U(VI) retention in compact Callovo-Oxfordian clay stone at temperature (20-80 °C); What is the applicability of adsorption models?

In the context of the radioactive waste management in deep geological formations, U(VI) retention by intact Callovo-Oxfordian claystone (COx) was studied by percolation-type experiments at 20 and 80 degrees C. The experimental results were confronted with modelling prediction based on a published adsorption model developed from dispersed media in the 20-80 degrees C temperature range. For the experiments at 20 degrees C, the adsorption model allowed to explain the results for the intact system; the retention was weak (R-d similar to 10 L.kg(-1)) and the analysis of the COx phases at the end of the experiment confirmed a retention of U by the clay fraction. The adsorption model in temperature also explained the observed trend of increasing retention with increasing temperature. However, it underestimated the temperature effect on the adsorption of U(VI) by the COx clay fraction, and other phases contributed to the retention. Solid-state analysis of the percolation-doped samples indicated a reactivity in the order pyrite>clay>calcite phases. The transposition of the knowledge at 20 degrees C from the dispersed system to the intact medium was therefore not possible at 80 degrees C for the studied U(VI)/COx system.