EXAFS study of the interaction of lanthanide cations with layered clays upon hydrothermal treatments

Abstract

The interaction of ions intercalated in layered clays with the host lattice is of interest concerning the preparation and applications of new clay-based materials. Intercalation reactions are usually reversible, and the structural integrity of the host lattice is formally conserved in the course of the forward and reverse processes. Many studies of this type involving rare earth cations have been carried out because these ions simulate radionuclides in the process of retention of nuclear wastes by clays. In relation with this process, thermal and hydrothermal treatments are known to prevent the reversibility of the exchange reaction with diverse cations, resulting in their “irreversible” fixation. This is of importance for the protection of the biosphere. With the aid of EXAFS, we have studied the reaction of lutetium ions intercalated within montmorillonite layers. We have followed the structural metamorphosis of the initial Lu(III) aquocomplex to the oxide-type environment, or disilicate phase. This process is of fundamental importance in evaluating the safety of the future nuclear waste repositories. Up to now, the hydrothermal treatments have been applied in our laboratory at temperatures higher than the real expected ones. The aim of this paper is to report the first results from a sample submitted to hydrothermal treatment at 200°C and for a long period of time (two months). These results are compatible with the coexistence of Lu(H2O)83+ and Lu(OH)3, what implies that significant structural changes have been induced by the hydrothermal treatment at 200°C. Since these conditions are very similar to those expected in the projected nuclear waste repositories of the bentonite backfill, the structural effects, when extrapolated to geologic timescales, support the safe fixation of radionuclides.