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Effet de la température sur la rétention de U(VI) par SrTiO$_3$

Abstract : The purpose of this research was the study of the interaction mechanisms between U(VI) ions and SrTiO$_3$ surfaces versus pH and temperature: 25, 50, 75 and 90°C. Firstly, a physicochemical characterization was realized (DRX, MEB, FTIR) and the surface site density was determined. The potentiometric titration data were simulated, for each temperature, using the constant capacitance model and taking into account bath protonation of the $\equiv$Sr-OH surface sites and deprotonation of the $\equiv$Ti-OH alles (one pK$_a$ model). Both enthalpy and entropy changes, corresponding to the surface acid-base reactions, were evaluated using the van't Hoff relation. U(VI) was sorbed onto SrTiO$_3$ powder in the pH range 0.5-5.0 with an U(VI) initial concentration 1.10$^{-4}$ M. By TRLIFS two U(VI) complexes were detected associated with two lifetime values (60 $\pm$ 5 and 12 $\pm$ 2 $\mu$s at 25°C). The sorption edges were simulated using FITEQL 4.0 software. The surface complexation constants constants of the system SrTiO$_3$/U(VI) between 25 and 90°C temperature range were thus obtained with the constant capacitance model considering two reactive surface sites. It reveals that two types of surface complex, namely [($\equiv SrOH)(\equiv TiOH)UO_2]^{2+}$ and [($\equiv TiOH)(\equiv TiO)UO^+_2]^{2+}$, are needed to properly describe the experimental observations. By application of the van't Hoff equation, $\Delta_RS°$ and $\Delta_RH°$ were obtained, which indicated an endothermic sorption process. Finally, an energy transfer study was realised by TRLIFS. The energy transfer between Tb$^{3+}$ and EU$^{3+}$ ions sorbed onto SrTiO$_3$ powders were investigated. The results showed that the energy transfer between Tb$^{3+}$ and EU$^{3+}$ is a non-radiative process and follows a dipole-dipole type interaction. A formalism based on the Dexter and the Inokuti-Hirayama theories was used to calculate the distances (2,7-3,4Å) between Tb$^{3+}$ and EU$^{3+}$ onto SrTiO$_3$ surface.
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G. Garcia-Rosales. Effet de la température sur la rétention de U(VI) par SrTiO$_3$. Radiochimie. Université Paris Sud - Paris XI, 2007. Français. ⟨tel-00200303⟩

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