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 Pb 2+, Cd 2+, Zn 2+, and Sr 2+ ions was studied and compared using a batch technique. The results strongly indicated the difference between the sorption mechanism of Pb 2+ and other investigated cations: the removal of Pb 2+ was pH-independent and almost complete in the entire pH range (3–12), while the sorption of Cd 2+, Zn 2+ and Sr 2+ generally increased with an increase of pH; the contact time required for attaining equilibrium was 30 min for Pb 2+ versus 24 h needed for other cations; maximum sorption capacity of HAP sample was found to be an order of magnitude higher for Pb 2+ (3.263 mmol/g), than for Cd 2+ (0.601 mmol/g), Zn 2+ (0.574 mmol/g) and Sr 2+ (0.257 mmol/g); the selectivity of HAP was found to decrease in the order Pb 2+ > Cd 2+ > Zn 2+ > Sr 2+ while a decrease of pH PZC, in respect to the value obtained in inert electrolyte, followed the order Cd 2+ > Zn 2+ > Pb 2+ > Sr 2+; neither of investigated competing cations (Ca 2+, Mg 2+, Na + and K +) influenced Pb 2+ immobilization whereas the sorption of other cations was reduced in the presence of Ca 2+, in the order Sr 2+ > Cd 2+ ≥ 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 Ca 2+ release from the HAP crystal lattice and pH decrease. The ion exchange and specific cation sorption mechanisms were anticipated for Cd 2+, Zn 2+ and Sr 2+, while dissolution of HAP followed by precipitation of hydroxypyromorphite (Pb 10(PO 4) 6(OH) 2) was found to be the main operating mechanism for Pb 2+ immobilization by HAP, with the contribution of specific cation sorption.