Hydroxy- and fluorapatite as sorbents in Cd(II)-Zn(II) multi-component solutions in the absence/presence of EDTA

Apatites are suitable sorbent materials for contaminated soil and water remediation because of their low solubility and ability to bind toxic metals into their structure. Whereas in soil/water systems different complexing ligands are present, it is important to examine how these ligands affect apatite metal sorption process. The removal of cadmium (Cd) and zinc (Zn) ions from aqueous solutions by hydroxyapatite (HAP) and fluorapatite (FAP) was investigated by batch experiments with and without EDTA being present in the pH range 4-11. The surface composition of the solid phases was analyzed by X-ray photoelectron spectroscopy (XPS). The surface layer of apatites (AP), according to the (Ca + Cd + Zn):P atomic ratio, remained constant (1.4 +/- 0.1) through an ion exchange. The amount of Cd2+ and Zn2+ removed increased with increasing pH. The removed amount of Zn2+ was higher than Cd2+. In the Cd-Zn binary system, competitive sorption reduced the individual removed amounts but the total maximum sorption was approximately constant. In the presence of EDTA, Cd2+ and Zn2+ removal was reduced because of the formation of [CdEDTA](2-) and [ZnEDTA](2-) in solution. XPS revealed an enrichment of AP surface by Cd2+ and Zn2+ and formation of new surface solid-solution phase with the general composition Ca8.4-xMex(HPO4)(1.6)(PO4)(4.4)(OH)(0.4). (c) 2013 Elsevier B.V. All rights reserved.