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

• DOI: 10.1351/pac-con-12-12-06
• Publication Date: 2013-08-04T19:01:09Z
• Authors: Xavier Gaona; David Fellhauer; Marcus Altmaier
• Keywords: Anhydrous; Thermogravimetric analysis; Solubility equilibrium

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