Showing papers on "Iron oxide published in 2002"

Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides.

Abstract: After the evolution of oxygen-producing cyanobacteria at some time before 2.7 billion years ago, oxygen production on Earth is thought to have depended on the availability of nutrients in the oceans, such as phosphorus (in the form of orthophosphate). In the modern oceans, a significant removal pathway for phosphorus occurs by way of its adsorption onto iron oxide deposits. Such deposits were thought to be more abundant in the past when, under low sulphate conditions, the formation of large amounts of iron oxides resulted in the deposition of banded iron formations. Under these circumstances, phosphorus removal by iron oxide adsorption could have been enhanced. Here we analyse the phosphorus and iron content of banded iron formations to show that ocean orthophosphate concentrations from 3.2 to 1.9 billion years ago (during the Archaean and early Proterozoic eras) were probably only approximately 10-25% of present-day concentrations. We suggest therefore that low phosphorus availability should have significantly reduced rates of photosynthesis and carbon burial, thereby reducing the long-term oxygen production on the early Earth--as previously speculated--and contributing to the low concentrations of atmospheric oxygen during the late Archaean and early Proterozoic.

Arsenic(III) and arsenic(V) reactions with zerovalent iron corrosion products.

Abstract: Zerovalent iron (Fe0) has tremendous potential as a remediation material for removal of arsenic from groundwater and drinking water. This study investigates the speciation of arsenate (As(V)) and arsenite (As(III)) after reaction with two Fe0 materials, their iron oxide corrosion products, and several model iron oxides. A variety of analytical techniques were used to study the reaction products including HPLC-hydride generation atomic absorption spectrometry, X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray analysis, and X-ray absorption spectroscopy. The products of corrosion of Fe0 include lepidocrocite (γ-FeOOH), magnetite (Fe3O4), and/or maghemite (γ-Fe2O3), all of which indicate Fe(II) oxidation as an intermediate step in the Fe0 corrosion process. The in-situ Fe0 corrosion reaction caused a high As(III) and As(V) uptake with both Fe0 materials studied. Under aerobic conditions, the Fe0 corrosion reaction did not cause As(V) reduction to As(III) but did cause As(III) oxidation ...

Reduction of Polyhalogenated Methanes by Surface-Bound Fe(II) in Aqueous Suspensions of Iron Oxides

Abstract: Uptake of ferrous iron from aqueous solution by iron oxides results in the formation of a variety of reactive surface species capable of reducing polyhalogenated methanes (PHMs). Pseudo-first-order reaction rate constants, kobs, of PHMs increased in the order CHBrCl2 < CHBr2Cl < CHBr3 < CCl4 < CFBr3 < CBrCl3 < CBr2Cl2. The kobs values increased with the exposure time, teq, of Fe(II) to suspended iron oxides which was attributed to the rearrangement of initially sorbed Fe(II) species to more reactive surface species with time. At pH 7.2, the kobs values of PHMs also increased with the concentration of surface-bound ferrous iron, Fe(II)sorb, particularly when Fe(II)tot was increased to concentrations where surface precipitation becomes likely. At fixed total Fe(II) concentrations, kobs values increased exponentially with pH. The highest reactivities were associated with pH conditions where surface precipitation of Fe(II) is expected. Fe(II)sorb and pH, however, had opposite effects on the product formation ...

Interactions of CO2 and CO at fractional atmosphere pressures with iron and iron oxide surfaces: one possible mechanism for surface contamination?

Abstract: The interactions of carbon dioxide and carbon monoxide at moderately high pressures with clean iron surfaces have been studied using x-ray photoelectron spectroscopy (XPS) and static time-of-flight secondary ion mass spectrometry (ToF-SIMS). Clean iron surfaces were analysed after exposure to CO2 or CO gas having pressures of 10–104 Pa at 25 °C. Exposure to either gas results first in a thin surface film of FeO. In addition, this oxide is completely or partially covered with a carbonaceous layer. The thickness of the carbon layer is influenced by the type of gas used and its pressure. The ToF-SIMS studies using isotopically labelled 13CO2 under the same reaction conditions show that the labelled carbon is not enriched on the surface and therefore no direct reaction occurs between the labelled carbon and the surface. It is proposed that the influence of either gas on the carbonaceous layer formed may be indirect: these gases may alter the equilibrium partial pressures of other adsorbates on the walls of the vacuum chamber. Copyright © 2002 John Wiley & Sons, Ltd.

