Temperature effects on the interaction mechanisms between U(VI) and Eu(III) and ZrP2O7: experiment and modelling

In the field of nuclear waste management, prediction of the radionuclides migration through the geosphere has to take into account the effects of temperature. The chosen substrate, zirconium diphosphate, was previously described concerning its surface acidity properties for several temperatures (25 C, 50 C, 75 C, 90 C). Edges for uranyl sorption were obtained in NaClO4 medium, and the europium(III) sorption edges were realized in KNO3 medium to evaluate the influence of the complexing nitrate ions. Influence of the temperature, the nature of the cation and the nature of the substrate were pointed out by comparing the edges for the different systems. A structural investigation (TRLFS) was carried out to define experimentally, at the microscopic level, the sorption equilibria. Only UO22+ and Eu3+ were sorbed on both ZrP2O7 surface sites (triple bond Zr-O and triple bond P-O), the nitrate medium having no influence on the sorbed species. Furthermore, temperature did not change the sorption mechanisms. The surface complexation modelling of both investigated systems was realized using (i) the constant capacitance model (ii) the results of the substrate characterizations (iii) uranyl sorbed complex determinations. The formation of bidentate surface complexes onto the triple bond Zr-O site is not accompanied by a proton release, whereas the formation of bidentate surface complexes onto the triple bond P-O site is followed by a release of 2 protons, for both cations and at all temperatures studied. The determined sorption constants increase with temperature, as was expected from the analysis of the sorption edge positions. (orig.)