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Abstract: The purpose of this work is the study of the interaction mechanisms between U(VI) ions and SrTiO3 surfaces as a function of pH and temperature (25, 50, 75 and 90 °C) by coupling thermodynamic and spectroscopic approaches. First, the reactivity towards U(VI) for both surface sites of the strontium titanate (ph name="sbnd" />O and ph name="sbnd" />O) has been investigated as a function of the temperature. The N2-BET specific area was measured: . The surface site density has been determined from potentiometric titrations (6 sites/nm2 for each site ph name="sbnd" />O and ph name="sbnd" />O). The potentiometric titration data have been simulated, for each temperature, using the FITEQL 4.0 software and the constant capacitance model, taking into account both protonation of the ph name="sbnd" />OH surface sites and deprotonation of the ph name="sbnd" />OH ones (one pK model). The intrinsic strontium protonation constant increases with an increasing temperature, while the titanate deprotonation one decreases. Moreover, both enthalpy and entropy changes corresponding to the surface acid–base reactions have been evaluated using the van''t Hoff relation. The uranium(VI) ions are sorbed onto SrTiO3 surfaces in the 0.5–5.0 pH range with an initial cation concentration equal to . The U(VI) surface complexes were identified by using time-resolved laser-induced fluorescence spectroscopy (TRLFS). For all the studied samples, the fluorescence spectra and the corresponding lifetime values do not change with the pH and the temperature. Two U(VI) complexes sorbed onto SrTiO3 were detected and the corresponding lifetimes are and whatever the temperature (25, 50, 75 and 90 °C). The sorption edges were simulated with the FITEQL 4.0 code. The sorption equilibrium constants of the U(VI)/SrTiO3 system between 25 and 90 °C were obtained with the constant capacitance model (CCM), considering two reactive surface sites. According to the spectroscopic characterization, two types of surface complexes, namely [(:glyph name="tbnd" />TiOH)UO2]2+ and [(:glyph name="tbnd" />TiO)UO2]2+, were considered. Finally, enthalpy () and entropy () changes were calculated from the temperature-dependent sorption constants, by the application of the van''t Hoff formalism. The formation of the [(:glyph name="tbnd" />TiOH)UO2]2+ surface complex was found to present an endothermic character associated to an increase in the disorder of the system. On the contrary, the formation of the [(:glyph name="tbnd" />TiO)UO2]2+ surface complex led to an exothermic process with only a slight increase in the disorder of the system. [Copyright &y& Elsevier]

In this paper, geometric bulk parameters, bulk moduli, energy gaps and relative stabilities of the TiO2 anatase and rutile phases were determined from periodic DFT calculations. Then, for the rutile phase, structures, relaxations and surface energies of the (110), (100), (101) and (001) faces were computed. The calculated surface energies are consistent with the natural rutile powder composition, even if a dependence on the number of layers of the slab used to model the surface was identified. Internal constraints, consisting in freezing some internal layers of the slab to atomic bulk positions, were thus added to mimic the bulk hardness in order to stabilise the computed surface energies for thinner systems. In parallel, the influence of pseudopotentials was studied and it appears that four valence electrons for titanium atoms are sufficient. The aim of this study was to optimise accurate rutile TiO2 surface models that will be used in further calculations to investigate water and uranyl ion sorption mechanisms. [ABSTRACT FROM AUTHOR]