Gas and humidity sensors based on iron oxide–polypyrrole nanocomposites

Abstract: Nanocomposites of iron oxide and polypyrrole were prepared by simultaneous gelation and polymerization process. This resulted in the formation of mixed iron oxide phase for lower polypyrrole concentration, stabilizing to a single cubic iron oxide phase at higher polypyrrole concentration. The composites in the pellet form were used for humidity and gas sensing investigations. Their sensitivity to humidity was found to increase with increasing concentration of polypyrrole. Gas sensing was performed for CO2, N2 and CH4 gases at varying pressures. The sensors showed a linear relationship between sensitivity and pressures for all the gases studied. The sensors showed highest sensitivity to CO2 gas.

Synthesis, characterization, and catalytic performance of single-site iron(III) centers on the surface of SBA-15 silica.

Abstract: A new molecular precursor strategy has been used to prepare a series of single-site catalysts that possess isolated iron centers supported on mesoporous SBA-15 silica. The iron centers were introduced via grafting reactions of the tris(tert-butoxy)siloxy iron(III) complex Fe[OSi(O(t)Bu)(3)](3)(THF) with SBA-15 in dry hexane. This complex reacts cleanly with the hydroxyl groups of SBA-15 to eliminate HOSi(O(t)Bu)(3) (as monitored by (1)H NMR spectroscopy) with formation of isolated surface species of the type identical with SiO-Fe-[OSi(O(t)Bu)(3)](2)(THF). In this way, up to 21% of the hydroxyl sites on SBA-15 were derivatized (0.23 Fe nm(-)(2)), and iron loadings in the range of 0.0-1.90% were achieved. The structure of the surface-bound iron species, as determined by spectroscopic methods (electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), UV-vis, and in situ infrared measurements) and by elemental analyses, contains a pseudotetrahedral iron(III) center. The THF ligand of this surface-bound complex was quantitatively displaced by acetonitrile (by (1)H NMR spectroscopy). Calcination of these materials at 300 degrees C for 2 h under oxygen resulted in removal of all organic matter and site-isolated iron surface species that are stable to condensation to iron oxide clusters. Spectroscopic data (UV-vis and EPR) suggest that the iron centers retain a mononuclear, pseudotetrahedral iron(III) structure after calcination. The calcinated, iron-grafted SBA-15 materials exhibit high selectivities as catalysts for oxidations of alkanes, alkenes, and arenes, with hydrogen peroxide as the oxidant.

Occurrence and prevention of photodissolution at the phase junction of magnetite and titanium dioxide

Abstract: A stable magnetic photocatalyst was prepared by coating a magnetic core with a layer of photoactive titanium dioxide. A direct deposition of titanium dioxide onto the surface of magnetic iron oxide particles proved ineffective in producing a stable magnetic photocatalyst, with high levels of photodissolution being observed with these samples. This observed photodissolution is believed to be due to the dissolution of the iron oxide phase, induced by the photoactive the titanium dioxide layer due to electronic interactions at the phase junction in these magnetic photocatalysts. The introduction of an intermediate passive SiO2 layer between the titanium dioxide phase and the iron oxide phase inhibited the direct electrical contact and hence prevented the photodissolution of the iron oxide phase. The magnetic photocatalyst is for use in slurry-type reactors from which the catalyst can be easily recovered by the application of an external magnetic field.

Sonochemical Synthesis of Mesoporous Iron Oxide and Accounts of Its Magnetic and Catalytic Properties

Abstract: Synthesis of mesoporous iron oxides has been reported using a sonochemical technique. Iron(III) ethoxide was used as an inorganic precursor and CTAB as an organic structure directing agent. After sonication the surfactant was removed by calcination and solvent extraction methods. FTIR spectra demonstrated the removal of the surfactant from the pores of the mesoporous iron oxide. The calcinated material was characterized by XRD, TEM, TGA, and BET surface area measurements. The surface area after solvent extraction is found to be 274 m2/g. The magnetic and catalytic properties of the materials have been studied. The as-prepared amorphous Fe2O3 has shown a paramagnetic behavior, while after calcination at 350 °C it changes to γ-Fe2O3 with good magnetic properties. The catalytic activity of mesoporous iron oxide was studied in the reaction of cyclohexane oxidation under mild conditions. The mesoporous Fe2O3 catalyst has shown high conversion of cyclohexane into cyclohexanone and cyclohexanol, with a high sele...

