The effect of pH, contact time, initial metal concentration and presence of common competing cations, on hydroxyapatite (HAP) sorption properties towards Ph2+, Cd2+, Zn2+, and Sr2+ ions was studied and compared using a batch technique. The results strongly indicated the difference between the sorption mechanism of Pb2+ and other investigated cations: the removal of Pb2+ was pH-independent and almost complete in the entire pH range (3-12), while the sorption of Cd2+, Zn2+ and Sr2+ generally increased with an increase of pH; the contact time required for attaining equilibrium was 30 mm for Pb 2+ versus 24h needed for other cations; maximum sorption capacity of HAP sample was found to be an order of magnitude higher for Pb2+ (3.263 mmol/g), than for Cd2+ (0.601 mmol/g), Zn (2+) (0.574 mmol/g) and Sr2+ (0.257 mmol/g); the selectivity of HAP was found to decrease in the order Ph2+> Cd2+> Zn2+> Sr2+ while a decrease of pHpzc, in respect to the value obtained in inert electrolyte, followed the order Cd2+ > Zn2+> Ph2+> Sr2+; neither of investigated competing cations (Ca2+, Mg2+, Na+ and K+) influenced Pb2+ immobilization whereas the sorption of other cations was reduced in the presence of Ca2+, in the order Sr2+ > Cd2+ > Zn (2+). The pseudo-second order kinetic model and Langmuir isotherm have been proposed for modeling kinetic and equilibrium data, respectively. The sorption of all examined metals was followed by Ca2+ release from the HAP crystal lattice and pH decrease. The ion exchange and specific cation sorption mechanisms were anticipated for Cd2+, Zn2+ and Sr2+, while dissolution of HAP followed by precipitation of hydroxypyromorphite (Pb-10(PO4)(6)(OH)(2)) was found to be the main operating mechanism for Pb 2+ immobilization by HAP, with the contribution of specific cation sorption. (c) 2007 Elsevier B. V. All rights reserved.