A crystallographic study of the sorption of cadmium on calcium hydroxyapatites: Incidence of cationic vacancies

Sorption of cadmium from aqueous solutions on several calcium hydroxyapatites was investigated as a function of pH and of the concentration of cationic vacancies, using elemental and structural analysis. The main result is that the maximum uptake of cadmium into the apatitic framework is about 0.8 mol/mol and does not depend on the stoichiometry of the starting solid. Sorbed cadium is localized in the 6h sites of the structure adjacent to the channels including the OH groups. For the less stoichiometric apatite (1.1 cationic vacancy per mol), the same phenomenon takes place, but a dissolution-precipitation mechanism is added, thus increasing the total quantity of cadmium immobilized in the solid. The additional sorbed quantity corresponds to the precipitation of Cd5H2(PO4)(4). Such results may be used for the treatment of contaminated waters and industrial wastes. (C) 1996 Academic Press, Inc.