Hydroxyapatite interactions with copper complexes

Hydroxyapatite was used to remove dilute copper(II) from aqueous solutions containing organic ligands. Both synthetic apatite and natural apatite originating from animal bones retained copper(II) in a flow-through column experiment but lost much of this ability when glycine, ethylenediamine or ethylenediaminetetraacetic acid (EDTA) was present in the solution. The amounts of copper(II) withheld by apatite was compared to the concentrations of free cupric ions present in equilibrium with complexed forms as a function of pH using complex formation constants derived from the IUPAC data base. It was found that most of the copper retained on the apatite was correlated with free metal ion concentrations. The strongest ligand, EDTA, formed a soluble stoichiometric copper(II) complex yet small amounts of copper could be sorbed on the apatite particles. The role of surface complexes remains to be determined to fully explain the metal uptake mechanism.