To experimentally identify both clay sorption sites and sorption equilibria and to understand the retention mechanisms at a molecular level, we have characterized the structure of hexavalent uranium surface complexes resulting from the interaction between the uranyl ions and the surface retention groups of a montmorillonite clay. We have performed laser-induced fluorescence spectroscopy (LIFS) and X-ray photoelectron spectroscopy (XPS) on uranyl ion loaded montmorillonite. These structural results were then compared to those obtained from the study of uranyl ions sorbed onto an alumina and also from U(VI) sorbed on an amorphous silica. This experimental approach allowed for a clear determination of the reactive surface sites of montmorillonite for U(VI) sorption. The lifetime values and the U4f XPS spectra of uranium(VI) sorbed on montmorillonite have shown that this ion is sorbed on both exchange and edge sites. The comparison of U(VI)/ clay and U(VI)/oxide systems has determined that the interaction between uranyl ions and montmorillonite edge sites occurs via both ≡AIOH and ≡SiOH surface groups and involves three distinct surface complexes. The surface complexation modeling of the U(VI)/montmorilloruite sorption edges was determined using the constant capacitance model and the above experimental constraints. The following equilibria were found to account for the uranyl sorption mechanisms onto montmorillonite for metal concentrations ranged from 10-6 to 10-3 M and two ionic strengths (0.1 and 0.5 M): 2≡XNa + UO22+ ↔ (≡X)2- UO2 + 2Na+, log kexch0 = 3.0; ≡ Al(OH)2+ UO22+ ↔ ≡ Al(OH)2- UO22+ log Kal0 = 14.9; ≡ Si(OH)2 + UO22+ ↔ ≡ SiO2UO2 + 2H+ , log ksit0 = -3.8; and ≡ Si(OH)2 + 3UO22+ + 5H2O ↔ ≡ SiO2(UO2)3(OH)5 + 7H+, log ksi20 = -20.0. [ABSTRACT FROM AUTHOR]

Abstract: The energy transfer at room temperature between Tb3+ and Eu3+ ions sorbed onto SrTiO3 powders is investigated, using Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS). Several published works deal with the energy transfer between two lanthanide ions in co-doped matrices but it is the first time that transfer processes between two lanthanide ions sorbed on a solid surface is reported. The results show that the energy transfer between sorbed Tb3+ and Eu3+ ions on strontium titanate is a non-radiative process and follows a dipole–dipole type interaction. Moreover, the higher the acceptor ions Eu3+ concentration, the more efficient the energy transfer. It is shown that no energy migration between the Tb3+ donor ions occurs. A formalism based on the model of Inokuti–Hirayama is used and allows one to fit the non-exponential Tb3+ fluorescence decay. It is thus possible to evaluate the critical radius (R 0) of the influence sphere of the sorbed Tb3+ ions. According to the previous works, two sorption sites are considered for the sorbed rare-earth. The calculated radii are similar to those obtained for other couples of donor–acceptor lanthanide ions reported in the literature. [Copyright &y& Elsevier]

Abstract: XPS and periodic DFT calculations have been used to investigate water sorption on the TiO2 rutile (110) face. Two sets of XPS spectra were collected on the TiO2 (110) single crystal clean and previously exposed to water: the first set with photoelectrons collected in a direction parallel to the normal to the surface; and the second set with the sample tilted by 70°, respectively. This tilting procedure promotes the signals from surface species and reveals that the first hydration layer is strongly coordinated to the surface and also that, despite the fact that the spectra were recorded under ultra-high vacuum, water molecules subsist in upper hydration layers. In addition, periodic DFT calculations were performed to investigate the water adsorption process to determine if molecular and/or dissociative adsorption takes place. The first step of the theoretical part was the optimisation of a dry surface model and then the investigation of water adsorption. The calculated molecular water adsorption energies are consistent with previously published experimental data and it appears that even though it is slightly less stable, the dissociative water sorption can also take place. This assumption was considered, in a second step, on a larger surface model where molecular and dissociated water molecules were adsorbed together with different ratio. It was found that, due to hydrogen bonding stabilisation, molecular and dissociated water molecules can coexist on the surface if the ratio of dissociated water molecules is less than ≈33%. These results are consistent with previous experimental works giving a 10–25% range. [Copyright &y& Elsevier]

The sorption of uranyl cations and water molecules on the basal (001) face of gibbsite was studied by combining vibrational and fluorescence spectroscopies together with density functional theory (DFT) computations. Both the calculated and experimental values of O-H bond lengths for the gibbsite bulk are in good agreement. In the second part, water sorption with this surface was studied to take into account the influence of hydration with respect to the uranyl adsorption. The computed water configurations agreed with previously published molecular dynamics studies. The uranyl adsorption in acidic media was followed by time-resolved laser-induced fluorescence spectroscopy and Raman spectrometry measurements. The existence of only one kind of adsorption site for the uranyl cation was then indicated in good agreement with the DFT calculations. The computation of the uranyl adsorption has been performed by means of a bidentate interaction with two surface oxygen atoms. The optimized structures displayed strong hydrogen bonds between the surface and the -yl oxygen of uranyl. The uranium-surface bond strength depends on the protonation state of the surface oxygen atoms. The calculated U-O(surface) bond lengths range between 2.1-2.2 and 2.6-2.7 A for the nonprotonated and protonated surface O atoms, respectively.

