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.

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.

A new method for gaining insight into the chemistry of drying mineral surfaces using ATR-FTIR.

Although it is understood that the chemical environment at a drying surface is likely to be quite different from that at a fully hydrated surface, the difficulty of quantitative measurement has meant that this potentially crucial aspect of surface chemistry has gone largely overlooked. As a result, most of our understanding comes from measurement before and after drying, with a gray region of speculation in between. An interesting natural example is the paradoxical reduction of Mn oxides in moist soils as they dry, because drying is usually considered an oxidative process. This phenomenon indicates that important chemical changes are occurring during drying and an approach is needed to probe the chemistry of drying surfaces. Here we show the suitability of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy for real-time, in situ investigation of the drying solid-water interface, using the change in surface pH as an example. This was achieved by adsorbing thymol blue pH indicator (pK(a)=1.65) onto a natural Mn-rich clay and observing the real-time pH change, which dropped from pH 5 to below pH 1.65 with the removal of free water from the surface.

A novel vertical attenuated total reflectance photochemical flow-through reaction cell for Fourier transform infrared spectroscopy.

A unique photochemical cell design and two experiments are presented, which illustrate the usefulness of flow-through attenuated total reflectance (ATR) Fourier transform infrared (FT-IR) spectroscopy as a technique for investigating photochemical reactions at the mineral-water interface. The kinetics of the photolysis reaction of potassium oxalate (K(2)C(2)O(4)) in a ferric iron solution and oxalate adsorbed onto goethite (alpha-FeOOH) were investigated to show the capabilities of the cell. Due to complicated kinetics, the adsorption experiment demonstrates not only the types of complex problems, that may exist at the mineral-water interface, but also the ability for this novel cell design to address them.

Pyrite oxidation inhibition by a cross-linked lipid coating.

The effect of a diacetylene-containing phospholipid on the oxidation of pyrite, FeS2, was investigated. Earlier work reported by our research group showed that the adsorption of l,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine on pyrite suppressed the extent of its oxidation by about 75% over a specific time period. Results presented here show that the pre-exposure to UV radiation of this lipid after sorption onto pyrite results in a 90% suppression. Attenuated total reflection (ATR) Fourier transform infra-red spectroscopy (FTIR) suggests that the UV irradiation of the lipid does not result in degradation of the adsorbed layer. It is believed that the UV exposure results in the cross-linking and polymerization of the adsorbed phospholipid into a relatively impermeable barrier that separates the pyrite from the aqueous phase. The results of this study might have implications for the protection of pyrite from oxidation in the environment.