A P-31 AND H-1 MAS NMR-STUDY OF PHOSPHATE SORPTION ONTO CALCIUM-CARBONATE

The sorption of inorganic phosphate by two calcium carbonates, characterized by specific surface areas equivalent to 0.37 and 22 m2 g-1, is studied using 1H and 31P MAS NMR spectroscopy. The 31P MAS and CP-MAS NMR spectra of the low-phosphate-concentration (0.79 μmol P sorbed g-1 CaCO3) sample show that the sorbed phosphate is most likely unprotonated, but the 31P nucleus has a dipolar coupling to nearby protons. At this low concentration, the sorbed phosphate is not in the form of hydroxyapatite, nor does its spectrum resemble the 31P CP-MAS NMR spectra of amorphous calcium phosphates previously characterized by other investigators. The solid state 31P NMR spectra of samples with 3.33-36.72 μmol P sorbed g-1 CaCO3 suggest the formation of a carbonated, apatitic-like phase. At phosphate addition levels that exceeded the computed monolayer coverage on the calcium carbonate surface, an apatitic-like phase and brushite were detected using 31P CP-MAS NMR. The 1H MAS NMR spectra of the phosphorus-free, high-surface-area synthetic calcium carbonate sample show a peak at ca. 5 ppm due to physisorbed water and a sharp peak at 6.7 ppm that is tentatively assigned to HCO3- or CaOH at the surface. In the samples containing high levels of sorbed phosphate, the 1H MAS NMR spectra exhibit the characteristic spectral features of brushite. The sorption of phosphate onto calcium carbonate is described by fitting the NMR-characterized, surface-sorbed P species using the constant capacitance model. © 1992.