO₂(OH)₅]^{2<em>y-</em>4} with <em>y</em> ≈ 2.4 ± 1.5 (EXAFS result) as predominant Np(V) hydrolysis species in solution for [CaCl₂] ≥ 0.25 and pH_m > 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>