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Recently. it has come to our attention, that a polemical note by Milonjic [S.K. Milonjic, Comments on "factors influencing the removal of divalent cations by hydroxyapatite", J. Hazard. Mater. 162 (2009) 1588-1589]. has been published raising several critical comments on some experimental procedures, as well as the presentation and interpretation of some data found in the cited paper by Smiciklas et al. [L. Smiciklas, A. Onjia. S. Raicevic, Dj. Janackovic, M. Mitric, Factors influencing the removal of divalent cations by hydroxyapatite, J. Hazard. Mater. 152 (2008) 876-884]. Since we have not received any prior copy of this discussion through the process involved in the acceptance of these comments, we now take the opportunity of responding. We have reviewed some scientific papers dealing with cation sorption, in order to demonstrate variety of experimental approaches used for batch sorption tests, regarding solution pH. Finally, we justified experimental methods and data interpretation methods we chose to perform in the mentioned manuscript.

Recently. it has come to our attention, that a polemical note by Milonjic [S.K. Milonjic, Comments on factors influencing the removal of divalent cations by hydroxyapatite, J. Hazard. Mater. 162 (2009) 1588-1589]. has been published raising several critical comments on some experimental procedures, as well as the presentation and interpretation of some data found in the cited paper by Smiciklas et al. [L. Smiciklas, A. Onjia. S. Raicevic, Dj. Janackovic, M. Mitric, Factors influencing the removal of divalent cations by hydroxyapatite, J. Hazard. Mater. 152 (2008) 876-884]. Since we have not received any prior copy of this discussion through the process involved in the acceptance of these comments, we now take the opportunity of responding. We have reviewed some scientific papers dealing with cation sorption, in order to demonstrate variety of experimental approaches used for batch sorption tests, regarding solution pH. Finally, we justified experimental methods and data interpretation methods we chose to perform in the mentioned manuscript. (C) 2009 Elsevier B.V. All rights reserved.

Recently. it has come to our attention, that a polemical note by Milonjic [S.K. Milonjic, Comments on factors influencing the removal of divalent cations by hydroxyapatite, J. Hazard. Mater. 162 (2009) 1588-1589]. has been published raising several critical comments on some experimental procedures, as well as the presentation and interpretation of some data found in the cited paper by Smiciklas et al. [L. Smiciklas, A. Onjia. S. Raicevic, Dj. Janackovic, M. Mitric, Factors influencing the removal of divalent cations by hydroxyapatite, J. Hazard. Mater. 152 (2008) 876-884]. Since we have not received any prior copy of this discussion through the process involved in the acceptance of these comments, we now take the opportunity of responding. We have reviewed some scientific papers dealing with cation sorption, in order to demonstrate variety of experimental approaches used for batch sorption tests, regarding solution pH. Finally, we justified experimental methods and data interpretation methods we chose to perform in the mentioned manuscript. (C) 2009 Elsevier B.V. All rights reserved.

Recently. it has come to our attention, that a polemical note by Milonjic [S.K. Milonjic, Comments on "factors influencing the removal of divalent cations by hydroxyapatite", J. Hazard. Mater. 162 (2009) 1588-1589]. has been published raising several critical comments on some experimental procedures, as well as the presentation and interpretation of some data found in the cited paper by Smiciklas et al. [L. Smiciklas, A. Onjia. S. Raicevic, Dj. Janackovic, M. Mitric, Factors influencing the removal of divalent cations by hydroxyapatite, J. Hazard. Mater. 152 (2008) 876-884]. Since we have not received any prior copy of this discussion through the process involved in the acceptance of these comments, we now take the opportunity of responding. We have reviewed some scientific papers dealing with cation sorption, in order to demonstrate variety of experimental approaches used for batch sorption tests, regarding solution pH. Finally, we justified experimental methods and data interpretation methods we chose to perform in the mentioned manuscript.

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+ gt Cd2+ gt Zn2+ gt Sr2+ while a decrease of pHpzc, in respect to the value obtained in inert electrolyte, followed the order Cd2+ gt Zn2+ gt Ph2+ gt 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+ gt Cd2+ gt 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.

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+ GT Cd2+ GT Zn2+ GT Sr2+ while a decrease of pHpzc, in respect to the value obtained in inert electrolyte, followed the order Cd2+ GT Zn2+ GT Ph2+ GT 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+ GT Cd2+ GT 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.

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+ GT Cd2+ GT Zn2+ GT Sr2+ while a decrease of pHpzc, in respect to the value obtained in inert electrolyte, followed the order Cd2+ GT Zn2+ GT Ph2+ GT 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+ GT Cd2+ GT 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.

