J. PHYS. ZV FRANCE 7 (1997) Colloque C2, Supplkment au Journal de Physique III d'avril 1997 Nanoscale Encapsulation: The Structure of Cations in Hydrophobic Microporous Aluminosilicates S.R. Wassew*, S.E. Yuchs, D. Giaquinta, L. Soderholm and K. Song Chemistry Division, Argonne National Laboratory, 9700 S. Cuss Ave., Argonne, IL 60439, U.S.A. Abstract: Hydrophobic microporous aluminosilicates created by the modification of zeolites and clays are currently being investigated as storage media for hazardous cations. Addition of an organic monolayer to the surface of an aluminosilicate after introduction of an ion into the zeolite or clay reduces the interaction of water with the material. The resuItant systems are approximately 20 times more resistant to leaching of the stored ion than the unmodified ion-exchanged materials. XAS spectra demonstrate that byproducts from the organic modifier can complex with the encapsulated cation. This complexation can result in a decreased affinity of the cation for the aluminosilicate matrix. Changing the organic modifier eliminates this problem. XAS spectra also indicate that the reactivity and subsequent speciation of the encapsulated ion alters upon application of the hydrophobic layer. 1. INTRODUCTION Microporous aluminosilicates, including clay minerals and zeolites, are ion-exchange materials 11, 21. In their most common forms, they have the ability to incorporate cationic species within their matrices. Because of this property, microporous aluminosilicates have been proposed as storage media for hazardous waste. In this paper we use XAS to examine the structure of cations held within smectite clay minerals and to determine how modification of the surface of the clay affects the coordination of the stored cation. Smectite Clay Encapsulation do01 1 Metal ion vm \surface ,ification - Figure 1: Structure of smectite clay minerals and schematic representation of ion exchange and organic encapsulation of ions within a smectite. The structure of smectite clays is illustrated in Figure 1. They consist of aluminosilicate sheets which contain a central section of octahedrally coordinated ~ 1 3 + that is sandwiched between two layers of tetrahedrally coordinated Si4+ [3]. Random substitutions, generally of M ~ ~ + for AI3+ and A13f for si4+, result in a net negative charge in the lattice. This charge is balanced by cations in the interlayer between the sheets. The interlayer also contains water, some of which is complexed to the cations. The interlayer cations can be replaced by ion-exchange techniques with other positively charged species. For this study Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1997240