Study of the reversibility on the local La3+ environment after thermal and drying treatments in lanthanum-exchanged smectites

Changes produced by thermal treatments (300 C, 500 C, and 700 C) and after an ''in situ'' drying treatment on the hydration sphere of the lanthanum ions located in the interlamellar space of montmorillonites have been investigated. X-ray absorption spectroscopy (XAS) spectra for these materials have been recorded with this purpose at room and liquid nitrogen temperature by using a special cell where the temperature and the atmosphere could be controlled. The dry atmosphere employed during the drying treatment produced either drastic or no significant effects on the La LIII-edge EXAFS spectra, depending upon the previous thermal treatment given to the sample. These results have allowed us to postulate a dehydration reaction mechanism for these samples and to show how XAS measurement is a very useful tool to be employed in the study of the local environments adopted by the exchangeable cations in clays. (orig.)