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The complexation of AnIII/LnIII with nitrate was investigated in dilute to concentrated Na-Mg-Ca-Cl-NO3 solutions at 2.94 ≤ pHm ≤ 13.2 and T = 22 ± 2 °C. Comprehensive solubility experiments with Nd(OH)3(s) were performed from undersaturation conditions and complemented by extensive spectroscopic studies using Cm(III)-TRLFS (time resolved laser fluorescence spectroscopy) and Nd-LIII EXAFS (extended X-ray absorption fine structure) under analogous pHm, Im and mNO3 - conditions. Solid phases recovered from batch solubility experiments were characterized by XRD and SEM-EDS. A significant influence of nitrate on the solubility of Nd(OH)3(s) is observed in concentrated weakly alkaline MgCl2-Mg(NO3)2 solutions with total salt concentration ≥ 2.83 m and mNO3 - ≥ 1.13 m. Nitrate has no effect in any of the studied NaCl-NaNO3 and CaCl2-Ca(NO3)2 mixtures, thus highlighting the key role of Mg2+ in the Nd(III)-NO3 interaction. The formation of inner-sphere complexes with the participation of Mg2+ is further confirmed by Cm(III)-TRLFS and Nd-LIII EXAFS. Based on these experimental evidences, the proposed chemical model includes the definition of two new aqueous species Mg[AnIII/LnIIINO3OH]3+ and Mg[AnIII/LnIIINO3(OH)2]2+ in equilibrium with solid Nd(OH)3(s) and allows to derive the thermodynamic and activity models (Pitzer) for the system Nd3+/Cm3+-H+-Mg2+-OH--Cl--NO3--H2O. [ABSTRACT FROM AUTHOR]

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 = pH = ~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 Na2 Np2O7(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⋅2HM2O), 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. [ABSTRACT FROM AUTHOR]

Known data on the solubility of Am(OH)3(s) and the hydrolysis of Am(III) and Cm(III), additional information from an extensive solubility study with Nd(OH)3(s) in NaCl, MgCl2, and CaCl2 media of various ionic strengths and spectroscopic (time-resolved laser fluorescence spectroscopy, TRLFS) data for Cm(III) in alkaline CaCl2 solutions are used to evaluate a comprehensive set of standard-state equilibrium constants and ion interaction parameters for the specific ion interaction theory (SIT) and Pitzer equations at 25 °C. The thermo dynamic model takes into account the analogous solubility and hydrolysis behavior of trivalent actinides and Nd(III) and covers the entire pH range in dilute to concentrated NaCl, MgCl2, and CaCl2 solutions. In alkali chloride/hydroxide solutions, the formation of the tetrahydroxide complex M(OH)4- requires OH- concentration above 3 mol l-1, whereas in alkaline CaCl2 solutions (at pHc < 12) M(III) complexes with four and six hydroxide ligands are formed. Similar as the recently detected ternary Ca-M(IV)-OH complexes Ca3[Zr(OH)6]4+ and Ca4[Th(OH)8]4+, these complexes are stabilized by the association of Ca2+ ions. The solubility and hydrolysis of Am(III), Cm(III), and Nd(III) in both Ca-free and -containing solutions is consistently described with a model including the ternary Ca-M(III)-OH complexes Ca[M(OH)3]2+, Ca2[M(OH)4]3+, and Ca3[M(OH)6]3+. [ABSTRACT FROM AUTHOR]