Investigation of natural lanthane-containing zeolites by the method of raster electronic microscopy

Abstract

      This paper presents the results of the study of lanthanum-containing forms of natural zeolites (stilbit, analcime, shabazite, mesolite) using scanning electron microscopy and energy dispersive X-ray spectroscopy. In order to study the sorption properties of monominerals of zeolites, the distribution of lanthanum ions in the structure of natural zeolites and on their surface was studied. To obtain a cut image of the samples, thin sections were prepared by pouring sorbent grains into epoxy resin, followed by grinding and mechanical polishing with a diamond paste. In electron microscopic photographs of a slice of zeolite grains, an insignificant content of lanthanum is established at local sites. SEM studies of the surface of the grains revealed the concentration of lanthanum ions in the form of clusters, aggregates of rather large sizes, unevenly distributed to a greater extent on the gaps and roughnesses. It is assumed that the deposition of lanthanum ions on the surface of zeolites is associated with the manifestation of the ion-sieve effect due to the obstruction into the pores of the zeolites of large lanthanum complex compounds. The lanthanum aquacomplex binds to the aluminosilicate surface of the zeolite due to oxygen atoms to form hydrogen bonds. A similar formation on the surface of zeolites of the crystalline phase of lead, copper, silver is described in the work of D. Breck. Its appearance is explained by the destabilization of metal ions in the zeolitic structure, as a result of which metal cations diffuse to the surface of the zeolite and agglomerate into small crystallites. The formation of various aggregations on the surface of zeolite grains can also be explained by the Fayans – Peskov – Paneth rule. According to this rule, those ions that are capable of completing the crystal lattice, are isomorphic or form poorly soluble compounds with ions entering the crystal lattice are predominantly adsorbed on the surface of a solid.
       Thus, electron microscopic examination and energy dispersive analysis of the elemental composition of natural zeolites made it possible to establish a predominantly surface, uneven distribution of lanthanum ions. On the surface of the monomineral zeolite grains, lanthanum ions are found in the composition of the crystalline phase localized at various structural defects. The qualitative composition of the aggregations re vealed only lanthanum atoms. It is assumed that lanthanum ions concentrate on the surface of zeolites in the form of aqua and hydroxo complexes.