Interaction between U(VI) and SrTiO3 surfaces versus temperature

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 (TiO and SrO) has been investigated as a function of the temperature. The N2-BET specific area was measured: 2.4±0.2 m2g−1. The surface site density has been determined from potentiometric titrations (6 sites/nm2 for each site TiO and SrO). 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 SrOH surface sites and deprotonation of the TiOH 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 10−4 M10−4 M. 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 60±560±5 and 12±2 μs12±2 μs 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 [(SrOH)(TiOH)UO2]2+ and [(TiOH)(TiO)UO2]2+, were considered. Finally, enthalpy (ΔrH°ΔrH°) and entropy (ΔrS°ΔrS°) changes were calculated from the temperature-dependent sorption constants, by the application of the van't Hoff formalism. The formation of the [(SrOH)(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 [(TiOH)(TiO)UO2]2+ surface complex led to an exothermic process with only a slight increase in the disorder of the system.

Graphical abstractEffect of the temperature on the SrTiO3 retention properties towards U(VI). Surface species remained the same but their relative proportions are affected by the temperature.Figure optionsDownload full-size imageDownload as PowerPoint slide