Standard Thermodynamic Properties and Heat Capacity Equations for Rare Earth Element Hydroxides

Abstract

Lanthanum / Neodymium / Hydroxide / Thermodynamic properties / Solubility SummaryStandard thermodynamic properties at 298.15 K, 1 bar and heat capacity equations for La(0H)3(s) and Nd(0H)3(s) were generated from thermochemical data available in the literature. Calculated solubility products of these crystalline hydroxides at 298.15 K were compared to those derived from critically assessed solubility and Potentiometrie measurements. Good agreement between the calculated and measured solubility was found for La(OH)3(s). In contrast, the Nd(OH)3(s) solubility at 298.15 K, calculated from thermochemical data, was more than one and half Orders of magnitude higher than that derived from experimental solubility data. This result shows that Nd^"^ is much less mobile in natural environments than previously thought and suggests that accepted Standard thermodynamic properties for the Nd'^ aqueous ion at 298.15 K, 1 bar [2] need to be re-examined. By calculating equilibria between La-and Nd-crystalline hydroxides, hydroxycarbonates and carbonates we show in this study that Ln-hydroxides are the stable phases at low CO2 partial pressures typical for concrete backfill environments in proposed radioactive waste storage sites.

Thermodynamics of trivalent actinides and neodymium in NaCl, MgCl2, and CaCl2 solutions: Solubility, hydrolysis, and ternary Ca-M(III)-OH complexes

“…The solubility constant of the aged Nd(OH) 3 (s) phase used in the present study, lg*K°s ,0 = 17.2 ± 0.4, and the value of 16.0 ± 0.4 proposed by Diakonov et al [35,36] for aged and crystalline Nd(OH) 3 (s) are close to the values selected in the NEA-TDB [6] for amorphous and aged/crystalline Am(OH) 3 (s). The very high values of lg*K°s ,0 [Nd(OH) 3 (am)] = 18.66 ± 0.55 [35,36] or 18.6 [15] are probably not appropriate for longterm model calculations. The hydrolysis constants lg*β°1 1 = -7.4 ± 0.4 and lg*β°1 2 = -15.7 ± 0.7 derived from our solubility study with Nd(OH) 3 (s) are in the range of the values selected in the NEA-TDB [6] from numerous studies and different experimental methods for Am(III) and Cm(III).…”

“…The solid Nd(III) hydroxide used in the present study was prepared under CO 2 -free argon atmosphere by hydration of crystalline Nd 2 O 3 (cr) (Merck) in pure water (actinide and lanthanide oxides M 2 O 3 (cr) are not stable in aqueous solution and transform into the hydroxides [4,35,36]). The solid was stored for about three months under water.…”

“…In later reviews of thermodynamic data for the solid lanthanide hydroxides, Diakonov et al [35,36] pointed out that the solubility constant lg*K°s ,0 = 18.6 recommended by Baes and Mesmer [15] refers to highly soluble amorphous precipitates. From their compilation of published data, they calculated mean values of lg*K°s ,0 = 18.66 ± 0.55 for fresh, amorphous precipitates of Nd(OH) 3 (s) and 16.02 ± 0.37 for aged and crystalline Nd(OH) 3 (s) [35,36].…”

“…From their compilation of published data, they calculated mean values of lg*K°s ,0 = 18.66 ± 0.55 for fresh, amorphous precipitates of Nd(OH) 3 (s) and 16.02 ± 0.37 for aged and crystalline Nd(OH) 3 (s) [35,36]. The solubility constant of the Nd(OH) 3 (s) phase used in the present work (lg*K°s ,0 = 17.2 ± 0.4) lies between these values.…”

Complex formation and lactonization reactions in aqueous solutions containing Ca2+ or Nd3+ ions and sugar-type ligands

“…for the first years in the time-scale of the cement degradation [5,113]. With regard to Nd(OH)3(s), 3 The dissolution reaction with the corresponding solubility product, Ksp, is the following: (15) At 25 °C, log Ksp 0 was reported to be 18.66 for amorphous, freshly precipitated hydroxide [114] as well as 16.02, 16.0 and 14.96 [114][115][116] The Nd 3+ aqua ion is known to form various hydroxido complexes in the general reaction of: (16) In this equation, Nd 3+ represents the Nd(H2O)x 3+ aqua ion and the NdpH−r (3p−r)+ species are equivalent to the Ndp(OH)r (3p−r)+ ones.…”