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

Abstract The aqueous chemistry of Np(V) in alkaline 0.01 to 5.5 M CaCl 2 solutions at T = 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 2 solutions differs completely from the one reported for alkaline NaCl and NaClO 4 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 2 solutions analyzed: CaNpO 2 (OH) 2.6 Cl 0.4 · 2H 2 O(s) (I) (metastable), Ca 0.5 NpO 2 (OH) 2 · 1.3H 2 O(s) (II) (long-term metastable) and Ca 0.5 NpO 2 (OH) 2 (s) (III) (stable). The considerably higher stability of these (qua)ternary phases which readily form in alkaline CaCl 2 solutions from oversaturation (addition of NpO 2 + ) and undersaturation (addition of binary NpO 2 OH(am)) limit the Np(V) equilibrium concentrations to values that are up to 3 log-units lower compared to those for NpO 2 OH(am) in NaCl and NaClO 4 systems. Based on systematic evaluation of the pH dependence (solubility curve slopes), Vis/NIR and EXAFS spectroscopic information, unhydrolysed NpO 2 + and innersphere chloro complexes, NpO 2 Cl(aq) and Ca z [NpO 2 Cl] 2 z + with z = 1 (EXAFS, Vis/NIR), were identified as prevailing Np(V) species in less alkaline solutions with pH m < 10.5 for [CaCl 2 ] ≤ 2.0 M and [CaCl 2 ] > 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 (I) ) and + 3 (for solids (II) and (III) ) as well as the Np L3-edge EXAFS results for the aqueous Np(V) species in 4.5 M CaCl 2 /pH m ≈ 12 confirm the presence of ternary Ca-Np(V)-OH complexes Ca x [NpO 2 (OH) 2 ] 2 x– 1 ( x ≈ 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 2 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 y [NpO 2 (OH) 5 ] 2 y– 4 with y ≈ 2.4 ± 1.5 (EXAFS result) as predominant Np(V) hydrolysis species in solution for [CaCl 2 ] ≥ 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]).