Thermodynamic description of Np(VI) solubility, hydrolysis, and redox behavior in dilute to concentrated alkaline NaCl solutions

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 ≤ pHm ≤ ~9, showing a steep decrease in Np solubility with a slope (log [Np] vs. pHm) of –3 or –2 (depending on NaCl concentration); (II) ~9 ≤ pHm ≤ ~10.5, with a nearly pH-independent [Np]; (III) ~10.5 ≤ pHm ≤ ~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 pHm ≥ ~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)42– + 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)42–] 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.