Surface phase transformations, surface complexation and solubilities of hydroxyapatite in the absence/presence of Cd(II) and EDTA

The removal of Cd from aqueous solutions by hydroxyapatite (HAP) was investigated with and without EDTA being present. Batch experiments were carried out using synthetic hydroxyapatite with Ca/P 1.57 and a specific surface area of 37.5 m2/g in the pH range 4–9 (25 °C; 0.1 M KNO3). The surface composition of the solid phases were analysed by X-ray Photoelectron Spectroscopy (XPS). The surface layer of HAP was found to undergo a phase transformation with a (Ca + Cd)/P atomic ratio of 1.4 and the involvement of an ion exchange process (Ca2+ ↔ Cd2+). The amount of Cd removed from the solution increased with increasing pH, reaching ≈100% at pH 9. In the presence of EDTA Cd removal was reduced due to the formation of [CdEDTA]2− in solution. The solubility of HAP increases in the presence of EDTA at pH values above 5, mainly due to the formation of [CaEDTA]2−. In contrast to this, the solubility was found to decrease in the presence of Cd2+ and CdEDTA2−. Using XPS the formation of a Cd-enriched HAP surface was found, which was interpreted as the formation of a solid solution of the general composition: Ca8.4-xCdx(HPO4)1.6(PO4)4.4(OH)0.4Ca8.4-xCdx(HPO4)1.6(PO4)4.4(OH)0.4.The information from the chemical analyses and XPS data was used to design an equilibrium model that takes into account dissolution, solution and surface complexation, as well as possible phase transformations. The total concentration of Ca, phosphate, EDTA, and Cd in solution were used in the equilibrium analysis. In the calculations the computer code WinSGW, which is based on the SOLGASWATER algorithm, was used. The following equilibria and compositions of the solid solutions were found to give the best fit to experimental data: logKs(Ca7.6Cd0.8(HPO4)1.6(PO4)4.4(OH)0.4(s)+4.8H+⇋7.6Ca2++0.8Cd2++6HPO42-+0.4H2O)=-28.03±0.07. The corresponding value for the composition Ca5.6Cd2.8(HPO4)1.6(PO4)4.4(OH)0.4(s) is −27.39 ± 0.06.The proposed model can be used to predict HAP dissolution and surface phase transformations in the presence of Cd and EDTA.

► EDTA impact on hydroxyapatite Cd removal in pH range 4–9. ► XPS proof of the formation of Cd-enriched hydroxyapatite surface. ► The thermodynamic model of equilibrium of hydroxyapatite surface and Cd2+ in solution. ► More insoluble surface phase formation with the composition Ca8.4−xCdx(HPO4)1.6(PO4)4.4(OH)0.4.