Sorption of radium onto early cretaceous clays (Gault and Plicatules Fm). Implications for a repository of low-level, long-lived radioactive waste

Abstract

The repository of low-level and long-lived radioactive waste from REE purification and treatment is presently under investigation by the French National Radioactive Waste Management Agency (Andra). This waste consists of Ra-bearing solids, mainly Ra-barite (Ba,Ra)(SO4), along with other salts such as NH4NO3 and monazite. The current repository concept under study is based on a shallow (~30 m deep) geological disposal in a clay formation. This work deals with the experimental study of the sorption of Ra onto two clay rocks from Lower Cretaceous geological formations in the Department of Aube, in France, the Gault (tegulines) Formation and Plicatules Formation. Clay samples were extracted from different boreholes from the both geological clay formations, and consisted mainly of illite, illite-smectite mixed layers (35-65% and 25-70%). Sorption experiments were carried out on raw materials as well as on samples exchanged with different cations (homoionic forms of Na, NH4, Ba and Ca, respectively). The results indicate that Ra sorption in raw materials is linear within the range of concentration investigated (10-8 - 10-11 M). In most of the samples, Kd values from 170 to 300 g·mL-1 were measured for Ra and from 30 to 70 g·mL-1 for Ba. The sorption experiments (from 1 h to 92 days) of Ra showed small kinetic effects as Kd values tend to increase with time from approximately logKd 2.15 to 2.65. In all exchanged samples, a dependence of sorption on the ionic strength was observed -the higher the ionic strength, the lower Kd values-; except for Na, this dependence is consistent with an ionic exchange process and selectivity coefficients (Ra-Ca; Ra-Ba and Ba-Ca) have been determined. A non-negligible effect of pH on sorption was also observed that could be explained considering surface complexation on the amphoteric groups present at the solid surface. In these materials, the selectivity of Ra was higher than that of Ba (half an order of magnitude). The experimental results indicate that sorption of Ra in the studied clay formations is high enough to limit the radium migration in the near field of a low-level, long lived disposal site.