Ion adsorption behaviour of hydroxyapatite with different crystallinities

This study aimed to correlate crystallinity of hydroxyapatite (HA) with the ion adsorption behaviour of the material. Hydroxyapatite powders of various crystallinities (X_c) and specific surface area (SSA) were prepared by precipitation following heat treatment. Adsorption experiments were carried out by using (i) multi-component ion solutions containing a broad range of light and heavy ions to study competitive adsorption and (ii) lead and zinc solutions with concentrations up to 250 ppm to determine the adsorption isotherms of the material. While as-prepared HA powders of low crystallinity (X_c = 0%) and a high SSA of 170 m²/g showed quantitative removal for divalent Pb, Zn, Be, U, Bi, V, Al, Cu and Ga ions, calcined powders with higher crystallinity (X_c = 65–95%) and lower SSA between 5 and 30 m²/g led to a quantitative removal only for a few elements (Pb, Bi, Ga). The time and concentration dependant ion removal capacity for Pb²⁺ and Zn²⁺ single element solutions showed quantitative removal even after short immersion times of less than 10 min for as-prepared HA powders. XRD analysis of the powders after ion adsorption revealed the presence of pyromorphite (Pb₅(PO₄)₃OH) and hopeite (Zn₃(PO₄)₂) phases, respectively.