title: Factors influencing the removal of divalent cations by hydroxyapatite.
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 24h 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+)>or=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.
authors.affiliation: Vinca Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia. ivanat@vin.bg.ac.yu
authors.forename: I
authors.initials: I
authors.lastname: Smiciklas
authors.forename: A
authors.initials: A
authors.lastname: Onjia
authors.forename: S
authors.initials: S
authors.lastname: Raicević
authors.forename: D
authors.initials: D
authors.lastname: Janaćković
authors.forename: M
authors.initials: M
authors.lastname: Mitrić
mesh_terms: D000327:Adsorption; D002413:Cations, Divalent / Q000302:isolation & purification*; D017886:Durapatite; D004785:Environmental Pollutants / Q000302:isolation & purification*; D019216:Metals, Heavy / Q000302:isolation & purification*
chemical_list: D002413:Cations, Divalent; D004785:Environmental Pollutants; D019216:Metals, Heavy; D017886:Durapatite