Solid Iron Oxide Research Articles

Fractionation of oxygen isotopes in phosphate during its interactions with iron oxides

Abstract Iron (III) oxides are ubiquitous in near-surface soils and sediments and interact strongly with dissolved phosphates via sorption, co-precipitation, mineral transformation and redox-cycling reactions. Iron oxide phases are thus, an important reservoir for dissolved phosphate, and phosphate bound to iron oxides may reflect dissolved phosphate sources as well as carry a history of the biogeochemical cycling of phosphorus (P). It has recently been demonstrated that dissolved inorganic phosphate (DIP) in rivers, lakes, estuaries and the open ocean can be used to distinguish different P sources and biological reaction pathways in the ratio of 18O/16O (δ18OP) in PO43−. Here we present results of experimental studies aimed at determining whether non-biological interactions between dissolved inorganic phosphate and solid iron oxides involve fractionation of oxygen isotopes in PO4. Determination of such fractionations is critical to any interpretation of δ18OP values of modern (e.g., hydrothermal iron oxide deposits, marine sediments, soils, groundwater systems) to ancient and extraterrestrial samples (e.g., BIF’s, Martian soils). Batch sorption experiments were performed using varied concentrations of synthetic ferrihydrite and isotopically-labeled dissolved ortho-phosphate at temperatures ranging from 4 to 95 °C. Mineral transformations and morphological changes were determined by X-Ray, Mossbauer spectroscopy and SEM image analyses. Our results show that isotopic fractionation between sorbed and aqueous phosphate occurs during the early phase of sorption with isotopically-light phosphate (P16O4) preferentially incorporated into sorbed/solid phases. This fractionation showed negligible temperature-dependence and gradually decreased as a result of O-isotope exchange between sorbed and aqueous-phase phosphate, to become insignificant at greater than ∼100 h of reaction. In high-temperature experiments, this exchange was very rapid resulting in negligible fractionation between sorbed and aqueous-phase phosphate at much shorter reaction times. Mineral transformation resulted in initial preferential desorption/loss of light phosphate (P16O4) to solution. However, the continual exchange between sorbed and aqueous PO4, concomitant with this mineralogical transformation resulted again in negligible fractionation between aqueous and sorbed PO4 at long reaction times (>2000 h). This finding is consistent with results obtained from natural marine samples. Therefore, 18O values of dissolved phosphate (DIP) in sea water may be preserved during its sorption to iron-oxide minerals such as hydrothermal plume particles, making marine iron oxides a potential new proxy for dissolved phosphate in the oceans.

Reduction of Solid Iron Oxide by Carbon

The reduction of pre‐reduced iron oxide by carbon was investigated from a kinetic viewpoint. The experimental procedure included thermogravimetric analysis to measure the weight change of the iron oxide pellet. The activation energy of the overall reaction was calculated to be 210 kJ/mol in the temperature range from 1273 to 1573 K. The overall reduction rate of FeO by carbon is believed to be mixed‐controlled by the Boudouard reaction and by the FeO reduction by CO gas in the temperature range investigated. The critical carbon concentration for the FeO reduction was estimated under the assumption that the activities of FeO and Fe are unity.

Semiconductivity and physicochemical properties of iron oxide-containing slags

Solid and liquid iron oxides exhibit characteristics of semiconductors. The semiconductive behavior also features in some slags containing iron oxide. This paper aims at highlighting some of the experimental indications of the role of semiconductivity in electrical conduction, the chemical diffusion and in reaction kinetics at slag surfaces. It is not a thorough review of the subject matter but an attempt at shedding some light on the connection between these properties.

