Pure and Applied Chemistry

Abstract: The solubility of Np(VI) was investigated in carbonate-free NaCl solutions (0.1 M ≤ I ≤ 5.0 M) at T = 22 ± 2 °C to derive thermodynamic properties of aqueous species and solid compounds formed under alkaline conditions. The experimentally derived solubility curves can be divided into four main regions: (I) ~7 ≤ pH_m ≤ ~9, showing a steep decrease in Np solubility with a slope (log [Np] vs. pH_m) of –3 or –2 (depending on NaCl concentration); (II) ~9 ≤ pH_m ≤ ~10.5, with a nearly pH-independent [Np]; (III) ~10.5 ≤ pH_m ≤ ~13.5, showing an increase in the solubility with a well-defined slope of +1. A region (IV) with a slope ≥ +2 was only observed at I ≥ 1.0 M NaCl and pH_m ≥ ~13.5. The solubility-controlling solid Np phases were characterized by X-ray diffraction (XRD), quantitative chemical analysis, thermogravimetric analysis and scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS), confirming the presence of anhydrous Na₂Np₂O₇(cr) in regions II and III. The same solid phase was identified in region I except for the system in 0.1 M NaCl, where a NpO₂(OH)₂·H₂O(cr) phase predominates. XRD patterns of this solid phase show a very good agreement with that of metaschoepite (UO₃·2H₂O), highlighting the similarities between Np(VI) and U(VI) with respect to solid phase formation and structure. Based on the analysis of solubility data, solid phase characterization and chemical analogy with U(VI), the equilibrium reactions 0.5 Na₂Np₂O₇(cr) + 1.5 H₂O ⬄ Na⁺ + NpO₂(OH)₃^– and 0.5 Na₂Np₂O₇(cr) + 2.5 H₂O ⬄ Na⁺ + NpO₂(OH)₄^2– + H⁺ were identified as controlling Np(VI) solubility in regions II and III, respectively. The predominance of NpO₂⁺ in the aqueous phase of region I (quantified by UV–vis/NIR) indicates the reductive dissolution of Np(VI) [either as Na₂Np₂O₇(cr) or NpO₂(OH)₂·H₂O(cr)] to Np(V)_aq. Oxidation to Np(VII) can explain the experimental observations in region IV, although it is not included in the chemical and thermodynamic models derived. The conditional equilibrium constants determined from the solubility experiments at different ionic strengths were evaluated with both the specific ion interaction theory (SIT) and Pitzer approaches. Thermodynamic data for aqueous Np(VI) species [NpO₂(OH)₃^– and NpO₂(OH)₄^2–] and solid compounds [Na₂Np₂O₇(cr) and NpO₂(OH)₂·H₂O(cr)] that are relevant under alkaline conditions were derived. These data are not currently included in the Nuclear Energy Agency-Thermochemical Database (NEA-TDB) compilation.