Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA.
Langmuir. 2010 Apr 6;26(7):4753-61. doi: 10.1021/la903484m.
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 (gamma-AlOOH) by solid-state (31)P 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 (31)P 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 (31)P{(1)H} cross-polarization magic-angle-spinning (CP/MAS) data, and yield nearly identical (31)P{(27)Al} 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 (31)P chemical shift anisotropy (CSA) and intensity variations with pH, the peak near 0 ppm being dominant at pH > 7. (31)P 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 (31)P NMR chemical shifts.
磷酸盐与粘土和金属氢氧化物表面的相互作用对于自然和农业系统中的养分循环很重要。我们通过固态（31）P NMR 光谱研究了磷酸盐对水铝石（γ-AlOOH）的特异性吸附，该光谱提供了存在两种不同质子化的桥联双齿表面络合物的证据。对于在 pH 5 下沿着吸附等温线制备的样品，在（31）P NMR 光谱中观察到两个主要的磷酸盐环境，表现为两个主要的峰（化学位移为 0 和-6 ppm），其相对强度随吸附剂负载变化很小。两个峰都对应于与 H 紧密接近的刚性磷酸盐，这一点由（31）P{(1)H}交叉极化魔角旋转（CP/MAS）数据表明，并且在旋转回波绝热通道双共振（REAPDOR）实验中产生几乎相同的（31）P{(27)Al}去相位曲线。REAPDOR 结果表明，两种磷酸盐环境与 Al 的配位相似，最佳拟合是通过对桥联双齿构型进行去相位曲线模拟得到的。两种分辨出的磷酸盐物种表现出不同的（31）P 化学位移各向异性（CSA）和强度随 pH 的变化，在 pH > 7 时，近 0 ppm 的峰占主导地位。水化双齿簇模型的量子化学计算表明，具有不同质子化状态的水合双齿簇模型的（31）P CSA，表明单质子化磷酸盐的 CSA 是独特的，并且与-6 ppm 处的峰非常匹配。0 ppm 处峰的 CSA 与二价和非质子化磷酸盐一致，但基于在高 pH 下制备的样品中该峰的主导地位以及（31）P NMR 化学位移的趋势，建议将其分配给后者。
