Daniel R. Strongin

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.

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.

The importance of C7 sites and surface roughness in the ammonia synthesis reaction over iron

In this note the authors present data which further supports the contention that C/sub 7/ sites (Fe atoms with seven nearest neighbors) are those most active in iron ammonia synthesis catalysts. Previous work in their laboratory showed that the synthesis of ammonia from its elements is a very structure sensitive reaction. It was found that the relative order of activity for ammonia formation was Fe(111) > Fe(100) > Fe(110). High pressure kinetic results presented here of ammonia synthesis over Fe(210) and Fe(211) surfaces in addition to the Fe(111), Fe(100), and Fe(110) surfaces show that the presence of highly coordinated sites is more important than surface roughness for high catalytic activity. 11 references.

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 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.

The effects of potassium on ammonia synthesis over iron single-crystal surfaces

Abstract The effects of potassium on ammonia synthesis over model iron single-crystal catalysts of (111), (100), and (110) orientation have been studied under high-pressure reaction conditions (20 atm reactant pressure of nitrogen and hydrogen). Under these conditions, no more than 0.15 monolayers (ML) of potassium can be stabilized on the iron surfaces. The Fe(110) surface shows no activity for ammonia synthesis in this study with or without adsorbed potassium. The presence of potassium on the Fe(111) and Fe(100) surfaces increases the rate of ammonia synthesis markedly. At a low reaction conversion of 0.3% the rate over Fe(111) and Fe(100) is enhanced by a factor of two in the presence of potassium. The effect of potassium on Fe(111) and Fe(100) is enhanced as higher reaction conversions (i.e., increasing ammonia partial pressures) are achieved because potassium induces changes in the reaction orders for both ammonia and hydrogen. No change in the activation energy for the reaction is observed with potassium, suggesting that the reaction mechanism has not been altered. Temperature-programmed desorption shows that the adsorption energy of ammonia is significantly reduced when coadsorbed with potassium. A model is proposed in which the decrease of ammonia adsorption energy, induced by potassium, reduces the concentration of ammonia on the iron surface. This effect decreases the number of active sites blocked by the ammonia product, thereby increasing the rate of ammonia synthesis. The model also suggests that an additional effect of potassium is to increase the rate of nitrogen dissociative chemisorption by about 30% over Fe(111) and Fe(100) under ammonia synthesis conditions.

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.

An introduction to geocatalysis

Abstract Heterogeneous catalysis at mineral surfaces, or geocatalysis, may be important in natural systems. This contribution reviews how catalyst may influence reactions and several examples, mostly taken from the geochemical literature, are presented to illustrate the most important concepts. Two key characteristics that define a catalyst are activity and selectivity. Activity is the extent to which the catalyst can facilitate the rate of conversion of reactant to product. The selectivity of the catalyst refers to the ability of a catalyst to facilitate a specific reaction pathway where a reaction may proceed via multiple pathways. Photocatalysis is presented as a separate class of catalysis because reactions are only catalyzed when the system is illuminated with light of suitable wavelength. Three mechanisms of photocatalysis (photolysis, photoelectrochemical reaction, and charge injection) are discussed. Geocatalysis is particularly important for reactions with high activation energies, such as isotopic exchange reactions involving SO 4 –H 2 S, H 2 O–SO 4 , H 2 O–PO 4 , CO 2 –CH 4 and transformations of humic acids and kerogen. While photocatalysis is limited to surficial aquatic environments, it may be of great importance in regulating the concentrations of redox-sensitive elements in the photic zones of lakes, streams, hot spring, and oceans. Photoelectrochemical reactions, such as the reduction of dinitrogen and carbon dioxide are thought to have been important in the origin of life on Earth. Geocatalysis may also prove to be useful in designing new environmental remediation technologies. For example, it is shown that sphalerite and ilmenite are capable of degrading chlorinated carbon compounds via a photocatalytic mechanism.

Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200°C), borosilicate septum bottles (70°C) and HDPE tubes (22°C) in the presence of Fe and Ni metal, awaruite (Ni80Fe20) and tetrataenite (Ni50Fe50). In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites) were found to contain up to 22 ppm Ntot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N3-). This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic) ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts.

ATR-FTIR and density functional theory study of the structures, energetics, and vibrational spectra of phosphate adsorbed onto goethite.

Periodic plane-wave density functional theory (DFT) and molecular cluster hybrid molecular orbital-DFT (MO-DFT) calculations were performed on models of phosphate surface complexes on the (100), (010), (001), (101), and (210) surfaces of α-FeOOH (goethite). Binding energies of monodentate and bidentate HPO(4)(2-) surface complexes were compared to H(2)PO(4)(-) outer-sphere complexes. Both the average potential energies from DFT molecular dynamics (DFT-MD) simulations and energy minimizations were used to estimate adsorption energies for each configuration. Molecular clusters were extracted from the energy-minimized structures of the periodic systems and subjected to energy reminimization and frequency analysis with MO-DFT. The modeled P-O and P---Fe distances were consistent with EXAFS data for the arsenate oxyanion that is an analog of phosphate, and the interatomic distances predicted by the clusters were similar to those of the periodic models. Calculated vibrational frequencies from these clusters were then correlated with observed infrared bands. Configurations that resulted in favorable adsorption energies were also found to produce theoretical vibrational frequencies that correlated well with experiment. The relative stability of monodentate versus bidentate configurations was a function of the goethite surface under consideration. Overall, our results show that phosphate adsorption onto goethite occurs as a variety of surface complexes depending on the habit of the mineral (i.e., surfaces present) and solution pH. Previous IR spectroscopic studies may have been difficult to interpret because the observed spectra averaged the structural properties of three or more configurations on any given sample with multiple surfaces.