Miscellaneous Materials

A number of specific materials have been discussed in this chapter. There are, however, numerous inorganic materials possessing ion exchange properties which have not been mentioned. In this section, a few of those materials which exhibit interesting ion exchange properties are introduced briefly. The list is far from complete, but serves to illustrate the diversity of ion exchange materials.

• Hydroxyapatites may undergo limited ion exchange reactions. While the calcium form (Ca10(PO4)6(OH)2) is the most common (it is a major component of teeth and bones), pure exchange end-members of Sr2 +, Cd2 + and Pb2 + are known, while various cations may form intermediate mixed-cation phases. The Sr2+ end-member, due to a slight lattice expansion, possesses superior ion exchange properties compared to Ca-hydroxyapa-tite. Of the Sr-hydroxyapatites, that with a (non-stoichiometric) Sr/P ratio of 1.73 has the highest ion exchange capacity of those measured. It is interesting that the presence of HCl may assist the ion exchange reaction by formation of a chlorapatite phase. This may be an example of simultaneous anion and cation exchange.

• Copper hexacyanoferrates, Cu2IFeII(CN)6 • xH2O and related compounds show quite promising exchange properties for Cs+, and have been investigated as agents for nuclear waste treatment. On passing caesium-containing waste through a column of Cu2IFeII(CN)6 • xH2O at room temperature, decontamination factors (ratios of pre-column to post-column Cs+ concentrations) of 103 can be achieved.

• Lithium manganate containing mixed-valence manganese ions exhibits unusual ion exchange properties, in that it undergoes combined ion exchange and redox reactions. Upon acid treatment of LiMnIIIMnIVO4, the MnIII is oxidized to MnIV

and Li is displaced from the structure thus:

4 LiMnIIIMnIVO4 + 8 H + p 3 Mn2VO4 + 4 Li+ +2 Mn2 ++ 4 H2O

The resulting spinel structure (y-MnO2) is highly selective for Li, and will readily re-insert Li# to regain the Li-manganate spinel:

Mn2VO4 + (n)LiOH p Li^Mn^MnJV-nO + (n/2)H2O + (n/4)O2

This type of exchange reaction is often referred to as the ion memory effect.

• Iodide ions may be efficiently exchanged for nitrate ion using BiPbO2NO3 in solutions of pH ^13. Under such conditions, the theoretical exchange capacity of 2 mmoL g 1 is approached.

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