Sorption of ${\mathrm{Pb}}^{2+}$ , ${\mathrm{Cd}}^{2+}$ , and ${\mathrm{Sr}}^{2+}$ Ions on Calcium Hydroxyapatite Powder Obtained by the Hydrothermal Method

The sorption of ${\mathrm{Pb}}^{2+}$ , ${\mathrm{Cd}}^{2+}$ , and ${\mathrm{Sr}}^{2+}$ ions from aqueous solutions by carbonate-substituted calcium hydroxyapatite (HAP) obtained by the hydrothermal decomposition of urea and calcium–ethylenediaminetetraacetic acid chelates was investigated. The shift of the point of zero charge $\left({\mathrm{pH}}_{\mathrm{pzc}}\right)$ of HAP toward lower pH values in solutions of ${\mathrm{Pb}}^{2+}$ and ${\mathrm{Cd}}^{2+}$ ions, which was more pronounced for ${\mathrm{Pb}}^{2+}$ ions than for ${\mathrm{Cd}}^{2+}$ ions, indicates that specific adsorption of these cations on HAP had occurred. There was no shift of the ${\mathrm{pH}}_{\mathrm{pzc}}$ in the solution containing ${\mathrm{Sr}}^{2+}$ ions, suggesting that specific adsorption of this cation on HAP had not occurred. The sorption isotherms suggest that the sequence of the efficiency of sorption by HAP is ${\mathrm{Pb}}^{2+}>{\mathrm{Cd}}^{2+}>{\mathrm{Sr}}^{2+}$ . Ion exchange is the main mechanism of removal of ${\mathrm{Sr}}^{2+}$ ions from aqueous solution by apatite, whereas during the sorption of ${\mathrm{Cd}}^{2+}$ ions, this mechanism occurs simultaneously with the process of specific adsorption. The interaction of the apatite with ${\mathrm{Pb}}^{2+}$ ions includes the previously mentioned mechanisms and dissolution of the apatite, followed by the precipitation of lead hydroxyapatite.