Martin A. A. Schoonen

The absolute energy positions of conduction and valence bands of selected semiconducting minerals

Abstract The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H2O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H2O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H2O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.

The structure of ferrihydrite, a nanocrystalline material.

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P63mc; a = ∼5.95 angstroms, c = ∼9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis δ-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.

Acid‐sulfate weathering of synthetic Martian basalt: The acid fog model revisited

The acid fog model has received considerable attention as a model of soil formation on Mars. Previous evaluations of this model have focused on experimental weathering of terrestrial basalt samples. However, these samples differ significantly from what now is thought to be typical of Martian basalt. The acid fog model is tested here using synthetic basaltic analogs derived from Mars Pathfinder soil and rock compositions. Reaction of synthetic basalt with various acidic solutions and subsequent evaporation has led to the formation of several putative secondary mineral phases. Many of these phases were not produced in prior experimental studies aimed at aqueous interactions on Mars. Of these alteration phases, Mg, Fe, Ca, and Al sulfates were identified. In addition, secondary ferric oxide phases formed via rapid Fe oxidation under relatively high pH levels buffered by basalt dissolution. Amorphous silica is a ubiquitous product in these experiments and has formed by precipitation from solution and by the dissolution of minerals and glasses leaving behind leached surface layers composed of residual silica. The secondary products formed in these experiments demonstrate the importance of primary mineralogy when testing models of aqueous interactions on Mars. New constraints are placed on both the reactivity of primary basalt and the secondary mineralogy present at the Martian surface. Copyright 2004 by the American Geophysical Union.

Surface Charge Development on Transition Metal Sulfides: An Electrokinetic Study

Abstract The isoelectric points, pH i.e.p. , of ZnS, PbS, CuFeS 2 , FeS, FeS 2 , NiS 2 , CoS 2 , and MnS 2 in NaCl supported electrolyte solutions are estimated to be between pH 3.3 and 0.6, with most of the isoelectric points below pH 2. The first electrokinetic measurements on NiS 2 , CoS 2 , and MnS 2 are reported here. Below pH i.e.p. the metal-sulfide surfaces are positively charged, above pH i.e.p. the surfaces are negatively charged. The addition of Me 2+ ions shifts the pH i.e.p. and changes the pH dependence considerably. The isoelectric points of the measured transition metal sulfides in the absence of metal ions or dissolved sulfide (H 2 S or HS − ) are in agreement with those found in earlier studies. The pH range of observed isoelectric points for metal sulfides (0.6–3.3) is compared to the considerably wider pH i.e.p. range (2–12) found for oxides. The correlation between pH i.e.p. and the electronegativities of the metal sulfides suggests that all metal sulfides will have an isoelectric point between pH 0.6 and 3.3. Compared to metal oxides, sulfides exhibit an isoelectric point that is largely independent of the nature of the metal cation in the solid.

Removal of dissolved oxygen from water : a comparison of four common techniques

Four common techniques for the removal of dissolved oxygen from water have been examined: boiling at 1 atm, boiling under reduced pressure, purging with N(2) and sonication under reduced pressure. After treatment, the residual oxygen in solution was analysed by the Winkler method. Nitrogen purging for 20-40 min at flow rate of 25 mL/s was found to be the most effective oxygen removal method. Boiling at 1 atm was found to be the least effective. None of the techniques evaluated here lead to complete removal of oxygen. The concentration of residual dissolved oxygen after purging for 20-40 minutes with nitrogen is 0.2-0.4 ppm.

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.

