Generation of uranyl/carbonate/hydroxide "coatings'' at the vaterite surface

The chemical behavior of the oxo-metallic cation UO22+ on pure vaterite sur faces in ultra-pure water has been examined at room temperature using sever al techniques: infrared (IR); confocal laser Raman microscopy (CLRM); scann ing electron microscopy (SEM); X-ray photoelectron spectroscopy (XPS) and a ngle-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 d ramatically as the concentration of uranyl ions added to the medium increas es. 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 varyi ng from 30 to 50 nm) on UO2/vaterite surfaces at the beginning of the react ion, and the progressive appearance (after a few days) of calcite germs tha t grow from these mesopores. The combined use of surface-analysis technique s (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)( x)Ca1+(z/2)-xCO3. 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 b een of great interest for the determination of the averaged chemical compos ition of these "coatings''. From XPS data and chemical analysis on the liqu id 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 complexe s and their direct implication on a kinetically-controlled solubility proce ss. This free energy is found to be lower than that of vaterite and even ca lcite, which is in good agreement with the experimental observations on the relative stability of these surface complexes.