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
Arnault Lassin, Sylvain Guignot, Adeline Lach, Christomir Christov, Laurent André, Benoı̂t Madé
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.