Y(III) Binding to the Smectite Hectorite

The smectite hectorite was synthesized in the presence of trivalent yttrium. The solid phases associated with the multi-step synthesis procedure were characterized by X-ray diffraction. The crystal-chemical environment surrounding Y(III) during distinct stages was probed by EXAFS spectroscopy. The trivalent cation is found 6-fold coordinated by oxygen atoms and located successively in a brucite-like and in a clay-like environment. The compelling formation of inner-sphere surface complexes or of an Y(III)-containing amorphous silica phase upon clay crystallization is ruled out. Hectorite was crystallized in the presence of Y(III). Data show that Y is 6-fold coordinated by oxygen and is successively located in a brucite-like and in a clay-like environment. The data suggest that trivalent cations may be incorporated in the octahedral sheet of clay minerals by co-precipitation in aqueous systems, but result in large strain. It can be anticipated that such a binding mechanism would be even more difficult for larger trivalent actinides. This study implements the molecular-level understanding of trivalent cation binding to clay minerals. (authors)