Web Release Date: October 13,
Effect of Dehydration on Sulfate Coordination and Speciation at the Fe-(Hydr)oxide-Water Interface: A Molecular Orbital/Density Functional Theory and Fourier Transform Infrared Spectroscopic Investigation
and
Department of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, Delaware 19716, and Department of Geosciences and Earth & Environmental Systems Institute, The Pennsylvania State University, University Park, Pennsylvania 16802
Received March 9, 2005
In Final Form: September 7, 2005
Abstract:
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 Å. The bond length substantially increases (bond strength decreases) to approximately 1.03 Å when the initial H-bond network is re-established through hydration, likely leading to deprotonation upon full mineral surface hydration.
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