Arsenic Removal Using Iron Oxide Loaded Alginate Beads

Abstract: The application of biopolymers (alginate), as sorbent supports, for the removal of arsenic from contaminated water has been investigated in the present study. Calcium alginate beads were placed in a column to form a fixed bed and treated (doped/coated) with hydrous ferric oxides. Three different types of modified alginate beads were examined for the removal of arsenic; the most efficient type was found to be doped with alginate and subsequently coated with iron oxides, whereas the other two types were calcium alginate beads doped or coated with iron oxides. The total amount of iron loaded on this material was found to be 3.9 mg of Fe/g of wet alginate bead. Approximately 230 bed volumes of a 50 μg/L As(V) solution were treated before the breakthrough point was reached, whereas the removal of As(III) was not as efficient, reaching the breakthrough point after the treatment of only 45 bed volumes. The results were modeled using the bed depth service time and empty bed residence time models.

Implications of heat treatment on the properties of a magnetic iron oxide–titanium dioxide photocatalyst

Abstract: Magnetic iron oxide–titania photocatalysts (Fe 3 O 4 –TiO 2 ) were prepared using a coating technique in which the photoactive titanium dioxide was deposited onto the surface of a magnetic iron oxide core. These particles had a core–shell structure. The prepared particles were heat treated at high temperature in order to transform the amorphous titanium dioxide into a photoactive crystalline phase. The heat treatment temperature and duration were varied, and the correlation between the heat treatment and the observed photoactivities was investigated. An increase in the applied heat treatment, either by increasing the temperature or increasing the heat treatment duration, led to a decrease in the activities of the catalyst particles. A decrease in surface area due to sintering, along with the diffusion of Fe ions into the titanium dioxide coating are seen as contributing factors to the decline in photoactivity which accompanied an increase in the heat treatment. Differential scanning calorimetry analysis (DSC) results confirmed that the presence of the iron oxide core did not have an effect on the phase transformation of titania under the experimental conditions investigated. In this study we also present surface charge measurements which show that the surface of the particles became more positive as the heat treatment was increased. This is an indication of changing surface properties as heat treatment is applied. For single-phase TiO 2 powders, this is postulated to be due to a decrease in the surface hydroxyl (OH) groups and/or residual organics (OR) groups. For the Fe 3 O 4 –TiO 2 powders, in addition to the loss of OH and OR groups, the diffusion of the Fe into the titania shell is postulated to also play a role in the changing surface properties with applied heat treatment. Experiments aimed at reducing the duration of the heat treatment revealed that a heat treatment duration of 20 min at 450 °C was sufficient to transform amorphous titanium dioxide into a photoactive crystalline phase. This does not only minimise loss of surface hydroxyl groups but it also has the potential to limit the oxidation of the magnetic core, which occurs due to the porosity of the coating. This has practical implications in terms of separating the magnetic particles from the treated waste waters under the application of an external magnetic field. It also presents an opportunity to produce photoactive composite particles while limiting the interactions between the core and the shell during the heat treatment.

SrFeO3-δ Perovskite Oxides: Chemical Features and Performance for Methane Combustion

Abstract: Oxygen-deficient SrFeO3-δ (0.02 < δ < 0.26) perovskites were prepared by soft-chemistry procedures, followed by annealing under different conditions, including high oxygen pressure. These materials contain Fe cations in the mixed Fe3+−Fe4+ valence state. The samples were characterized by X-ray and neutron powder diffraction, thermal analysis under reducing conditions, temperature-programmed reduction and desorption, specific surface area measurements, and XPS. The most deficient oxide, SrFeO2.74, shows a complex neutron diffraction diagram corresponding to a superstructure of perovskite in which half of the Fe cations are pentacoordinated to oxide anions in a pyramidal configuration. By contrast, the other half are octahedrally coordinated. The materials were tested as catalysts for methane oxidation. At moderate temperatures, a significantly higher catalytic activity was observed for the most oxygen-deficient sample, which is consistent with the oxygen desorption ability exhibited by this material, conta...

