Solubility properties of synthetic and natural meta-torbernite
Résumé
Meta-torbernite, Cu(UO2)2(PO4)2⋅8H2O, is one of the most common secondary minerals resulting from the alteration of pitchblende. The determination of the thermodynamic data associated to this phase appears to be a crucial step toward the understanding the origin of uranium deposits or to forecast the fate and transport of uranium in natural media. A parallel approach based on the study of both synthetic and natural samples of meta-torbernite (H3O)0.4Cu0.8(UO2)2(PO4)2⋅7.6H2O was set up to evaluate its solubility constant. The two solids were first thoroughly characterized and compared by means of XRD, SEM, X-EDS analyses, Raman spectroscopy and BET measurements. The solubility constant was then determined in both under- and supersaturated conditions: the obtained value appeared close to log(298 K) = −52.9 ± 0.1 whatever the type of experiment and the sample considered. The joint determination of Gibbs free energy (ΔRG°(298 K) = 300 ± 2 kJ mol−1) then allowed the calculation of ΔRH°(298 K) = 40 ± 3 kJ mol−1 and ΔRS°(298 K) = −879 ± 7 J mol−1 K−1. From these values, the thermodynamic data associated with the formation of meta-torbernite (H3O)0.4Cu0.8(UO2)2(PO4)2⋅7.6H2O were also evaluated and found to be consistent with those previously obtained by calorimetry, showing the reliability of the method developed in this work. Finally, the obtained data were implemented in a calculation code to determine the conditions of meta-torbernite formation in environmental conditions typical of a former mining site.