Cadmium substitution in HA

Cadmium is a very toxic metal, widely present in the environment due mostly to industrial sources, and responsible for a number of potentially fatal ailments in humans. Its uptake and release by bone has been the subject of numerous studies, with better knowledge of site preference and bond mechanisms needed for remediation and treatment. The article cited below contributions to understanding the underlying materials properties and hopefully resolves some of the contradictory explanations found in the literature for its behavior.

Theoretical and Experimental Studies of Substitution of Cadmium into Hydroxyapatite

Joice Terra¹, Gabriela Gonzalez², A. M. Rossi¹, J.G. Eon³, D. E. Ellis⁴

(J. Chem. Chem. Phys. 2010)

Substitution of cadmium into bulk hydroxyapatite Ca_(10-x)Cd_x(PO₄)₆(OH)₂ (CdHA: x= 0.12, 1.3, 2.5) is studied by combining X-ray diffraction data from synchrotron radiation, infra-red spectroscopy (FTIR) and density functional theory (DFT) calculations. Energetic and electronic analyses are carried out for several configurations of Cd substitution for Ca at both cationic sites. Rietveld analysis shows preferential occupation of Ca2 site by cadmium. FTIR data suggest a non-negligible covalent character of Cd-OH. The much-discussed cation site preference for substitution is determined on the basis of relaxed-lattice energetics, and interpreted in terms of chemical concepts; theory indicates that the Ca2 site is clearly favored and such preference is related to the more covalent character of this site compared to that of site1.

The figure shows a top view of HA with hexagonal OH-channels delimited by Ca2 (green) atom triangles. Large balls (red, blue, cyan) are distinct oxygen types, and small orange balls are P. Ca1 channels are seen in projection as gray balls.