STUDY OF THE REVERSIBILITY ON THE LOCAL LA3-EXCHANGED SMECTITES( ENVIRONMENT AFTER THERMAL AND DRYING TREATMENTS IN LANTHANUM)

Changes produced by thermal treatments (300 degrees C, 500 degrees C, and 700 degrees C) and after an ''in situ'' drying treatment on the hy dration sphere of the lanthanum ions located in the interlamellar spac e of montmorillonites have been investigated. X-ray absorption spectro scopy (XAS) spectra for these materials have been recorded with this p urpose 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 drast ic or no significant effects on the La L-III-edge EXAFS spectra, depen ding upon the previous thermal treatment given to the sample. These re sults have allowed us to postulate a dehydration reaction mechanism fo r 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 e xchangeable cations in clays. (C) 1997 Elsevier Science B.V.