Modeling the Solution Properties and Mineral–Solution Equilibria in Radionuclide-Bearing Aqueous Nitrate Systems: Application to Binary and Ternary Systems Containing U, Th, or Lanthanides at 25 °C

Abstract

This article focuses on the modeling of the thermodynamic properties of aqueous nitrate systems that contain radionuclides from low molalities to saturation and occasionally supersaturation with respect to the corresponding nitrate solid salts. It is an additional contribution following previous works dedicated to nitrate systems containing alkali and/or alkali-earth metals or lanthanides. Here, 18 chemical systems, mostly ternary, were studied at 25 °C: 5 contained actinides (Th or U­(IV)) and 13 contained lanthanum and/or lanthanides (Ce, Pr, Nd, Sm, and/or Er). Six of these systems also contained alkali (Na, K) or alkali-earth metals (Mg, Ca). The modeling approach was based on the standard Pitzer formulation for strong aqueous electrolytes and was used to reproduce the published experimental data on the osmotic coefficient of solutions and on the solubility diagrams of salts. Ion-specific ternary interaction parameters and nitrate salt solubility products are proposed. The results suggest that ternary nitrate systems containing two lanthanides with close atomic numbers, such as Pr and Nd or Nd and Sm, are favorable to the formation of solid solutions. Otherwise, pure salts precipitate in their own stability domain. This allows us to propose predictive solubility diagrams for Ln–Er–NO3–H2O and Ln–Ce–NO3–H2O systems (with Ln = La, Pr, or Nd), the former being controlled by pure salts and the latter being controlled by ideal solid solutions.

Thermodynamic Properties and Phase Equilibria in the H2O–HNO3–Ca(NO3)2–UO2(NO3)2 System

“…As can be seen (Figure ), the one of Ca­(NO 3 ) 2 ·4H 2 O has a characteristic bent which cannot be reproduced with the proposed thermodynamic model. The same problem was reported by Lassin et al who investigated the H 2 O–Ca­(NO 3 ) 2 –UO 2 (NO 3 ) 2 system in terms of classical Pitzer equations. According to Lassin, this phenomenon might be connected with not taking any ion pairing into account.…”

Pilot-Scale Lanthanide Precipitation from Sulfate-Based Spent Ni-MH Battery Leachates: Thermodynamic-Based Choice of Operating Conditions

“…33−37 Moreover, the existence of solid solutions containing at least two REEs has previously been reported in the literature for other types of compounds such as rare earth oxides, 38 nitrates, 39 or phosphates. 40 Additionally, several authors have reported that stable solid solutions are favorable for adjacent lanthanide pairs (i.e., La− Ce, Ce−Pr, Pr−Nd) in nitrate 39 and phosphate 40 systems. In this case, the solid solution stabilizes the solid phase as opposed to a mixture of pure phases; therefore, a smaller amount of the element remains in solution when it has precipitated in the form of a solid solution than as a mixture of pure phases.…”

“…The above discrepancies observed between experimental and thermodynamic concentrations of Ce and Pr in solution are most likely due to the formation of a solid solution, where Ce 3+ and/or Pr 3+ would be inserted or substituted within the crystal structure of NaLa­(SO 4 ) 2 ·H 2 O and/or NaNd­(SO 4 ) 2 ·H 2 O. Theoretically, a solid solution could form given that the crystal structures of single phases of NaREE­(SO 4 ) 2 ·H 2 O (REE = La, Ce, Nd, or Pr) are all trigonal (space group P 3 2 21 or the enantiomorphic analogue P 3 2 21 ). − Moreover, the existence of solid solutions containing at least two REEs has previously been reported in the literature for other types of compounds such as rare earth oxides, nitrates, or phosphates . Additionally, several authors have reported that stable solid solutions are favorable for adjacent lanthanide pairs (i.e., La–Ce, Ce–Pr, Pr–Nd) in nitrate and phosphate systems. In this case, the solid solution stabilizes the solid phase as opposed to a mixture of pure phases; therefore, a smaller amount of the element remains in solution when it has precipitated in the form of a solid solution than as a mixture of pure phases …”

“…Additionally, several authors have reported that stable solid solutions are favorable for adjacent lanthanide pairs (i.e., La–Ce, Ce–Pr, Pr–Nd) in nitrate and phosphate systems. In this case, the solid solution stabilizes the solid phase as opposed to a mixture of pure phases; therefore, a smaller amount of the element remains in solution when it has precipitated in the form of a solid solution than as a mixture of pure phases …”

“…In this case, the solid solution stabilizes the solid phase as opposed to a mixture of pure phases; therefore, a smaller amount of the element remains in solution when it has precipitated in the form of a solid solution than as a mixture of pure phases. 39 As mentioned in section 2.2, the OLI system software does not allow taking into account the thermodynamic properties of solid solutions, which leads to overestimation of the quantities of Ce and Pr in solution. However, even if other software can make it possible to describe equilibria between solid solutions and concentrated aqueous solutions (e.g., Phreeqc, FactSage), there is to our knowledge no consistent thermodynamic description available for quaternary or higher-order systems REE-REE′-Na 2 SO 4 -H 2 SO 4 -H 2 O (REE = La, Ce, Nd, or Pr).…”