Ion exchange behavior among metal trisilicates: probing selectivity, structures, and mechanism
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One model system for the investigation of selectivity in inorganic ion exchangers is a group of synthetic analogues of the mineral umbite. Hydrothermally synthesized trisilicates with the general form A2BSi3O9.H2O, where A is a monovalent cation, and B = Ti4+, Zr4+, and Sn4+ have been shown to have ion exchange properties. The extended three dimensional framework structure offers the ability to tune the selectivity based on the size of the cavities and channels. The unit cell volume, and therefore the pore size, can be altered by changing the size of the octahedral metal. The substitution of Ge for Si can also increase the pore size. A variety of cations have been exchanged into the trisilicates including alkali and alkaline earths, lanthanides, and actinides. The reason for the selectivity rests in the pocket of framework oxygens which make up the exchange sites. Close examination of the cation environments shows that the ions with the greatest affinity are those that have the closest contacts to the framework oxygens. For example, among alkali cations, zirconium trisilicate demonstrates the greatest affinity for Rb+ and has the most A-O contact distances approaching the sum of their ionic radii. The origins of selectivity also rely upon the valence of the incoming cation. When cations are of similar ionic radius, a cation of higher charge is always preferred over the lower valence. Ion exchange studies in binary solutions of cations of different valence, but similar size (1.0Å ) have proven the selectivity series to be Th4+ > Gd3+ > Ca2+ > Na+. Through structural characterization, kinetic studies, and use of in situ x-ray diffraction techniques the origins of selectivity in these inorganic ion exchangers has been further elucidated. The principles gleaned from these studies can be applied to other inorganic framework materials. The umbite system has the potential to be altered and tailored for specific separation needs. The trisilicate materials presented in this work are representative of the types of advances in inorganic materials research and prove their potential as applicable compounds useful for solving real world problems.
Fewox, Christopher Sean (2008). Ion exchange behavior among metal trisilicates: probing selectivity, structures, and mechanism. Doctoral dissertation, Texas A&M University. Texas A&M University. Available electronically from