Catalytic H2S Conversion and SO2 Production over Iron Oxide and Iron Oxide/γ-Al2O3 in Liquid Sulfur

A stirred-glass autoclave containing liquid sulfur and solid iron oxide catalyst was used to study low-tonnage sulfur recovery from H2S-containing gas streams. The objectives were to test the feasibility of using both liquid sulfur as a reaction medium and iron oxide as a direct oxidation catalyst for prolonged H2S conversion. Using a 1.60% H2S and 0.80% O2 (balance N2) feed gas, fresh iron oxide acted primarily as a scavenger for bulk H2S removal from the inlet gas stream. Following the scavenging phase, the steady-state iron oxide/sulfide was able to maintain low catalytic activity (30% conversion). The steady-state catalyst did, however, have a strong ability to generate significant amounts of SO2 in the presence of inlet feed O2. Data showed that this SO2 production resulted from the oxidation of the liquid sulfur over the steady-state iron oxide/sulfide. The rate of SO2 formation was shown to be directly proportional to the concentration of O2 in the inlet feed gas. Although H2S conversions over stea...

In Situ Observation of Reduction Behavior of Hematite with Solid Carbon and Crystallographic Orientation between Hematite and Magnetite

Recently, the composite pellets between iron ore and carbon are studied by many researchers because of its high reactivity. However, the reaction mechanism of the composite pellet is not yet clarified. It is obviously determined the reaction mechanism at initial stage as the direct reaction between solid carbon and iron oxide. However, in the course of reaction, the reaction mechanism will change to the indirect reduction of iron oxide with CO gas owing to the separation of the interface between solid carbon and iron oxide.In this study, samples of polycrystalline hematite (grain size was approximately 20 μm) were prepared. The hematite sample was coated by carbon with different thickness. The in situ observation by a laser microscope was carried out. The crystal orientation and morphology was investigated before and after the reaction using EBSP, SEM, AFM, XRD and Raman.The direct reduction began from about 650°C and the reaction proceeded rapidly until carbon was consumed. The behavior of reduction was different from the thickness of carbon layer (amount of carbon) which was related to whether the product gas (CO) could be passed through the carbon layer or not. After reduction, the lath magnetite having same direction was a grain was observed on the surface. The crystallographic orientation between magnetite and hematite was as follows;(0 0 0 1)H || (111)M, [1 1 0 0]H || [110]MIn addition, the growth direction of the lath magnetite formed on the hematite surface was [2 1 3 0]H || [101]M

Microbially Mediated Redox Cycling at the Oxic–Anoxic Boundary in Sediments: Comparison of Animal and Plants Habitats

Microorganisms are responsible for the bulk of transformations that occur in surficial sediments. They are most active at redox boundaries where they can benefit from access to various oxidants and reductants generated during redox cycling events. To illustrate the dynamics of microbially mediated processes, especially those involving sulfur and metal cycles, processes were compared in habitats either bioturbated by a capitellid worm or inhabited by a salt marsh grass. The presence of macrofauna and macroflora greatly altered the three-dimensional array of redox gradients in sediments, but the type and form of reductants and oxidants provided varied greatly; clastic sedimentary infauna subducted solid phase organic material and iron oxides, whereas plant roots released dissolved organic matter and oxygen. These differences resulted in a bioturbated system that exhibited a rapid sulfur cycle (residence time of minutes), but a slower iron cycle (days), whereas vegetation caused a slow sulfur cycle and rapid iron cycle. Alteration of sediments by higher life forms also greatly affected the composition and relative abundances of sedimentary bacteria, even on short time scales. Although redox cycling at interfaces can be somewhat predictable, variations in response to biological and physical perturbations demonstrated wide differences in the dynamics of redox-mediated processes.

Electron transfer in environmental systems: a frontier for theoretical chemistry

The advances in understanding the kinetic behavior of certain environmental electron transfer (ET) systems are presented. Emphasis is placed on the homogeneous ET chemistry of transition metals, particularly the FeII/III system, in various relevant forms. In the context of modern ET theory, we examine the utility of computational chemistry methods for the calculation of ET quantities such as the reorganization energy and electronic coupling matrix element. We discuss successful application of the methods to topics of homogeneous oxidation of dissolved metal ions by molecular oxygen in aqueous solution, as well as the prediction of electron mobility in solid phase iron oxide crystals. The examples illustrate the significant potential for many more advances in understanding environmental ET systems through the combination of ET theory and computational chemistry.

