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Factors influencing the removal of divalent cations by hydroxyapatite

Journal of Hazardous Materials
Publication Date
DOI: 10.1016/j.jhazmat.2007.07.056
  • Hydroxyapatite
  • Cation Sorption
  • Equilibrium
  • Kinetics
  • Sorption Mechanisms
  • Phpzc


Abstract 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.

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