『(Abstract)
Interaction of phosphate with the surfaces of clays and metal
oxyhydroxides is important for nutrient cycling in natural and
agricultural systems. We examined the specific adsorption of phosphate
to boehmite (γ-AlOOH) by solid-state 31P NMR spectroscopy,
which yields evidence for the presence of two bridging bidentate
surface complexes differing in protonation. For samples prepared
along the sorption isotherm at pH 5, distinct phosphate environments
are observed as two major peaks in 31P NMR spectra
(chemical shifts of 0 and -6 ppm) that show little change in relative
intensity with adsorbate loading. Both peaks correspond to rigid
phosphate in close proximity to H, as indicated by 31P{1H}
cross-polarization magic-angle-spinning (CP/MAS) data, and yield
nearly identical 31P{27Al} dephasing curves
in rotational echo adiabatic passage double resonance (REAPDOR)
experiments. The REAPDOR results indicate that both phosphate
environments have similar coordination to Al and are best fit
by dephasing curves simulated for bridging bidentate configurations.
The two resolved phosphate species exhibit distinct 31P
chemical shift anisotropy (CSA) and intensity variations with
pH, the peak near 0 ppm being dominant at pH>7. 31P
CSA's from quantum chemical calculations of hydrated bidentate
cluster models with varying protonation state show that the CSA
for monoprotonated phosphate is unique and closely matches that
for the peak at -6 ppm. The CSA for the peak at 0 ppm is consistent
with both di- and nonprotonated phosphate, but assignment to the
latter is suggested based on the dominance of this peak in samples
prepared at high pH and with trends in 31P NMR chemical
shifts.』
1. Introduction
2. Materials and methods
2.1. Materials
2.2. Phosphate sorption
2.3. NMR data collection
2.4. NMR shielding calculations
3. Results
3.1. Phosphate adsorption
3.2. 31P SP/MAS NMR spectra
3.3. 31P{1H} CP/MAS results
3.4. 31P{1H}-27Al REAPDOR and
TRAPDOR results
3.5. 31P chemical shift anisotropy
4. Discussion
Acknowledgment
Supporting information available