Surface complexation modeling of uranium(VI) sorbed onto zirconium oxophosphate versus temperature: Thermodynamic and structural approaches

This work presents an investigation of the interaction mechanisms between uranyl ions and a solid phosphate, the zirconium oxophosphate: Zr2O(PO4)2. Both thermodynamic and structural points of view are developed. Indeed, prior to any simulation of the retention data, it is necessary to precisely characterize the system under study in order to gain information at a molecular scale. First, the intrinsic surface properties of this synthetic compound have been investigated for different temperatures ranging from 25 to 90 ◦C. Mass and potentiometric titrations show that the surface site density remains constant between 25 and 90 ◦C, while the experimental point of zero charge slightly decreases from 4.8 to 4.5 with an increasing temperature. The potentiometric titration data are simulated, for each temperature, using the constant capacitance model and taking into account two surface sites (≡Zr–O and ≡P–O) with a total surface site density equal to 7.0 sites nm−2. For both reactive sites, the intrinsic protonation constants do not change with the temperature, while the deprotonation ones increase. These results led to the determination of the associated enthalpy and entropy changes according to the van't Hoff relation. Second, the speciation of U(VI) at the solid/solution interface has been studied using two complementary spectroscopic techniques probing the sorbed uranyl ions: time-resolved laser-induced fluorescence spectroscopy (TRLFS) and X-ray absorption spectroscopy (EXAFS). The substrate presents two different reactive surface sites against uranium retention, which are constituted by the oxygen atoms of the surface PO4 groups and the oxygen atoms linked to the zirconium atoms. Two inner-sphere complexes are thus present on the substrate, their relative proportion depending on the pH value of the suspension. The effects of the temperature (25–90 ◦C) on the surrounding uranium were checked using the TRLFS technique. The uranyl sorption constants onto the Zr2O(PO4)2 substrate were determined taking into account the structural investigation. The surface complexation modeling was performed using the constant capacitance model included in the FITEQLv4.0 code. The four adsorption edges obtained at 25, 50, 75, and 90 ◦C were simulated. The modeling of these experimental data was realized considering two surface complexes ((≡ZrOH)2UO2+ 2 , (≡PO)2UO2) according to the structural investigation. The constant value associated with the ≡ZrO site does not change with the temperature, while the one corresponding to the ≡PO site increases. Finally, the enthalpy and entropy changes associated with the uranyl sorption constants have been determined using the van't Hoff relation