Temperature effect of U(VI) retention on the Callovo-Oxfordian clay rock

In the context of management of the radioactive waste in deep geological formations, the effect of temperature (20–80 °C) on U(VI) adsorption by Callovo-Oxfordian claystone (COx) was studied. A step-by-step approach was followed, starting with the single mineral, illite, followed by an increase in the complexity of the system, through the analysis of the clay fraction and the natural samples of the Callovo-Oxfordian formation. Depending on the study conditions, and the speciation of U(VI) in solution (hydrolysed species, carbonate species and presence of ternary U(VI)-Ca(Mg)‑carbonate complexes), the temperature effect was either negligible, or positive (where the increase in temperature favours retention). The most important positive effect was observed for the U(VI)/COx system in the presence of ternary complexes. The data were modelled considering an existing sorption model at 20 °C and the thermodynamic data available to describe the evolution of the speciation of U(VI) in solution in function of temperature. The enthalpy values associated with the surface complexes were fitted from the experimental data following a stepwise approach based on the van't Hoff equation.