Analytical utility of valence band X-ray photoelectron spectroscopy of iron and its oxides, with spectral interpretation by cluster and band structure calculations

Abstract: The valence band and core-level X-ray photoelectron spectroscopy (XPS) of iron and its oxides are reported, and the valence band spectra interpreted by various calculation models. The paper focuses upon the valence band region, which shows significant differences between the metal and the following oxidized iron species: FeO, Fe3O4, α-Fe2O3, γ-Fe2O3, α-FeOOH and γ-FeOOH. The core region is of little analytical value as a means of distinguishing between these species, but the valence band region shows significant differences. These differences are consistent with spectra predicted by cluster and band structure calculations. Cluster calculations are valuable as a means for interpreting the spectra of iron oxides with multiple iron sites and defect characteristics.

Intracellular Iron Minerals in a Dissimilatory Iron-Reducing Bacterium

Abstract: Among prokaryotes, there are few examples of controlled mineral formation; the formation of crystalline iron oxides and sulfides [magnetite (Fe3O4) or greigite (Fe3S4)] by magnetotactic bacteria is an exception. Shewanella putrefaciens CN32, a Gram-negative, facultative anaerobic bacterium that is capable of dissimilatory iron reduction, produced microscopic intracellular grains of iron oxide minerals during growth on two-line ferrihydrite in a hydrogen-argon atmosphere. The minerals, formed at iron concentrations found in the soil and sedimentary environments where these bacteria are active, could represent an unexplored pathway for the cycling of iron by bacteria.

Influence of the silica based matrix on the formation of iron oxide nanoparticles in the Fe2O3–SiO2 system, obtained by sol–gel method

Abstract: Composite iron oxide–SiO2 materials were prepared by a sol–gel method starting with two types of precursors, tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS), as the SiO2 source. As the iron source a soluble Fe2+ salt, mainly Fe(SO4)2·7H2O, was used, the iron oxides were generated during the sol–gel process. The amorphous gels obtained were thermally treated up to 1000 °C in order to obtain iron oxides with different structures and grain size. The initial gels and the thermally treated samples were characterised by DTA/TGA analysis, DR-UV–VIS and IR-spectroscopy, EPR measurements, transmission electron microscopy (TEM) and BET surface area methods. The matrices obtained from the precursors play a major role in the evolution of the process. In both cases the initial gels are amorphous. In the non-porous matrix obtained by thermal treatment using methyltriethoxysilane (MTEOS), the tendency for crystallisation increases, and the iron oxide particle size is increased.

Association of uranium with iron oxides typically formed on corroding steel surfaces

Abstract: Decontamination of metal surfaces contaminated with low levels of radionuclides is a major concern at Department of Energy facilities. The development of an environmentally friendly and cost-effective decontamination process requires an understanding of their association with the corroding surfaces. We investigated the association of uranium with the amorphous and crystalline forms of iron oxides commonly formed on corroding steel surfaces. Uranium was incorporated with the oxide by addition during the formation of ferrihydrite, goethite, green rust II, lepidocrocite, maghemite, and magnetite. X-ray diffraction confirmed the mineralogical form of the oxide. EXAFS analysis at the U L(III) edge showed that uranium was present in hexavalent form as a uranyl oxyhydroxide species with goethite, maghemite, and magnetite and as a bidentate inner-sphere complex with ferrihydrite and lepidocrocite. Iron was present in the ferric form with ferrihydrite, goethite, lepidocrocite, and maghemite; whereas with magnetite and green rust II, both ferrous and ferric forms were present with characteristic ferrous:total iron ratios of 0.65 and 0.73, respectively. In the presence of the uranyl ion, green rust II was converted to magnetite with concomitantreduction of uranium to its tetravalent form. The rate and extent of uranium dissolution in dilute HCl depended on its association with the oxide: uranium present as oxyhydroxide species underwent rapid dissolution followed by a slow dissolution of iron; whereas uranium present as an inner-sphere complex with iron resulted in concomitant dissolution of the uranium and iron.

CO and NO2 sensing properties of doped-Fe2O3 thin films prepared by LPD

Abstract: The CO and NO2 sensing properties of iron oxide thin films doped with Au and Zn and prepared by a liquid-phase deposition method (LPD) have been investigated. The undoped Fe2O3 sensor was found to be sensitive to NO2 but not to CO. The addition of Zn increases the sensitivity at low temperature towards NO2 and decreases strongly the intrinsic resistance of the iron oxide film. The addition of gold was necessary to obtain a detectable response to CO, the Au-doped ZnO sensor was found the most sensitive to CO. By operating at an appropriate temperature, these sensors could be able to detect selectively CO and NO2, with negligible humidity cross-sensitivity.

