A zeolite is a crystalline, hydrated aluminosilicate of alkali and alkaline earth cations having an infinite, open, three-dimensional structure. It is further able to lose and gain water reversibly and to exchange extraframework cations, both without change of crystal structure. The large structural cavities and the entry channels leading into them contain water molecules, which form hydration spheres around exchangeable cations. On removal of water by heating at 350–400°C, small molecules can pass through entry channels, but larger molecules are excluded—the so called “molecular sieve” property of crystalline zeolites. The uniform size and shape of the rings of oxygen in zeolites contrasts with the relatively wide range of pore sizes in silica gel, activated alumina, and activated carbon, and the Langmuir shape of their adsorption isotherms allows zeolites to remove the last trace of a particular gas from a system (e.g., H2O from refrigerator Freon lines). Furthermore, zeolites adsorb polar molecules with high selectivity. Thus, polar CO2 is adsorbed preferentially by certain zeolites, allowing impure methane or natural gas streams to be upgraded. The quadrupole moment of N2 contributes to its selective adsorption by zeolites from air, thereby producing O2-enriched products. The adsorption selectivity for H2O, however, is greater than for any other molecule, leading to uses in drying and solar heating and cooling.
The weakly bonded extraframework cations can be removed or exchanged readily by washing with a strong solution of another cation. The CEC of a zeolite is basically a function of the amount of Al that substitutes for Si in the framework tetrahedra; the greater the Al content, the more extraframework cations needed to balance the charge. Natural zeolites have CECs from 2 to 4 milliequivalents/g (meq/g), about twice the CEC of bentonite clay. Unlike most noncrystalline ion exchangers, e.g., organic resins and inorganic aluminosilicate gels (mislabeled in the trade as “zeolites”), the framework of a crystalline zeolite dictates its selectivity toward competing ions. The hydration spheres of high field-strength cations prevent their close approach to the seat of charge in the framework; hence, cations of low field strength are generally more tightly held and selectively exchanged by the zeolite than other ions. Clinoptilolite has a relatively small CEC (≈2.25 meq/g), but its cation selectivity is Cs > Rb > K > NH4 > Ba > Sr > Na > Ca > Fe > Al > Mg > Li.
This preference for larger cations, including NH4+, was exploited for removing NH4-N from municipal sewage effluent and has been extended to agricultural and aquacultural applications (1, 2). Clinoptilolite and natural chabazite have also been used to extract Cs and Sr from nuclear wastes and fallout.
Most zeolites in volcanogenic sedimentary rocks were formed by the dissolution of volcanic glass (ash) and later precipitation of micrometer-size crystals, which mimic the shape and morphology of their basalt counterparts (Fig. 1; ref. 3). Sedimentary zeolitic tuffs are generally soft, friable, and lightweight and commonly contain 50–95% of a single zeolite; however, several zeolites may coexist, along with unreacted volcanic glass, quartz, K-feldspar, montmorillonite, calcite, gypsum, and cristobalite/tridymite. Applications of natural zeolites make use of one or more of the following properties: (i) cation exchange, (ii) adsorption and related molecular-sieving, (iii) catalytic, (iv) dehydration and rehydration, and (v) biological reactivity. Extrinsic properties of the rock (e.g., siliceous composition, color, porosity, attrition resistance, and bulk density) are also important in many applications. Thus, the ideal zeolitic tuff for both cation-exchange and adsorption applications should be mechanically strong to resist abrasion and disintegration, highly porous to allow solutions and gases to diffuse readily in and out of the rock, and soft enough to be easily crushed. Obviously, the greater the content of a desired zeolite, the better a certain tuff will perform, ceteris paribus. (See Table 1 for more information on the properties of zeolites.)