Michael J. Borda

A mechanism for the production of hydroxyl radical at surface defect sites on pyrite

Abstract A previous contribution from our laboratory reported the formation of hydrogen peroxide (H2O2) upon addition of pyrite (FeS2) to O2-free water. It was hypothesized that a reaction between adsorbed H2O and Fe(III), at a sulfur-deficient defect site, on the pyrite surface generates an adsorbed hydroxyl radical (OH•). ≡Fe(III) + H 2 O (ads) → ≡Fe(II) + OH • (ads) + H + The combination of two OH• then produces H2O2. In the present study, we show spectroscopic evidence consistent with the conversion of Fe(III) to Fe(II) at defect sites, the origin of H2O2 from H2O, and the existence of OH• in solution. To demonstrate the iron conversion at the surface, X-ray photoelectron spectroscopy (XPS) was employed. Using a novel mass spectrometry method, the production of H2O2 was evaluated. The aqueous concentration of OH• was measured using a standard radical scavenger method. The formation of OH• via the interaction of H2O with the pyrite surface is consistent with several observations in earlier studies and clarifies a fundamental step in the oxidation mechanism of pyrite.

Pyrite-Induced Hydrogen Peroxide Formation as a Driving Force in the Evolution of Photosynthetic Organisms on an Early Earth

The remarkable discovery of pyrite-induced hydrogen peroxide (H2O2) provides a key step in the evolution of oxygenic photosynthesis. Here we show that H2O2 can be generated rapidly via a reaction between pyrite and H2O in the absence of dissolved oxygen. The reaction proceeds in the dark, and H2O2 levels increase upon illumination with visible light. Since pyrite was stable in most photic environments prior to the rise of O2 levels, this finding represents an important mechanism for the formation of H2O2 on early Earth.

Effect of temperature and illumination on pyrite oxidation between pH 2 and 6

The effect of heat and illumination with visible light on the oxidation of pyrite with dissolved molecular oxygen in solutions between pH 2 and 6 has been investigated using a combination of surface science experiments and batch oxidation experiments. The rate of the oxidation of pyrite is strongly dependent on temperature. It is, however, not possible to cast the temperature dependence in a simple Arrhenius equation because the magnitude of the activation energy depends on the progress variable chosen. Activation energies based on proton release rate, sulfate release rate, and total iron release rate vary by as much as 40 kJ mol-1, suggesting that the oxidation mechanism of the sulfur and iron component of pyrite are largely independent of each other. This difference in mechanism can also explain why the reaction rates on the basis of these three different progress variables do not show the same pH dependence. Exposed to visible light, the rate of pyrite oxidation is under most conditions accelerated by less than a factor of two. Some of this acceleration may be accounted for by a light-induced heating of the pyrite surface. Surface science experiments employing photoelectron spectroscopy show no evidence for significant changes in the chemical composition of the surface as a function of exposure to visible light. The batch sorption experiments show, however, that the reaction stoichiometry changes somewhat, which indicates that there might be a change in reaction mechanism as a result of exposure to visible light.

Suppression of pyrite oxidation in acidic aqueous environments using lipids having two hydrophobic tails

Abstract Acid mine drainage (AMD) resulting from the oxidation of metal-sulfides, primarily pyrite, is a significant environmental problem. This environmental impact includes acidification of rivers and streams as well as leaching of toxic metals from the metal-sulfide material. This study shows that the rate of pyrite oxidation under low pH conditions can be reduced by as much as 80% by treating pyrite with lipids having two hydrocarbon tails per polar head group. The formation of a passivating layer on the pyrite surface after exposure to the lipid presumably suppresses pyrite oxidation by either interrupting the advection of aqueous oxidants or the electron transfer between oxidants and the pyrite. The results presented here may form the basis for a new AMD suppression strategy.

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.

A vibrational spectroscopic study of the oxidation of pyrite by ferric iron

Abstract The step-wise oxidation of the disulfide group during pyrite oxidation by ferric iron was investigated using in situ attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy. Pyrite oxidation by ferric iron was investigated at pH 2.0, in the dark, and with both ultraviolet and visible wavelength illumination. We concluded that the spectra collected during pyrite oxidation are consistent with the existence of more than a single sulfur species at the pyrite surface. Exact assignment of all of the sulfur species was not realistic. However, the assignment of sulfate in an outer-sphere complex and thiosulfate in a monodentate complex was possible. The presence of multiple sulfur species at the pyrite surface directly confirms the step-wise oxidation of the disulfide group, and the presence of sulfate suggests that this species is the surface release group. Photochemical experiments verify an increase in the rate of pyrite oxidation with illumination and suggest that either the mechanism or the rate of sulfur oxidation is affected by illumination

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.

Characterization of the Structure and the Surface Reactivity of a Marcasite Thin Film.

Abstract A thin film of marcasite, FeS 2 , was synthesized under vacuum and its structure and reactivity under oxidizing conditions was investigated by means of diffraction and surface analytical techniques, respectively. Synthesis of the film was carried out by codepositing Fe and S 2 onto a Ta support. The thickness of the film could be varied from approximately 10 A to 1 μm. High-resolution S 2p synchrotron-based photoemission showed S22−, with undetectable amounts of S 2− impurity that is typically present on natural sample surfaces. X-ray diffraction of the micron-thick films showed that the film crystallized in the marcasite phase of FeS 2 . Atomic force microscopy indicated that the thin film had a nanometer-scale roughness suggesting the film contained defects such as steps and kinks. X-ray photoelectron spectroscopy studies found the thin marcasite film to be more reactive than natural pyrite (the most ubiquitous FeS 2 dimorph) after exposure to a gaseous O 2 /H 2 O environment on the basis of the amount of sulfate formation. Likely the oxidation of marcasite was dominated by its short-range order (e.g., presence of steps), because the density of nonstoichiometric defect sites (e.g., S 2− ) was low as assessed by photoelectron spectroscopy.