Structural studies of synthetic calcium and lead apatites

Full text: Hydroxyapatite, Ca10(PO4)6(OH)2, is a major mineral component of calcified tissues, including bones and teeth. It is a good model system for natural apatites, having a structure which can easily accommodate a great variety of anionic and cationic substitutions. It acts as a natural sink for heavy metals, especially Pb and Cd, in biological systems. The hydroxyl ion can be replaced by other anions and one aim of the present work is to establish how such substitutions influence the ability of apatites to sequest and immobilise heavy metals. Environmentally speaking, there is still a lot of industrial use for Pb and the halide substituted apatite as lead chlorapatite, Pb10(PO4)6Cl2, which is found in Pb contaminated soils. This leads to interest in looking at the ability to release Pb encapsulated in mineral bodies and to develop methods that can selectively remove Pb from contaminated sites. We have used a combination of synchrotron X-ray diffraction and high resolution powder neutron diffraction methods for our structural studies of some substituted apatites. We have refined the structures of eight apatites M10(PO4)6X2, where M = Pb, Ca and X = OH, F, Cl, Br, using the Rietveld method. In these there are two cation sites, a channel of M-I atoms and a triangle of M-II atoms. The anion interacts most strongly with the M-II atoms at (↓, deg , 1/4). For the Ca compounds, the F ion sits within the triangles at (0, 0, 1/4), while the larger OH and Cl anions are disordered above and below the M-II triangles. The Br is at (0, 0, 1/2). Despite the larger size of the isostructural Pb compounds, no anions are found in the triangles. The F, Cl and Br ions are at (0, 0, 1/2) and the OH ion is disordered at (0, 0, z). This difference in behaviour is possibly related to the stereochemical activity of the Pb 6s electrons. While synchrotron X-ray methods have been important in determining the positional parameters of the heavy elements (eg. Pb, Ca), neutron scattering methods have been essential in determining the positional parameters of the lighter elements (eg. H, F, O) in the presence of the heavy Pb atom. In this paper we present both the experimental results and discuss the possible reasons for the observed differences