Surface speciation of phosphate on Boehmite (γ-AlOOH) Determined from NMR Spectroscopy

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.