Modeling the Osmotic and Activity Coefficients of Lanthanide Nitrate Aqueous Solutions at 298.15 K from Low Molalities to Supersaturation

Abstract

This article is a contribution to the modeling of the thermodynamic properties of 15 lanthanide (including lanthanum)-nitrate aqueous binary solutions, from low molalities to saturation (for Ce, Gd, Tb, and Tm) and supersaturation (for La, Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb, and Lu) with respect to the corresponding trinitrate lanthanide solid salts. A critical compilation of the experimental and previous modeling data available in the literature is presented. The modeling approach is based on the standard Pitzer formulation for strong aqueous electrolytes, with two cases considered: three (β(0), β(1), and Cφ ) or five (β(0), β(1), β(2), α2, and Cφ ) parameters. The best fits are obtained for the latter case, leading to the representation of unprecedented accuracy for the osmotic and activity coefficients over the whole range of experimental data up to the saturation/supersaturation points. On the basis of the models developed, the thermodynamic solubility product constants and the standard molar Gibbs free energy of formation of precipitating hexahydrate or pentahydrate lanthanide–nitrate solids are calculated.

Thermodynamic Modeling of Multicomponent Rare Earth Nitrates Aqueous Systems

“…Published efforts of thermodynamical modeling of H 2 O-RE(NO 3 ) 3 systems where RE = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, have been based mainly on vapor-liquid equilibria and employed the Pitzer model [20]. The authors of [2][3][4] managed to describe their results with acceptable accuracy only for a narrow range of concentrations with a maximum at 1.9 mol/kg.…”

“…Pérez-Villaseñor et al [19] succeeded in describing the properties of H 2 O-RE(NO 3 ) 3 systems up to a salt content of 6 mol/kg with an accuracy better than 1%. In [20], a five-parameter version of the Pitzer model was used to calculate not only the osmotic coefficients above the saturation point but the solubility of salt at 25°C as well. The molar Gibbs energies of RE(NO 3 ) 3 ⋅nH 2 O crystalline hydrate formation, where n = 5 and 6, were estimated simultaneously.…”

“…The molar Gibbs energies of RE(NO 3 ) 3 ⋅nH 2 O crystalline hydrate formation, where n = 5 and 6, were estimated simultaneously. The complete dissociation of RE nitrates in water without the formation of intermediate species or associates was assumed in [2,19,20].…”

Liquid–Liquid Equilibria in the Water–Nitric Acid–Europium Nitrate–Gadolinium Nitrate–Tributyl Phosphate System at 298.15 K

“…It is known that data on phase equilibria in these systems is very important for modeling the extraction separation of rare earth elements in a nitric acid medium with tributyl phosphate. Phase equilibria and thermodynamic properties in the subsystems H 2 O–HNO 3 , H 2 O–TBP, H 2 O–HNO 3 –TBP, H 2 O–Eu­(NO 3 ) 3 , − H 2 O–Gd­(NO 3 ) 3 , − H 2 O–HNO 3 –Eu­(NO 3 ) 3 , and H 2 O–HNO 3 –Gd­(NO 3 ) 3 are well studied. Although liquid–liquid equilibria in the H 2 O–HNO 3 –Ln­(NO 3 ) 3 –TBP (Ln = Eu, Gd) systems were investigated, the data are insufficient. − In some studies, authors used trace concentrations of REEs. − In other studies, authors investigated a wide range of concentrations of nitric acid and rare earth elements but did not present the total compositions of the systems. − Thus, these data cannot be used to construct the rigorous thermodynamic model.…”

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

“…In this context, a series of works have been initiated to explore the modeling of the chemical behavior of radionuclide-bearing aqueous nitrate systems, whose salinity can be as high as several moles of dissolved nitrate salt per kg of solution. Relying on Pitzer equations, these works focused on binary and ternary aqueous systems composed of nitrate and of major elements such as H, Na, K, Mg, and Ca, as well as on binary aqueous systems composed of nitrate and a list of lanthanides at 25 °C.…”