Summary Review of Zeolites in Adsorptive Separation

Another application for adsorption of metal impurities is in the nuclear power industry. Radioactive cesium is one of the compounds that is difficult to remove from radioactive waste. This is because ordinary resins and zeolites do not effectively adsorb radioactive cesium. In 1997, lONSlV lE-911 crystalline silicotitan-ate (CST) ion exchangers were developed and effectively used to clean up radioactive wastes in the Melton Valley tanks at Oak Ridge [268, 269], CST was discovered [270] by researchers at Sandia National Laboratories and Texas A M University, with commercial manufacture carried out by UOP. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

The survey summaries show that zeoHte adsorbents are most often employed for non-aqueous systems. This is because the material generally used as a binder to fabricate an agglomerated zeoHte, is a clay comprising silicon dioxide and aluminum oxide which tends to dissolve in water. This dissolution results in negative changes in physical characteristics of the adsorbent as well as silicon contamination of the solution which manifests itself as turbidity in the product. 

A proven solution to the binder problem is to use water insoluble organic polymer binders instead of clay. For example cellulose acetates and cellulose ethers binders are successfully employed to make commercial zeolitic adsorbents for sugar separation in aqueous solutions [154, 205, 218-223, 225-226, 231-232, 238]. This technique allows the use of zeoHte adsorbents in aqueous separation processes. 

Silicalite is another crystaUine material whose use in liquid adsorptive separation is found in the surveys. Applications for siHcaHte are noted in categories 1 

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