Synthetic hydroxyapatites as inorganic cation exchangers. Part 2

The removal of cations such as Pb²⁺, Mn²⁺, Co²⁺ and Cu²⁺ in aqueous solution by four synthetic hydroxyapatites (S-1, S-2, S-3, S-4) has been investigated using both batch and column methods. The removal is due not only to an adsorption effect but also to an ion-exchange reaction between the cations in solution and the Ca²⁺ ions of the apatites. The order of the ions according to the amount exchanged was as follows: Pb²⁺ > Cu²⁺ > Mn²⁺ ≃ Co²⁺. Pb²⁺ ions were readily removed by the apatites and the maximum value for the exchange of Pb²⁺ ions was 230 mg per g of S-4 apatite. The apatites, particularly S-4, would seem to be possible agents for the removal of toxic Pb²⁺ ions. The selectivity of the apatites for the cations can be explained by considering the radii and the electronegativities of the ions.