Measurement and interpretation of molecular-level forces of interaction between the siderophore azotobactin and mineral surfaces

The forces of interaction were measured between the siderophore azotobactin and the minerals goethite (α-FeOOH) and diaspore (α-AlOOH) in aqueous solution using force microscopy. Azotobactin, a pyoverdin-type siderophore, was covalently linked to a hydrazide terminated atomic force microscope tip using a standard active ester protein coupling technique. Upon contact with each mineral surface, the adhesion force between azotobactin and goethite was two to three times the value observed for the isostructural Al-equivalent diaspore. The affinity for the solid iron oxide surface reflected in the force measurements correlates with the specificity of azotobactin for aqueous ferric iron. Further, the adhesion force between azotobactin and goethite significantly decreases (4 nN to 2 nN) when small amounts of soluble iron (0.1 μM FeCl 3 · 6H 2O) are added to the system at pH 3.5 suggesting a significant specific interaction between the chelating reactive center of azotobactin and the mineral surface. Changes in the force signature with pH and ionic strength were fairly predictable when considering mineral solubility, the charge character of the mineral surfaces, the molecular structure of azotobactin, and the intervening solution. For example, azotobactin–goethite adhesion values were consistently smaller at pH 3.5 relative to the forces at pH 7. At the lower pH, the large number of protons and the increase in the mineral solubility provides additional electron acceptors (e.g., H + and Fe 3+(aq)) that are free to compete for the basic oxygen chelating sites in the azotobactin structure. It is believed that this competition disrupts siderophore affinity for the surface resulting in decreased adhesion values.

The influence of sulfides on soluble organic-Fe(III) in anoxic sediment porewaters

Solid and colloidal iron oxides are commonly involved in early diagenesis. More readily available soluble Fe(III) should accelerate the cycling of iron (Fe) and sulfur (S) in sediments. Experiments with synthetic solutions (Taillefert et al. 2000) showed that soluble Fe(III) (i.e., <50 nm diameter) reacts at a mercury voltammetric electrode at circumneutral pH if it is complexed by an organic ligand. The reactivity of soluble organic-Fe(III) with sulfide is greatly increased compared to its solid equivalent (e.g., amorphous hydrous iron oxides or goethite). We report here data from two different creeks of the Hackensack Meadowlands District (New Jersey) collected with solid state Au/Hg voltammetric microelectrodes and other conventional techniques, which confirm the existence of soluble organic-Fe(III) in sediments and its interaction with sulfide. Chemical profiles in these two anoxic sediments show the interaction between iron and sulfur during early diagenesis. Soluble organic-Fe(III) and Fe(II) are dominant in a creek where sulfide is negligible. This dominance suggests that the reductive dissolution of iron oxides goes through the dissolution of solid Fe(III), then reduction to Fe(II), or that soluble organic-Fe(III) is formed by chemical or microbial oxidation of organic-Fe(II) complexes. In a creek sediment where sulfide occurs in significant concentration, the reductive dissolution of Fe(III) is followed by formation of FeS(aq), which further precipitates. Dissolved sulfide may influence the fate of soluble organic-Fe(III), but the pH may be the key variable behind this process. The high reactivity of soluble organic-Fe(III) and its mobility may result in the shifting of local reactions, at depths where other electron acceptors are used. These data also suggest that estuarine and coastal sediments may not always be at steady state.

The evolution of multicomponent systems at high pressures: VI. The thermodynamic stability of the hydrogen-carbon system: the genesis of hydrocarbons and the origin of petroleum.