Canister integrity and radionuclide retention is of first importance for assessing the long-term safety of nuclear waste stored in engineered geologic depositories. Uranyl ion sorption on the TiO(2) rutile (110) face is investigated using periodic density functional theory (DFT) calculations. From experimental observations, only two uranyl surface complexes are observed and characterized. When the pH increases (from 1.5 to 4.5), the relative ratios of these two surface complexes are modified. From a crystallographic point of view, three sorption sites can be considered and have been studied with different protonation states of the surface to account for very acidic and low acidic conditions. The two surface complexes experimentally observed were calculated as the most stable ones, while the evolution of their sorption energies agrees with experimental data.

This paper is devoted to the study of the mechanisms of interaction between uranyl ion and rutile TiO2. Among the radionuclides of interest, U(VI) can be considered as a model of the radionuclides oxo-cations. The substrate under study here is the rutile titanium dioxide (TiO2) which is an interesting candidate as a methodological solid since it can be easily found as powder and as manufactured single crystals. This material presents also a wide domain of stability as a function of pH. Then, it allows the study of the retention processes on well-defined crystallographic planes, which can lead to a better understanding of the surface reaction mechanisms. Moreover, it is well-established that the (110) crystallographic orientation is dominating the surface chemistry of the rutile powder. Therefore, the spectroscopic results obtained for the U(VI)/rutile (110) system and other relevant crystallographic orientations were used to have some insight on the nature of the uranium surface complexes formed on rutile powder. This goal was achieved by using time-resolved laser-induced fluorescence spectroscopy (TRLFS) which allows the investigation, at a molecular scale, of the nature of the reactive surface sites as well as the surface species. For rutile surfaces, oxygen atoms can be 3-fold, 2-fold (bridging oxygens), or single-fold (top oxygens) coordinated to titanium atoms. However, among these three types of surface oxygen atoms, the 3-fold coordinated ones are not reactive toward water molecules or aqueous metallic cations. This study led to conclude on the presence of two uranium(VI) surface complexes: the first one corresponds to the sorption of aquo UO22+ ion sorbed on two bridging oxygen atoms, while the second one, which is favored at higher surface coverages, corresponds to the retention of UO22+ by one bridging and one top oxygen atom. Thus, the approach presented in this paper allows the establishment of experimental constraints that have to be taken into account in the modeling of the sorption mechanisms.

Periodic density functional theory calculations have been performed in order to study the uranyl ion sorption on the TiO2 rutile (110) face. From experimental measurements, two uranyl surface complexes have been observed and the two corresponding sorption sites have been identified. However, from a crystallographic point of view, three different sorption sites can be considered on this face. The corresponding three surface bidentate complexes were modeled and optimized, and their relative energies were calculated. Only 5 kJ/mol separates the two most stable structures, which correspond to the experimental ones. The third surface complex is nearly 10 kJ/mol less stable, in agreement with the fact that it was not observed experimentally.

The rotational anisotropy of second-harmonic generation at the surface of a (001) single-crystal rutile is obtained in the presence of uranyl cations sorbed at the surface from acidic solutions at various concentrations. Surface second-harmonic generation appears to be sensitive to the presence of uranyl cations on the rutile samples. Evolution of the anisotropy pattern with initial uranyl concentration is analyzed through a phenomenological model. The elements obtained for the nonlinear susceptibility tensor Chi(2) for each sample significantly constrain the geometry of the possible sorption complexes between uranyl cations and rutile and lead to the proposition of two sorption sites involving different oxygen atoms of the rutile surface.