The structure and solubility of carbonated hydroxyl and chloro lead apatites

Abstract

The properties of carbonated hydroxyl and chloro lead apatites, Pb 10(PO 4) 6(OH) 2 and Pb 10(PO 4) 6Cl 2, serve as models for the incorporation of carbonate into their medically important calcium analogs, and there is likely incorporation of carbonate in an insoluble lead phosphate phase during lead remediation. We have synthesized a series of carbonated lead hydroxyl- and lead chloro-apatites at 60–80 °C. The incorporation of carbonate into the apatite structure was documented by X-ray powder diffraction, IR and Raman spectroscopy, 207Pb solid state NMR spectroscopy, and elemental analysis. The carbonate content was determined by combustion analysis and confirmed by Raman spectroscopic analysis. As carbonate content increases in hydroxyl lead apatite, Raman spectra show changes in the phosphate stretching modes at 925 and 950 cm −1, an increase in intensity and downshift of a new peak at 1050 cm −1, and changes in the spectral features of the O–H stretch at about 3560 cm −1. The variation in unit cell parameters for the chloro lead apatite as a function of carbonate content is similar to that documented for B-type substitution in calcium apatites. The 207Pb NMR spectra corroborate B-type substitution. For the hydroxyl lead apatite, the changes in cell parameters suggest a combination of A- and B-type substitution. Solubilities of the carbonated lead apatites, determined by ICP-MS, increase slightly at low to moderate carbonate content, but more strongly at ca. 5.0 wt.% carbonate content. K sp values extrapolated to zero carbonate content reveal that the chloro lead apatite is indeed less soluble than the hydroxyl analog.