The sorption of Cd aqueous solutions onto several hydroxyapatites was investigated, as a function of the pH and the cation vacancy concentration, by using chemical and structural analyses. The principal conclusion was that the maximum uptake of Cd into the apatite framework was about 80%, and did not depend upon the stoichiometry of the original solid. It was deduced that the sorbed Cd was localized at the 6h sites of the structure, adjacent to the channels which included the OH groups. In less stoichiometric apatite (1.1 cationic vacancy per mol), the same phenomena occurred but dissolution-precipitation also occurred; thus increasing the total quantity of Cd which was immobilized in the solid. Non-stoichiometry was suggested to result from different distributions of vacancies between Ca(1) and Ca(2) sites. The cationic vacancies did not seem to have an effect upon the maximum uptake of Ca into the apatite phase. The reason for this was unclear, and was thought to be an intrinsic property of Cd ions, not to occupy neighboring sites. Thus, the present results showed that the use of calcium hydroxyapatites with a high cationic vacancy concentration increased the amount of Cd which was immobilized in the solid phase; due to a dissolution-precipitation process.

J.Jeanjean, S.McGrellis, J.C.Rouchaud, M.Fedoroff, A.Rondeau, S.Perocheau, A.Dubis: Journal of Solid State Chemistry, 1996, 126[2], 195-201