Factors influencing the removal of divalent cations by hydroxyapatite

The effect of pH, contact time, initial metal concentration and presence of common competing cations, on hydroxyapatite (HAP) sorption properties towards Pb2+, 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 min for Pb2+ versus 24 h 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), Zn2+ (0.574 mmol/g) and Sr2+ (0.257 mmol/g); the selectivity of HAP was found to decrease in the order Pb2+ > Cd2+ > Zn2+ > Sr2+ while a decrease of pHPZC, in respect to the value obtained in inert electrolyte, followed the order Cd2+ > Zn2+ > Pb2+ > 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+ ≥ Zn2+. 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 (Pb10(PO4)6(OH)2) was found to be the main operating mechanism for Pb2+ immobilization by HAP, with the contribution of specific cation sorption.