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

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 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.

Influence of pH on Initial Concentration Effect of Arsenate Adsorption on TiO2 Surfaces: Thermodynamic, DFT, and EXAFS Interpretations

“…This fragment was reasonably large to describe the typical adsorption sites for arsenate on Ti-(hydr)oxides. 1,3,5,21,22 The good agreement of calculated As-O and As-Ti distances with experimental EXAFS results implied that (100) surface was dominant in the TiO 2 powder used in this adsorption experiment. 10,14 There were two different oxygen atoms O(2) and O(3) in anatase TiO 2 , which were bonded by two and three titanium atoms, respectively.…”

“…1,[3][4][5][6][7][8] However, two crucial issues are neglected. First, the influence of adjacent surface functional group (-H 2 O and -OH) on adsorption reaction pathways and the stability of surface complex were not considered.…”

Soil Chemical Insights Provided through Vibrational Spectroscopy

“…Adamescu et al, 2010;Ha et al, 2008;Kubicki et al, 1999;Parikh et al, 2011) and inorganic (e.g. Baltrusaitis et al, 2007;Bargar et al, 2005;Paul et al, 2005) molecules with mineral surfaces. For example, quantum chemical calculations were used in conjunction with experimental collection of ATR-FTIR spectra to reveal that lactate binds to hematite nanoparticles through both outer-sphere and inner-sphere coordination .…”

Studies on the reaction pathway of arsenate adsorption at water–TiO2 interfaces using density functional theory

“…Electronic structure methods such as density functional theory (DFT) were widely used to calculate the adsorption structures of oxyanions on mineral surfaces [1][2][3][4]. However, the details of the formation of the surface complexes are little discussed.…”