Paul Kristian W, Borda Michael J, Kubicki James D, Sparks Donald L
Department of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, Delaware 19716, USA.
Langmuir. 2005 Nov 22;21(24):11071-8. doi: 10.1021/la050648v.
The effect of dehydration on the coordination and speciation of sulfate at the Fe-(hydr)oxide-H2O interface was investigated using molecular orbital/density functional theory (MO/DFT) and Fourier transform infrared (FTIR) spectroscopy. IR frequency calculations were performed at the UB3LYP/6-31+G(d) level of theory for potential sulfate (bidentate bridging, monodentate, and H-bonded) and bisulfate (bidentate bridging and monodentate) surface complexes. MO/DFT calculated IR frequencies were compared to available IR literature results and attenuated total reflectance (ATR) FTIR spectra collected in our laboratory of sulfate adsorbed at the hematite-H2O interface. IR frequency calculations performed using the larger 6-311+G(d,p) basis set resulted in minor frequency shifts that did not dramatically alter the agreement with experiment. This investigation proposes that sulfate undergoes a speciation change as a function of surface dehydration. A generalized model for the speciation change is proposed as follows. (1) At the Fe-(hydr)oxide-H2O interface, sulfate adsorbs as a bidentate bridging or monodentate surface complex under most experimental conditions. (2) Upon surface dehydration, sulfate changes speciation to form bidentate bridging and/or monodentate bisulfate. However, surface dehydration does not yield 100% speciation change but leads to a mixture of sulfate and bisulfate. (3) The speciation change is reversible as a function of rehydration. The reversibility of the sulfate-bisulfate speciation change is chiefly determined by the local hydration environment of the O-H bond in bisulfate. Under dehydrated conditions, the O-H bond length is approximately 0.98 A. The bond length substantially increases (bond strength decreases) to approximately 1.03 A when the initial H-bond network is re-established through hydration, likely leading to deprotonation upon full mineral surface hydration.
利用分子轨道/密度泛函理论（MO/DFT）和傅里叶变换红外（FTIR）光谱研究了脱水对铁（氢）氧化物 - 水界面处硫酸盐的配位和形态的影响。在UB3LYP/6 - 31 + G(d)理论水平上对潜在的硫酸盐（双齿桥连、单齿和氢键结合）和硫酸氢盐（双齿桥连和单齿）表面配合物进行了红外频率计算。将MO/DFT计算的红外频率与现有的红外文献结果以及我们实验室收集的赤铁矿 - 水界面吸附硫酸盐的衰减全反射（ATR）FTIR光谱进行了比较。使用较大的6 - 311 + G(d,p)基组进行的红外频率计算导致频率有微小偏移，但并未显著改变与实验结果的一致性。本研究表明，硫酸盐会随着表面脱水而发生形态变化。提出了一个关于形态变化的通用模型如下：（1）在铁（氢）氧化物 - 水界面，在大多数实验条件下，硫酸盐以双齿桥连或单齿表面配合物的形式吸附。（2）表面脱水后，硫酸盐形态发生变化，形成双齿桥连和/或单齿硫酸氢盐。然而，表面脱水并不会导致100%的形态变化，而是会产生硫酸盐和硫酸氢盐的混合物。（3）形态变化是可逆的，其取决于再水化过程。硫酸盐 - 硫酸氢盐形态变化的可逆性主要由硫酸氢盐中O - H键的局部水化环境决定。在脱水条件下，O - H键长度约为0.98 Å。当通过水化重新建立初始氢键网络时，键长显著增加（键强度降低）至约1.03 Å，这可能导致在矿物表面完全水化时发生去质子化。
