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 SrTiO(3) surfaces as a function of pH and temperature (25, 50, 75 and 90 degrees C) by coupling thermodynamic and spectroscopic approaches. First, the reactivity towards U(VI) for both surface sites of the strontium titanate ([triple bond]Ti-O and [triple bond]Sr-O) has been investigated as a function of the temperature. The N(2)-BET specific area was measured: 2.4+/-0.2 m(2)g(-1). The surface site density has been determined from potentiometric titrations (6 sites/nm(2) for each site [triple bond]Ti-O and [triple bond]Sr-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 [triple bond]Sr-OH surface sites and deprotonation of the [triple bond]Ti-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 SrTiO(3) surfaces in the 0.5-5.0 pH range with an initial cation concentration equal to 10(-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 SrTiO(3) were detected and the corresponding lifetimes are 60+/-5 and 12+/-2 micros whatever the temperature (25, 50, 75 and 90 degrees C). The sorption edges were simulated with the FITEQL 4.0 code. The sorption equilibrium constants of the U(VI)/SrTiO(3) system between 25 and 90 degrees 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 ([triple bond]SrOH)([triple bond]TiOH)UO(2) and ([triple bond]TiOH)([triple bond]TiO)UO(2), were considered. Finally, enthalpy (Delta(r)H(o)) and entropy (Delta(r)S(o)) changes were calculated from the temperature-dependent sorption constants, by the application of the van't Hoff formalism. The formation of the ([triple bond]SrOH)([triple bond]TiOH)UO(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 ([triple bond]TiOH)([triple bond]TiO)UO(2) surface complex led to an exothermic process with only a slight increase in the disorder of the system.