Contribution to lanthanide and actinide speciation: Thermodynamic studies of hydroxycarbonate species

The accurate prediction of the conditions for the formation of solid compounds, that may appear as the result of the long-term interaction of groundwater with the nuclear waste matrix, requires a better understanding of the thermodynamic parameters associated with the formation of such compounds. Studies reported by this laboratory on the enthalpy of formation of lanthanide and americium hydroxycarbonates M(OH)CO3.0.5H(2)O(cr) in their orthorhombic form [1] and their relation to the corresponding hydroxides [2] are contributions to this aim. The authors report here some thermodynamic parameters of the hexagonal form M(OH)CO3(cr) (M = Nd, Sm). An extrapolation to the hexagonal variety of AmOHCO3(cr) which was isolated for the first time more than a decade ago from J-13 groundwater of Yucca Mountain [3] is now possible. In this context, the authors have developed a new sealed micro calorimeter, the original version of which was previously designed in their laboratory. The new calorimetric chamber was made of 18-carat gold alloy in order to improve the overall performances. Thermodynamically meaningful results require well-defined solids. Each compound was prepared in triplicate and thoroughly characterised by X-ray diffraction, IR spectra, TGA, and gravimetry of the sesquioxide. The standard enthalpies of formation of the compounds were obtained from the measured heat effect associated with their dissolution in 6.00 mol.dm(-3) HCl, using relevant auxiliary data. These results are discussed in the light of literature data on the enthalpy of formation and solubility of the corresponding M(OH)CO3.0.5H(2)O(cr) (M = Nd, Sm, Am) hydroxycarbonates in their orthorhombic form and of other related compounds.