The spontaneous genesis of hydrocarbons that comprise natural petroleum have been analyzed by chemical thermodynamic-stability theory. The constraints imposed on chemical evolution by the second law of thermodynamics are briefly reviewed, and the effective prohibition of transformation, in the regime of temperatures and pressures characteristic of the near-surface crust of the Earth, of biological molecules into hydrocarbon molecules heavier than methane is recognized. For the theoretical analysis of this phenomenon, a general, first-principles equation of state has been developed by extending scaled particle theory and by using the technique of the factored partition function of the simplified perturbed hard-chain theory. The chemical potentials and the respective thermodynamic Affinity have been calculated for typical components of the H-C system over a range of pressures between 1 and 100 kbar (1 kbar = 100 MPa) and at temperatures consistent with those of the depths of the Earth at such pressures. The theoretical analyses establish that the normal alkanes, the homologous hydrocarbon group of lowest chemical potential, evolve only at pressures greater than approximately 30 kbar, excepting only the lightest, methane. The pressure of 30 kbar corresponds to depths of approximately 100 km. For experimental verification of the predictions of the theoretical analysis, a special high-pressure apparatus has been designed that permits investigations at pressures to 50 kbar and temperatures to 1,500 degrees C and also allows rapid cooling while maintaining high pressures. The high-pressure genesis of petroleum hydrocarbons has been demonstrated using only the reagents solid iron oxide, FeO, and marble, CaCO3, 99.9% pure and wet with triple-distilled water.

99/00666 Initiation of flame spreading on shallow sub-flash fuel layers

This is the first study in which the mechanism of burner slagging in a furnace firing pulverized coal has been investigated. The objective was to relate the in-flame transformations of the coal minerals to the nature of the slags formed on deposition probes designed to simulate a burner quarl and a superheater tube. The experiments were performed in the 2.5 MW refractory-lined IFRF furnace No. 1, using a swirl-stabilized pulverized coal burner, firing a pyrite-rich coal. In-flame samples and slag deposits were extracted and analysed for pyrite and pyrite decomposition products by electron probe microanalysis and X-ray diffraction. Immediately after injection into the flame, pyrite (FeS2) decomposed to pyrrhotite (FeS), molten droplets of which were oxidized to solid iron oxide (Fe3O4 and Fe2O3) particles in both the internal and external recirculation zones in the furnace. Owing to the complex flow pattern in the furnace, the kinetics of the pyrite conversion steps could not be directly determined but were indirectly derived from a comparison with the coal devolatilization and char oxidation steps. It is concluded that the decomposition of pyrite proceeds as fast as the coal devolatilization step and that the oxidation of pyrrhotite to iron oxide is as fast as the oxidation of the coal/char.

The Solubility of Oxygen in Liquid Iron Oxide During the Combustion of Iron Rods in High-Pressure Oxygen

The combustion of pure iron rods in oxygen is characterized by excess oxygen in the molten products. The amount of oxygen dissolved in the molten mass exceeds that required for stoichiometric haematite, Fe2O3, the highest oxidation state for solid iron oxide. Oxygen mass-balance calculations and post-test product analysis suggest that the average oxygen-to-iron molar ratio of the molten oxide product is as high as 2.1 and possibly higher. This corresponds to an increase in oxygen solubility in the molten iron oxide (over that of molten iron metal), which is compatible with the existence of various forms of ferrite ions such as FeO2−1, Fe2O5−4, FeO3−3, and Fe2O7−8. The presence of this excess oxygen confirms the proposed heterogeneous mechanism for the burning of iron, that is, that primary oxidation takes place at an interface between liquid iron and the molten combustion products containing the excess oxygen.

Coagulation as a structure-forming separation process in water and wastewater treatment

This paper presents the results of research on coagulation sludge properties in three waterworks. The investigation assumed: chemical composition, specific surface of coagulation structures, their sedimentation, gravity thickening and rheological parameters. The solid fraction of the examined structures consisted mostly of mineral substances, especially silica, aluminium and iron oxides. This composition is typical for most coagulation sludges formed in surface water treatment facilities. It has been determined, that the main factor determining structures properties is chemical composition of the solid fraction, mainly alum and iron oxides. Their presence in the sludge causes a high specific surface in coagulation structures, and significantly influences the rheological properties. It has been proved that non-Newtonian properties appear first in structures with the highest contents of alum and iron oxides, with limit concentrations of c. 1.5%. Investigations on sedimentation and gravitational thickening showed a high stability of coagulation structures vs. forces. The water amount in the investigated sludges during their compression was no lower than 94-98%, which means that gravitational forces did not destroy these structures. The factors stabilizing coagulation sludges are; fine dispersion of the solid phase and high affinity to water molecules. This is why in the range of hydrations lower than the compression hydration, the coagulation structures are classified as quasihomogenous systems consisting of two interpenetrating quasicontinuous media, which means; solution of liquid dispersed to colloidal and semicolloidal particles of the solid phase. They act on each other by intermolecular forces, which are the volume forces.

