Structural Basis of Selectivity in Tunnel Type Inorganic Ion Exchangers Academic Article uri icon


  • 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.

publication date

  • December 1999