Interdependence of Anion and Cation Exchanges in Calcium Hydroxyapatite: Pb(2+) and Cl(-)

The ion exchange of Pb(2+) and Cl(-) into calcium hydroxyapatite from aqueous solutions is studied at 293 K. Although the exchange of Cl(-) into CaHAp and PbHAp was not observed, in the presence of both Pb(2+) and Cl(-) in aqueous solution the exchange of both Pb(2+) and Cl(-) into CaHAp occurred, and, at least at intermediate concentrations of HCl, a proportionality between the sorbed quantities of these two ions was observed. The dissolution of CaHAp is shown to be dependent upon the pH, while the quantity of Pb(2+) exchanged into the solid is not a function of the proton content; consequently, the exchange is not primarily a dissolution-precipitation process. Copyright 1999 Academic Press.