Coagulation as a structure-forming separation process in water and wastewater treatment

This paper presents the results of research on coagulation sludge properties in three waterworks. The investigation assumed: chemical composition, specific surface of coagulation structures, their sedimentation, gravity thickening and theological parameters. The solid fraction of the examined structures consisted mostly of mineral substances, especially silica, aluminium and iron oxides. This composition is typical for most coagulation sludges formed in surface water treatment facilities. It has been determined, that the main factor determining structures properties is chemical composition of the solid fraction, mainly alum and iron oxides. Their presence in the sludge causes a high specific surface in coagulation structures, and significantly influences the theological properties. It has been proved that non-Newtonian properties appear first in structures with the highest contents of alum and iron oxides, with limit concentrations of c. 1.5%. Investigations on sedimentation and gravitational thickening showed a high stability of coagulation structures vs. forces. The water amount in the investigated sludges during their compression was no lower than 94–98%, which means that gravitational forces did not destroy these structures. The factors stabilizing coagulation sludges are; fine dispersion of the solid phase and high affinity to water molecules. This is why in the range of hydrations lower than the compression hydration, the coagulation structures are classified as quasihomogenous systems consisting of two interpenetrating quasicontinuous media, which means; solution of liquid dispersed to colloidal and semicolloidal particles of the solid phase. They act on each other by intermolecular forces, which are the volume forces.

Flame transformations and burner slagging in a 2.5 MW furnace firing pulverized coal: 1. Flame transformations

This is the first study in which the mechanism of burner slagging in a furnace firing pulverized coal has been investigated. The objective was to relate the in-flame transformations of the coal minerals to the nature of the slags formed on deposition probes designed to simulate a burner quarl and a superheater tube. The experiments were performed in the 2.5 MW refractory-lined IFRF furnace No. 1, using a swirl-stabilized pulverized coal burner, firing a pyrite-rich coal. In-flame samples and slag deposits were extracted and analysed for pyrite and pyrite decomposition products by electron probe microanalysis and X-ray diffraction. Immediately after injection into the flame, pyrite (FeS 2) decomposed to pyrrhotite (FeS), molten droplets of which were oxidized to solid iron oxide (Fe 3O 4 and Fe 2O 3) particles in both the internal and external recirculation zones in the furnace. Owing to the complex flow pattern in the furnace, the kinetics of the pyrite conversion steps could not be directly determined but were indirectly derived from a comparison with the coal devolatilization and char oxidation steps. It is concluded that the decomposition of pyrite proceeds as fast as the coal devolatilization step and that the oxidation of pyrrhotite to iron oxide is as fast as the oxidation of the coal/char.

Characterization of arsenate adsorption on hydrous iron oxide using chemical and physical methods

The adsorption of arsenate by hydrous iron oxide was investigated by both chemical and physical energy dispersive analysis of X-rays (EDAX) and Fourier transform infrared (FTIR) methods. The chemical methods for characterizing arsenate adsorption sites on hydrous iron oxide surfaces in this work were investigations of the effect of ionic strength and zeta potential measurements of iron oxide systems containing various arsenate concentrations over the pH range 4–10. An inner-sphere complexation at the aqueous arsenate/iron oxide interface was identified from the results of the ionic strength effect and the shift in isoelectric point of the system. Evidence for direct coordination of arsenate to the surface of iron oxide was obtained by EDAX and FTIR techniques. Results from these physical methods showed that arsenate was chemisorbed on the iron oxide surface. Specific adsorption could therefore be deduced, on the basis of both chemical and physical methods, to occur at the aqueous arsenate and solid iron oxide interfaces.