Immobilization of radionuclides and heavy metals through anaerobic bio-oxidation of Fe(II).

Abstract: Adsorption of heavy metals and radionuclides (HMR) onto iron and manganese oxides has long been recognized as an important reaction for the immobilization of these compounds. However, in environments containing elevated concentrations of these HMR the adsorptive capacity of the iron and manganese oxides may well be exceeded, and the HMR can migrate as soluble compounds in aqueous systems. Here we demonstrate the potential of a bioremediative strategy for HMR stabilization in reducing environments based on the recently described anaerobic nitrate-dependent Fe(II) oxidation by Dechlorosoma species. Bio-oxidation of 10 mM Fe(II) and precipitation of Fe(III) oxides by these organisms resulted in rapid adsorption and removal of 55 microM uranium and 81 microM cobalt from solution. The adsorptive capacity of the biogenic Fe(III) oxides was lower than that of abiotically produced Fe(III) oxides (100 microM for both metals), which may have been a result of steric hindrance by the microbial cells on the iron oxide surfaces. The binding capacity of the biogenic oxides for different heavy metals was indirectly correlated to the atomic radius of the bound element. X-ray absorption spectroscopy indicated that the uranium was bound to the biogenically produced Fe(III) oxides as U(VI) and that the U(VI) formed bidentate and tridentate inner-sphere complexes with the Fe(III) oxide surfaces. Dechlorosoma suillum oxidation was specific for Fe(II), and the organism did not enzymatically oxidize U(IV) or Co(II). Small amounts (less than 2.5 microM) of Cr(III) were reoxidized by D. suillum; however, this appeared to be inversely dependent on the initial concentration of the Cr(III). The results of this study demonstrate the potential of this novel approach for stabilization and immobilization of HMR in the environment.

In vitro characterization of two different ultrasmall iron oxide particles for magnetic resonance cell tracking.

Abstract: Fleige G, Seeberger F, Laux D, et al. In vitro characterization of two different ultrasmall iron oxide particles for magnetic resonance cell tracking. Invest Radiol 2002;37:482–488.Rationale and Objectives.Comparison of two different ultrasmall superparamagnetic iron oxide (USPIO) particles in terms

A model for early diagenetic processes in sediments of the continental shelf of the Black sea

Abstract: A numerical model for early diagenetic processes in the sediments of the north-western continental shelf of the Black Sea is presented. The north-western shelf area is influenced by nutrient, organic matter and reactive iron inputs from major rivers such as the Danube, Dnieper and Dniester. Low oxygen concentrations in the bottom water and high carbon deposition rates have resulted in a dominance of anoxic mineralization processes including manganese, iron and sulphate reduction. The model includes six solid phase variables (labile and less labile organic carbon, iron-and manganese oxides, iron monosulphide and pyrite) and eight solutes (oxygen, nitrate, ammonium, manganese, ferrous iron sulphate, hydrogen sulphide and methane). The cycling of iron and sulphur is explicitly incorporated in the model since these elements play an important role in the mineralization of organic matter on the continental shelf. The model is capable of performing both steady state and dynamic simulations and can be coupled to biogeochemical models for water-column processes. The output of the model is composed of depth profiles of the state variables and their biogeochemical reaction rates and fluxes of the solute substances across the sediment–water interface. The model has been applied to solid-phase and porewater data from a station near the Danube delta. A steady state stimulation reasonably reproduced the main features, but not the distribution of iron and the concentrations of oxygen and total organic carbon in the first 2 cm. A dynamic simulation revealed that a pulse flux of organic matter resulting from the spring bloom could be responsible for the deviation from steady state in the near-surface distribution of total organic carbon and oxygen. The effect of carbon loading on organic matter degradation pathways has been investigated. There is a gradual increase in the relative importance of anoxic mineralization pathways as carbon loadings increase, but the transitions from manganese-based respiration to iron-based respiration and from iron-based respiration to sulphate reduction are rather abrupt. The first jump is due to the positive feedback between manganese reduction coupled to ferrous iron oxidation and iron oxide reduction. The second swap is due to the formation of iron sulphides. This will consume part of the reduced iron, which is consequently not available for re-oxidation to iron oxide, and the iron-reducing bacteria become iron-oxide limited. The cycling efficiency of iron changes abruptly, and a larger part of the iron will be fixed as iron sulphides.