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+ gt Cd2+ gt Zn2+ gt Sr2+ while a decrease of pHpzc, in respect to the value obtained in inert electrolyte, followed the order Cd2+ gt Zn2+ gt Ph2+ gt 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+ gt Cd2+ gt 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.

The in situ (in place) immobilization of toxic metals, using inexpensive "reactive" amendments, is considered as a simple and cost-effective approach for the treatment of soils, contaminated by the presence of heavy metals, when these soils are difficult or costly to be removed and treated ex situ. Several application studies have demonstrated that the stabilization of contaminated soils and groundwaters by the addition of apatite minerals has the potential to be a successful and widely applicable remediation strategy for the case of Pb, Cd, as well as for other toxic metals, existing in polluted soils. On the other hand, the specific immobilization mechanism(s) of these toxic metals remains rather elusive. The present work involves an interdisciplinary theoretical and experimental approach, designed to gain at the fundamental (molecular) level the understanding of respective mechanisms, considering the immobilization of Pb and Cd by the addition of apatites. The theoretical analysis of stability, regarding the apatite/Pb or apatite/Cd systems and the relevant results of sorption experiments, pointed out two different mechanisms for the immobilization of Pb or Cd by the use of apatites. The possible practical consequences of these findings for the selection/application of natural apatites for the remediation of contaminated soils by the presence of heavy metals have been also discussed.

The in situ (in place) immobilization of toxic metals, using inexpensive reactive amendments, is considered as a simple and cost-effective approach for the treatment of soils, contaminated by the presence of heavy metals, when these soils are difficult or costly to be removed and treated ex situ. Several application studies have demonstrated that the stabilization of contaminated soils and groundwaters by the addition of apatite minerals has the potential to be a successful and widely applicable remediation strategy for the case of Pb, Cd, as well as for other toxic metals, existing in polluted soils. On the other hand, the specific immobilization mechanism(s) of these toxic metals remains rather elusive. The present work involves an interdisciplinary theoretical and experimental approach, designed to gain at the fundamental (molecular) level the understanding of respective mechanisms, considering the immobilization of Pb and Cd by the addition of apatites. The theoretical analysis of stability, regarding the apatite/Pb or apatite/Cd systems and the relevant results of sorption experiments, pointed out two different mechanisms for the immobilization of Pb or Cd by the use of apatites. The possible practical consequences of these findings for the selection/application of natural apatites for the remediation of contaminated soils by the presence of heavy metals have been also discussed. (C) 2004 Elsevier B.V. All rights reserved.

The in situ (in place) immobilization of toxic metals, using inexpensive reactive amendments, is considered as a simple and cost-effective approach for the treatment of soils, contaminated by the presence of heavy metals, when these soils are difficult or costly to be removed and treated ex situ. Several application studies have demonstrated that the stabilization of contaminated soils and groundwaters by the addition of apatite minerals has the potential to be a successful and widely applicable remediation strategy for the case of Pb, Cd, as well as for other toxic metals, existing in polluted soils. On the other hand, the specific immobilization mechanism(s) of these toxic metals remains rather elusive. The present work involves an interdisciplinary theoretical and experimental approach, designed to gain at the fundamental (molecular) level the understanding of respective mechanisms, considering the immobilization of Pb and Cd by the addition of apatites. The theoretical analysis of stability, regarding the apatite/Pb or apatite/Cd systems and the relevant results of sorption experiments, pointed out two different mechanisms for the immobilization of Pb or Cd by the use of apatites. The possible practical consequences of these findings for the selection/application of natural apatites for the remediation of contaminated soils by the presence of heavy metals have been also discussed. (C) 2004 Elsevier B.V. All rights reserved.

The in situ (in place) immobilization of toxic metals, using inexpensive "reactive" amendments, is considered as a simple and cost-effective approach for the treatment of soils, contaminated by the presence of heavy metals, when these soils are difficult or costly to be removed and treated ex situ. Several application studies have demonstrated that the stabilization of contaminated soils and groundwaters by the addition of apatite minerals has the potential to be a successful and widely applicable remediation strategy for the case of Pb, Cd, as well as for other toxic metals, existing in polluted soils. On the other hand, the specific immobilization mechanism(s) of these toxic metals remains rather elusive. The present work involves an interdisciplinary theoretical and experimental approach, designed to gain at the fundamental (molecular) level the understanding of respective mechanisms, considering the immobilization of Pb and Cd by the addition of apatites. The theoretical analysis of stability, regarding the apatite/Pb or apatite/Cd systems and the relevant results of sorption experiments, pointed out two different mechanisms for the immobilization of Pb or Cd by the use of apatites. The possible practical consequences of these findings for the selection/application of natural apatites for the remediation of contaminated soils by the presence of heavy metals have been also discussed.