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 CaCl₂ 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 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) (I) (metastable), Ca_0.5NpO₂(OH)₂ · 1.3H₂O(s) (II) (long-term metastable) and Ca_0.5NpO₂(OH)₂(s) (III) (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_z[NpO₂Cl]^2z+ with z = 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 (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 CaCl₂/pH_m ≈ 12 confirm the presence of ternary Ca-Np(V)-OH complexes Ca_x[NpO₂(OH)₂]^2x–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 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_y[NpO₂(OH)₅]^2y–4 with y ≈ 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]).