Thermochemistry of Selected Lanthanide and Actinide Hydroxycarbonates and Carbonates

Abstract

Lanthanide and actinide carbonates and hydroxycarbonates are, together with the hydroxides, important compounds in the problematics of high level waste disposal, since such and related compounds may appear as solid phases as a result of the interaction of the waste with natural waters. Experimental results on the stability and relationships between these compounds, particularly the hydroxycarbonates, are relatively fragmentary and arise mainly from solubility measurements, and, to a lesser extent, from enthalpy of formation determinations.We report here the enthalpies of formation of a number of well characterized hydroxycarbonates of trivalent lanthanides, namely those of neodymium, samarium, dysprosium, ytterbium, and of americium, as well as that of neodymium sesquicarbonate, using solution calorimetry in acidic media. The obtained values are discussed in the light of literature data on existing solubility and enthalpy of formation of related compounds.Our results have been obtained with a newly conceived sealed calorimeter, briefly described in the paper, in which the C0 2 produced during the dissolution reaction is maintained in solution, thus eliminating the problem of the kinetically slow evolution of the C0 2 (g) from the medium and the irregularities in the associated thermal effect.

Dissolution of LnYSiAlO glass (Ln=La or Ce) in aqueous media. I. Influence of the pH on hydrolysis mechanisms and kinetics

“…The compounds taken into account for these considerations are: vitreous SiO 2 [16], gibbsite Al 2 O 3 , Y and La hydroxides (Y(OH) 3 and Ln(OH) 3 ) (with Ln ¼ La or Ce) [17]. Calculations were done using solubility constants and enthalpy values at 298 K found in literature [17][18][19][20][21][22][23][24]. VanÕt Hoff equation was used to find the corresponding constants at 363 K. In cases where data were not available for this last step, a likely extrapolation of the curve was done (dotted lines in Fig.…”

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

“…All experimental studies performed without precautions against CO2 contamination could lead to the formation of hydroxycarbonates (see discussion on experimental design in [22,52,53]). Indeed, recent solubility [29], calorimetric [30], and computational [31] studies have demonstrated that REE-hydroxycarbonates are less soluble in aqueous solutions in equilibrium with atmospheric Pco2 compared to Ln-hydroxides. Orhanovic et al [54] performed tyndallometric measurements on La(0H)3(s) precipitation with CO2 precautions (Nj glove box, C02-free KOH) and without those.…”

“…Morss [2] pointed out in his review that "the calorimetric and equilibrium measurements for these hydroxides need to be reconciled". Recent experimental data [6,[26][27][28][29][30] and calculations [7,31,87] have shown that solid hydroxycarbonates (Lni0H)/C03X • q(H20(s)) and carbonates (Ln2(C03)2 • qHjOCs)) of the REE and actinides may control the solubility of these elements in surface and near-surface aqueous solutions in the environments such as proposed geological repositories for radioactive waste. Consequently, adequate knowledge of REE-hydroxide thermodynamics and solubility is needed to calculate REE-hydroxide, hydroxycarbonate and carbonate equilibria.…”