『Abstract
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 δP-31 = 0 and -6 ppm, 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 δP-31
= 2.65 ppm is observed, consistent with hydroxylapatite (Hap).
The NMR data indicate that after 30 days 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 2.65
ppm 31P peak correlates to a narrow 1H peak
at δH-1 = 0.2 ppm 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 15 min
at pH 9. These results that the xrystallization rate of Hap is
enhanced by the boehmite surface, although the detailed mechanisms
could bot be discerned from these data.』
1. Introduction
2. materials and methods
2.1. Materials and reagent
2.2. Sample preparation
2.3. 31P solid-state NMR
2.4. Powder X-ray diffraction
2.5. Scanning electron microscopy and energy-dispersive X-ray
spectroscopy
2.6. Speciation calculation
3. Results and discussion
3.1. Phosphorus speciation
3.2. Phosphate uptake
3.3. 31P single pulse NMR spectra
3.4. 31P{1H} CP/MAS spectra
3.5. 31P{1H} HetCor
3.6. Powder X-ray diffraction
3.7. 31P{27Al} REAPDOR
3.8. SEM and EDX analysis
3.9. Effect of reaction time
4. Conclusions
Acknowledgments
References