Effect of Ferrocene on soot in a prevaporized iso-octane/air diffusion flame

Measurements were carried out to determine the effect on soot when ferrocene is added to a prevaporized isooctane/air diffusion flame. Data were obtained for operation of the flame above and below its smoke-point. Mie scattering was used to determine soot size, number density, and volume fraction with and without ferrocene. In addition, quartz microprobe sampling and gas chromatographic analysis were used to determine ferrocene's effect on soot precursor hydrocarbon and other species. The flame temperature was measured using a novel method based on infrared emission tomography. Ferrocene shortened the time for soot to first appear in the flame, and caused the formation of solid iron oxide particulates. It perturbed the particulate's size and number density in a complex manner in that increases and decreases of the latter were observed at early and late residence times, respectively. Ferrocene was very effective in and near the flame's terminus, thereby appearing to enhance soot burnout. On the whole, ferrocene did not significantly affect the soot precursor chemistry. An exception to this was the observed simultaneous decrease and increase of acetylene and molecular hydrogen mole fractions, respectively, which may be interpreted as an enhancement of acetylene oxidation. Solid effluent was collected above the flame and subjected to an Auger-type chemical analysis in order to look for coating of iron and/or its oxides by soot, which, if present, would support a catalytic mechanism for enhanced soot burnout with ferrocene seeding; the Auger data were not fully consistent with the aforesaid coating. Ferrocene did not affect flame temperature, which ruled out any thermally related explanation of its behavior in relation to soot suppression.

Absolute Light Scattering Calibration of the Stopped-Flow Spectrophotometer with Application to the Kinetics of Formation of Monodispersed Colloidal Hematite

A calibration method based on absolute intensities of scattered light from colloidal silica was developed to standardize the stopped-flow temperature jump spectrophotometer (SFTJ). The need for such calibration was exemplified by the study of the temperature effect on the Rayleigh ratio changes during the formation of solid iron (hydrous) oxides.

The stimulatory effects of asbestos on NADPH-dependent lipid peroxidation in rat liver microsomes.

Lipid peroxidation in rat liver microsomes induced by asbestos fibres, crocidolite and chrysotile, is greatly increased in the presence of NADPH, leading to malondialdehyde levels comparable with those induced by CCl4, a very strong inducer of lipid peroxidation. This synergic effect only occurs during the first minutes and could be explained by an increase or a regeneration of the ferrous active sites of asbestos by NADPH, which in turn could rapidly be prevented by the adsorption of microsomal proteins on the surface of the fibres. It is not inhibited by superoxide dismutase, catalase and mannitol, indicating that oxygen radicals are not involved in the reaction. It is also not inhibited by desferrioxamine, indicating that it is not due to a release of free iron ions in solution from the fibres. Lipid peroxidation in NADPH-supplemented microsomes is also greatly increased upon addition of magnetite. This could be linked to the presence of ferrous ions in this solid iron oxide, since the ferric oxides haematite and goethite are completely inactive.

Reducing Rates of Molten Iron Oxide by Solid Carbon or Carbon in Molten Iron

In order to collect basic data on the smelting reduction process which consists of blowing powder iron ore and powder coal into molten iron bath with oxygen gas for a partial combustion, the reduction rates of molten iron oxide by the solid carbon or the carbon in molten iron were measured. Molten iron oxide in a steel or an alumina crucible was reduced by a rotating carbon rod. The carbon in molten iron in an alumina crucible was reacted with molten iron oxide which was melted in a steel container beforehand. The reduction rates were calculated from the amount of CO gas evolved. The following results were obtained:(1) The reduction rates of molten iron oxide by the solid carbon were 0.21-0.82×10-4mol-FeO/cm2•s at 1420-1620°C, and the activation energy of the reaction were 75 and 31kcal/mol for a steel and an alumina crucibles, respectively.(2) The reduction rates of molten iron oxide by the carbon in molten iron were 1.1-3.3×10-4mol-FeO/cm2•s at 142O- 1620°C, and the activation energy of the reaction was 44kcal/mol.(3) It was concluded that the reaction rate between the solid iron oxide and the carbon in molten iron was the highest among reactions between the solid or the molten iron oxide and the solid carbon, the carbon in molten iron, CO, or H2 gas based on the results so far.