Microemulsion-Assisted Synthesis of Tunable Superparamagnetic Composites

Abstract: Tunable superparamagnetic materials have been obtained by the thermal annealing of iron oxide/silica composites synthesized using microemulsions with a high content of the iron oxide precursor. The crystallochemical characteristics of the powders obtained after controlled hydrolysis within the reverse micelle nanocavities and their thermal evolution up to the formation of the γ-Fe2O3 crystalline phase were studied. The thermally annealed composites clearly exhibited superparamagnetic behavior with the value of the saturation magnetization easily tuned by changing the iron oxide content and the annealing temperature. Information on the influence of particle size and interparticle interactions on the magnetic properties of the composites was inferred from zero field and field cooling measurements and also from the decay of the reduced remanence with temperature. It was observed that the blocking temperature increased with content and particle size of γ-Fe2O3. We have also determined a strong dependence of t...

Catalyzed destructive adsorption of environmental toxins with nanocrystalline metal oxides. Fluoro-, chloro-, bromocarbons, sulfur, and organophosophorus compounds.

Abstract: In the temperature range of 300-500 degrees C, solid nanocrystalline oxides react nearly stoichiometrically with numerous halocarbons, sulfur, and organophosphorus compounds. In some cases, the reaction efficiencies can be improved by the presence of a small amount of transition-metal oxide as catalyst; for example, Fe2O3 on CaO and mobile intermediate species such as FeCl3 or Fe(SO3)x are important in the catalytic process. Herein, a series of environmentally problematic compounds are discussed, including CCl4, COS, CS2, C2Cl4, CHCl3, CH2Cl2, CH3Cl, and (CH3O)2P(O)CH3. Nanocrystals of CaO coated with a thin layer of Fe2O3 (or other transition metals) =[Fe2O3]CaO, or intimately mixed =Fe2O3/CaO were compared with pure CaO. It was found that (a) the presence of a small amount of surface [Fe2O3] or other transition-metal oxide can have a marked effect on the destructive adsorption activity, (b) for some reagents, such as CCl4, C2Cl4, SO2 and others, the nanocrystalline CaO can react in stoichiometric amounts, especially if a transition-metal oxide catalyst is present, (c) although the reaction with dimethylmethylphosphonate is surface-limited, the nanocrystalline calcium oxide performed well and in high capacity, (d) nanocrystalline calcium oxide exhibits near stoichiometric activitywith several interesting sulfur-containing compounds, such as COS and CS2, (e) unfortunately, most fluorocarbons were not destructively adsorbed at 500 degrees C under the conditions employed; however, some of these can be effectively mineralized over the calcium oxide at higher temperatures. These compounds include C2F6, C3F6, C2ClF3, and CHF3, and (f) upon reaction, surface areas decreased considerably, from about 100 to about 10 m2/g. The results of these experiments further demonstrate that, with the proper choice of catalytic material, some solid-gas reactions can be engineered to be rapid and essentially stoichiometric.

Iron Oxide Surface-Catalyzed Oxidation of Ferrous Iron by Monochloramine: Implications of Oxide Type and Carbonate on Reactivity

Abstract: The maintenance of monochloramine residuals in drinking water distribution systems is one technique often used to minimize microbial outbreaks and thereby maintain the safety of the water. Reactions between oxidizable species and monochloramine can however lead to undesirable losses in the disinfectant residual. Previous work has illustrated that the Fe(II) present within distribution systems is one type of oxidizable species that can exert a monochloramine demand. This paper extends this prior work by examining the kinetics of the reactions between Fe(II) and monochloramine in the presence of a variety of iron oxide surfaces. The identity of the iron oxide plays a significant role in the rate of these reactions. Surface area-normalized initial rate coefficients (kinit) obtained in the presence of each oxide at pH ≈6.9 exhibit the following trend in catalytic activity: magnetite > goethite > hematite ≈ lepidocrocite > ferrihydrite. The differences in the activity of these oxides are hypothesized to resul...