Reversibility of La and Lu sorption onto smectites: implications for the design of engineered barriers in deep geological repositories.

The sorption reversibility of La and Lu (considered as actinide analogues) onto a set of smectites (bentonite FEBEX; hectorite, HEC; MX80; saponite, SAP; Otay montmorillonite, SCa-3; and Texas montmorillonite, STx-1) was studied to estimate actinide retention by smectites that are candidates for use as engineered barriers in deep geological repositories. The sorption distribution coefficients (K(d)) and the reversibility parameters (desorption distribution coefficients (K(d,des)), adjusted distribution coefficients (K(d,adj)), and desorption rates (R(des))) were determined from batch tests in two ionic media: deionized water and Ca 0.02 mol L(-1). The latter simulates possible conditions due to the presence of concrete leachates. The results varied greatly depending on the ionic medium, the lanthanide concentration and the clay structure. The high values of K(d,des) obtained (up to 1.1 x 10(5) and 9.2 x 10(4) L kg(-1) for La and Lu in water, and 2.8 x 10(4) and 4.1 x 10(4)L kg(-1) for La and Lu in the Ca medium) indicate the suitability of the tested smectites for lanthanide (and therefore, actinide) retention. Based on all the data, SCa-3, HEC and FEBEX clays are considered the best choices for water environments, whereas in Ca environments the suitable clays depended on the lanthanide considered.