Generation of uranyl/carbonate/hydroxide ‘‘coatings’’ at the vaterite surface

Abstract

The chemical behavior of the oxo-metallic cation UO₂²⁺ on pure vaterite surfaces in ultra-pure water has been examined at room temperature using several techniques: infrared (IR); confocal laser Raman microscopy (CLRM); scanning electron microscopy (SEM); X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS [(AR)XPS]; and inductively coupled plasma atomic emission spectroscopy (ICP-AES ). IR and XRD investigations have revealed that the rate of the crystalline transformation of vaterite into calcite decreases dramatically as the concentration of uranyl ions added to the medium increases. This phenomenon is due to the generation of oxo-metallic ‘‘coatings’’ at the vaterite surface. Furthermore, SEM studies vs. time of these chemical combinations have shown clearly the presence of mesopores (with sizes varying from 30 to 50 nm) on UO₂/vaterite surfaces at the beginning of the reaction, and the progressive appearance (after a few days) of calcite germs that grow from these mesopores. The combined use of surface-analysis techniques (CLRM and XPS) ascertains the existence of compounds such as CaCO₃–UO₂CO₃–UO₂(OH)₂–H₂O or hydrated UO₂(II) complexes such as (H₂O)_y(OH)_z(UO₂)_xCa_{1 + (z/2)−x}CO₃. CLRM has been used particularly for the characterization of the surface minerals involved in recovered UO₂/vaterite solids, and for the identification of newly-generated specimens. In addition, (AR)XPS has been of great interest for the determination of the averaged chemical composition of these ‘‘ coatings’’. From XPS data and chemical analysis on the liquid and solid phases recovered, calculations realized on the uranyl/vaterite/water system have allowed us to obtain the free energy of formation of the generated coatings and to confirm the importance of these surface complexes and their direct implication on a kinetically-controlled solubility process. This free energy is found to be lower than that of vaterite and even calcite, which is in good agreement with the experimental observations on the relative stability of these surface complexes.