The stability of thiosulfate in the presence of pyrite in low-temperature aqueous solutions

The decomposition rate of thiosulfate (S2O32−) and the formation rates of its partial decomposition products, sulfite (SO32−), sulfate (SO42−), and tetrathionate (S4O62−), were measured in the absence and presence of pyrite in aqueous solution of pH 2.9–8.6 at 20°C. The pyrite-surface-catalyzed oxidation of S2O32− to S4062− by dissolved oxygen is the dominant S2O32− decomposition mechanism under the experimental conditions. The rate of tetrathionate formation is first order with respect to the pyrite surface concentration and has a fractional order (0 ≤ n ≤ 1) with respect to the S2032− concentration. This result is consistent with a surface-controlled heterogeneous mechanism and can be fitted with a Langmuir-Hinshelwood rate equation. The rate shows no pH dependence in the pH range between 2.9 and 6.6, but decreases in alkaline solution. The catalysis of pyrite in this reaction originates from its strong affinity for aqueous sulfur species and its semiconducting properties. Pyrite is thought to form an interfacial intermediate complex with the aqueous electron donor, S2O32−, on anodic sites, and the terminal electron acceptor, O2, on cathodic sites. The electrons can transfer from the anodic site to the cathodic site via the conduction band of pyrite. In essence, the presence of pyrite eliminates a symmetry mismatch between the frontier orbitals of thiosulfate and oxygen. In the absence of pyrite, this symmetry overlap precludes the progress of this reaction and thiosulfate decomposes via disproportionation to yield sulfite and elemental sulfur.

Surface structural controls on compositional zoning of SO2−4 and SeO2−4 in synthetic calcite single crystals

Abstract Coprecipitation experiments show that structural characteristics of growth surfaces on synthetic calcite single crystals are a primary control on SO2−4 and SeO2−4 incorporation. Electron probe microanalyses of sections through sectorally zoned crystals show that SO2−4 concentrations in 1014 sectors are 50% higher than in 0112 sectors. Electron probe and synchrotron X-ray fluorescence microanalyses within 1014 sectors document a twofold to threefold difference in SO2−4 and SeO2−4 contents between subsectors associated with nonequivalent vicinal faces of growth hillocks. This differential incorporation of SO2−4 and SeO2−4 documents path-dependent, nonequilibrium partitioning behavior. Models of nearest-neighbor and second-nearest-neighbor coordination environments of nonequivalent kink sites reveal differences in site sizes and geometries. Vicinal faces that consist of steps with larger and geometrically less constrained kink sites have greater SO2−4 and SeO2−4 incorporation, whereas vicinal faces with smaller, more constrained kink sites always have less SO2−4 and SeO2−4 incorporation. This correlation requires surface-structural, i.e., crystallographic, controls on SO2−4 and SeO2−4 incorporation and, therefore, is consistent with SO2−4 and SeO2−4 substitution for CO2−3. Our results clearly show that consideration of surface structural controls on trace ion partitioning is essential for a better understanding of trace ion studies in carbonate geochemistry and of crystal surface processes.

Pyrite-induced hydroxyl radical formation and its effect on nucleic acids

BackgroundPyrite, the most abundant metal sulphide on Earth, is known to spontaneously form hydrogen peroxide when exposed to water. In this study the hypothesis that pyrite-induced hydrogen peroxide is transformed to hydroxyl radicals is tested.ResultsUsing a combination of electron spin resonance (ESR) spin-trapping techniques and scavenging reactions involving nucleic acids, the formation of hydroxyl radicals in pyrite/aqueous suspensions is demonstrated. The addition of EDTA to pyrite slurries inhibits the hydrogen peroxide-to-hydroxyl radical conversion, but does not inhibit the formation of hydrogen peroxide. Given the stability of EDTA chelation with both ferrous and ferric iron, this suggests that the addition of the EDTA prevents the transformation by chelation of dissolved iron species.ConclusionWhile the exact mechanism or mechanisms of the hydrogen peroxide-to-hydroxyl radical conversion cannot be resolved on the basis of the experiments reported in this study, it is clear that the pyrite surface promotes the reaction. The formation of hydroxyl radicals is significant because they react nearly instantaneously with most organic molecules. This suggests that the presence of pyrite in natural, engineered, or physiological aqueous systems may induce the transformation of a wide range of organic molecules. This finding has implications for the role pyrite may play in aquatic environments and raises the question whether inhalation of pyrite dust contributes to the development of lung diseases.

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.