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 Cd(2+) and Zn(2+) removed increased with increasing pH. The removed amount of Zn(2+) was higher than Cd(2+). 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, Cd(2+) and Zn(2+) removal was reduced because of the formation of [CdEDTA](2-) and [ZnEDTA](2-) in solution. XPS revealed an enrichment of AP surface by Cd(2+) and Zn(2+) and formation of new surface solid-solution phase with the general composition Ca8.4-xMex(HPO4)1.6(PO4)4.4(OH)0.4.

EDTA impact on Cd2+ migration in apatite-water system.

The impact factors on Cd sorption and desorption in aqueous solution on apatite were studied. Batch experiments were carried out using synthetic hydroxyapatite with Ca/P 1.44, 1.66 and 1.94 in Cd(NO3)2 and Cd(NO3)2-EDTA equimolar complex solutions in the pH range from 4 to 7. It was established that Cd sorption on apatite depends not only on apatite specific surface area but also on Ca/P mole ratio in apatite as well as on the presence of chelating compounds. Presence of EDTA in the solution decreases the amount of Cd bound. [CdEDTA]2- prevents chemical sorption of Cd2+ ions on apatite. EDTA considerably decreases the sorption capacity of apatite with Ca excess. Impact of EDTA is smaller for the stoichiometric apatite and for the apatite with calcium deficiency. Cd bound due to adsorption is more easily removed from apatite. Ca2+ ions increase and presence of EDTA in a solution cause total Cd desorption from apatite.