Cd²⁺ and Zn²⁺ sorption on apatite in the presence of EDTA and humic substance

¹ Laboratory of Inorganic Materials, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, ESTONIA
² Department of Chemistry, Umeå University, SE-901 87 Umeå, SWEDEN

^a karin.viipsi@ttu.ee

Abstract

The sorption of Cd²⁺ and Zn²⁺ on hydroxyapatite [HAP- Ca₁₀(PO₄)₆(OH)₂] and fluorapatite [FAP- Ca₁₀( PO₄)₆(F)₂] with different specific surface area and stoichiometry was investigated in batch experiments in the pH range 4 to 11 (25 ^◦C; 0.1 M KNO₃). The impact of different conditions was concerned: solution pH, the presence of complexing ligands (EDTA and humic substance) and competing metal ions, as well as reaction kinetic and equilibrium conditions. To evaluate the reversibility of Cd²⁺ sorption onto HAP, desorption characteristics in water, Ca, EDTA, and HUM-solutions were determined. Additionally to solution analysis the surface composition of solid phases was analysed by X-Ray Photoelectron Spectroscopy XPS). The information from the chemical analyses was used to design an equilibration model that takes into account dissolution, surface potential, solution and surface complexation, as well as possible phase transformations. It was revealed that apatites effectively sorb Cd²⁺ and Zn²⁺ by ion exchange reactions on surface by formation of new surface phases. Using XPS the formation of a Me-enriched HAP surface was found, which was interpreted as the formation of a solid solution with the general formula: Ca_8.4-xMe_x(HPO₄)_1.6(PO₄)_4.4(OH)_0.4. In a binary solution (Cd+Zn) the competition of metals reduced individual sorbed amount compared with the single component solutions but the total adsorption maximum was approximately constant. The presence of EDTA reduces the metal sorption on apatite due to [CdEDTA]^2- and [ZnEDTA]^2- complexes and increases apatite solubility due to [CaEDTA]^2- complex formation. The dissolved humic substance was bound on apatite in suspensions but the amount of Cd²⁺ bound was not changed. The results showed that the solution pH and the presence of complexing ligands have a significant effect on heavy metal sorption on apatite and must be considered if apatites are used as remediation agent. The proposed model can be used to predict apatite dissolution and surface phase transformations in the presence of metal ions and EDTA.