Uranyl ion sorption mechanisms on titanium oxide: a multi-scale approach

Full text of publication follows: Radionuclides retention mechanisms onto mineral phases is of primary importance for nuclear waste management. The aim of the presented study is to demonstrate that it is possible to predict the retention properties of a methodological powdery substrate from the study of its natural crystallographic orientations. Among the radionuclides of interest, U(VI) can be seen as a model of the radionuclides oxo-cations. The substrate under study is the titanium oxide (TiO2). In fact, rutile can be found as powder and also as manufactured single crystal which allows to study the retention processes on perfectly known crystallographic planes. Since the repartition of the different crystallographic orientations are known for the powder, the results obtained for the single crystals can directly be used to account for the powder retention properties. By using combined spectroscopic techniques such as TRLFS, XPS, DRIFT and SHG, it is possible to determine the nature of the reactive surface sites and also the surface species. XPS and TRLFS measurements allowed to determine that two same uranyl surface species were formed on titania (110) and (001). Only, the relative intensities of these species vary with the surface coverage. Atomic Force Microscopy was carried out to verify that no surface precipitation occurs for the higher surface coverages. Moreover, these analysis have also evidenced that the U(VI) sorption is homogeneous. These observations were corroborated by SHG experiments (mainly for (001)) which have also shown that the sorption occurs, in a first step, onto preferential surface symmetry axis. For rutile powder, the preferential crystallographic orientations are (110), (100) and (101) in the ratio 60/20/20. TRLFS and XPS experiments have shown that two uranyl surface species are formed whatever the pH value ranged from 1 to 5. The spectroscopic characteristics of these species are the same as the ones observed on (110) and (001) planes. DRIFT experiments have been shown that the reactive oxygens remained protonated after uranyl sorption. For powders, potentiometric titration experiments were performed and were successfully modelled (Constant Capacitance Model) using CD-Music and the natural repartition of the crystallographic planes of titania. Thus, only the inner-capacitance value remained as an adjustable parameter. The sorption edges defined for powders were then fitted on the basis of the constraints brought by the spectroscopic investigation which allowed to perform the fit with a minimal number of adjustable parameters and then allowed a more accurate determination of the sorption constants values. (authors)