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Zeolite minerals are crystalline, hydrated aluminosilicates of alkali and alkaline earth cations characterized by an ability to hydrate/dehydrate reversibly and to exchange some of their constituent cations with aqueous solutions, both without a major change in structure. Because of their ion-exchange, adsorption, and molecular sieve properties, as well as their geographically widespread abundance, zeolite minerals have generated worldwide interest for use in a broad range of applications. Examples of these applications are discussed in other chapters of this book. Of particular interest in this chapter are the cation-exchange properties of zeolite minerals. Due to the favorable ion-exchange selectivity of natural zeolites for certain cations, such as Cs+, Sr2+, and NH4+, these minerals have been studied for potential use in the treatment of nuclear wastewaters (Howden and Pilot 1984; Baxter and Berghauser 1986; Robinson et al. 1995; Pansini 1996), municipal and industrial wastewaters (Kallo 1995; Pansini 1996), and acid mine drainage waters (Bremner and Schultze 1995; Zamzow and Schultze 1995). Natural zeolites have also been studied for potential use in the remediation of sites contaminated with fission products such as 90Sr and 135,137Cs (Leppert 1988; Valcke et al. 1997a; Valcke et al. 1997b) and in the remediation of soils contaminated with heavy metals (Ming and Allen, this volume). Additional interest resulted from the potential siting of a high-level nuclear waste repository at Yucca Mountain, Nevada, which is underlain by diagenetically altered, zeolite-rich (clinoptilolite, heulandite, and mordenite) rhyolitic tuffs (Broxton et al. 1986; Broxton et al. 1987) that could serve as barriers to radionuclide migration to the accessible environment.
Zeolites consist of three-dimensional frameworks of (Si,Al)O4 tetrahedra where all oxygen ions of each tetrahedron are shared with adjacent tetrahedra. The presence of Al3+ …