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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/86001 |
| Date | 10 October 2008 |
| Creators | Fewox, Christopher Sean |
| Contributors | Clearfield, Abraham |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation, text |
| Format | electronic, born digital |
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