Structural Basis of Selectivity in Tunnel Type Inorganic Ion Exchangers
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The crystal structures of two tunnel type inorganic ion exchangers are described. A knowledge of structure is necessary to understand the ion exchange properties of these compounds. The titanosilicate of composition Na1.64H0.36Ti2O3(SiO 4)·1.8H2O has a square framework structure outlining a tunnel parallel to the c-axis. In addition, the faces outlining the tunnel have cavities in which Na+ fits snugly but alkali metal ions larger than Na+ are excluded. Cs+ fits within the tunnels forming eight bonds with oxygen atoms of the silicate having distances of 3.183(5) Å and 3.057(6) Å. Because of its large diameter, Cs+ can only occupy half of the tunnel sites for a maximum uptake of 25% of the total exchange capacity. The remaining charge is satisfied by Na+ and protons within the tunnel. The affinity for Cs+ is much greater than for Na+ in the tunnel sites so that small amounts of Cs+ may be removed from concentrated sodium nitrate solutions making this exchanger useful for nuclear waste remediation. The second exchanger, K3H(TiO)4(SiO4)·4H2O, has a structure similar to the first but with a shorter c-axis. Thus, ions occupy the face centers but not the tunnels so that the selectivity series for alkali metal ions depends upon ion size.
author list (cited authors)
Clearfield, A., Poojary, D. M., Behrens, E. A., Cahill, R. A., Bortun, A. I., & Bortun, L. N.