Formation of hydroxylapatite from co-sorption of phosphate and calcium by boehmite

The interaction of calcium with phosphate at mineral/water interfaces is of importance for understanding both P sequestration and phosphate mineral formation. We investigated the effect of dissolved calcium on phosphate uptake by boehmite in batch sorption studies as a function of pH. Examination of the solids by ³¹P NMR spectroscopy and powder X-ray diffraction (XRD) shows evidence for formation of hydroxylapatite from pH 7 to pH 9, which is supported by correlation of Ca and P on particle surfaces at pH 9 observed by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) analysis. At pH 6, two major ³¹P NMR peaks are observed at δ _P-31=0 and -6ppm, indicating the presence of bidentate binuclear complexes with surface Al atoms, similar to those found in the absence of dissolved Ca. At higher pH, an additional ³¹P peak at δ _P-31=2.65ppm is observed, consistent with hydroxylapatite (Hap). The NMR data indicate that after 30days most of the phosphate (75%) remained as adsorption complexes at pH 7, but that Hap accounts for most of the phosphate at higher pH, although surface complexes were still evident in CP/MAS NMR spectra. The identification of crystalline Hap is further supported by ³¹P{ ¹H} heteronuclear correlation (HetCor) experiments in which the 2.65ppm ³¹P peak correlates to a narrow ¹H peak at δ _H-1=0.2ppm that is diagnostic of the hydroxyl groups of Hap. In powder X-ray diffraction patterns, two small peaks are observed at slow scan rates that match the major reflections of Hap. Nonetheless, Hap crystals could not be identified in SEM images suggesting small particle size, in agreement with broad XRD peaks. At short reaction times only adsorbed phosphate is observed at pH 7, whereas Hap forms within 15min at pH 9. These results indicate that the crystallization rate of Hap is enhanced by the boehmite surface, although the detailed mechanisms could not be discerned from these data.