Sorption of Pb2+, Cd2+, and Sr2+ ions on calcium hydroxyapatite powder obtained by the hydrothermal method

Abstract

The sorption of Pb2+, Cd2+, and Sr2+ 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 (pH(pzc)) of HAP toward lower pH values in solutions of Pb2+ and Cd2+ ions, which was more pronounced for Pb2+ ions than for Cd2+ ions, indicates that specific adsorption of these cations on HAP had occurred. There was no shift of the pH(pzc) in the solution containing Sr2+ 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 Pb2+ gt Cd2+ gt Sr2+. Ion exchange is the main mechanism of removal of Sr2+ ions from aqueous solution by apatite, whereas during the sorption of Cd2+ ions, this mechanism occurs simultaneously with the process of specific adsorption. The interaction of the apatite with Pb2+ ions includes the previously mentioned mechanisms and dissolution of the apatite, followed by the precipitation of lead hydroxyapatite.