Showing papers on "Iron oxide published in 2000"

Environmental Microbe-Metal Interactions

Abstract: Part I. Biogeochemical Cycling of Iron and Manganese 1. Dissimilatory Fe(III) and Mn(IV) Reduction 2. Microbial Oxidation Fe(III) and Mn(II) at Circumneutral pH 3. Phylogenetic and Biochemical Diversity Among Acidophilic Bacteria that Respire on Iron 4. Trace Metal-Phytoplankton Interactions in Aquatic Systems 5. Biologically Controlled Mineralization of Magnetic Iron Minerals by Magnetotactic Bacteria 6. The Role of Siderophores in Iron Oxide Dissolution 7. Microbially Influenced Corrosion of Steel Part II. Microbial Ineractions with Toxic Metals-Biomineralization and Bioremediation 8. Microbial Mercury Reduction 9. Dissimilatory Reduction of Selenate and Arsenate in Nature 10. Microbial Chromate Reduction 11. Influence of Fungi and Environmental Mobility Metals and Metalloids 12. Bacterial Surface-Mediated Mineral Formation 13. Bioremediation of Radioactive Metals 14. Biosorption Process for Heavy Metal Removal 15. Biodegradation of Synthetic Chelating Agents

Novel Photocatalyst: Titania-Coated Magnetite. Activity and Photodissolution

Abstract: Magnetic photocatalysts were synthesized by coating titanium dioxide particles onto colloidal magnetite and nano-magnetite particles. The photoactivity of the prepared coated particles was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment. These observations were explained in terms of an unfavorable heterojunction between the titanium dioxide and the iron oxide core, leading to an increase in electron−hole recombination. Interactions between the iron oxide core and the titanium dioxide matrix upon heat treatment were also seen as a possible cause of the observed low activities of these samples. Other issues considered include the physical and chemical characteristics of the samples, such as surface area and the presence of surface hydroxyl groups. Depending on the calcination conditions, these photocatalysts were found to suffer from varying degrees of photodissolution. Photodissolution tests revealed the greater the extent of the heat treatment, the low...

Modeling Cd and Zn Sorption to Hydrous Metal Oxides

Abstract: The mobility and bioavailability of Cd and Zn in soils and sediments are affected by contaminant distribution mechanisms. One important process is sorption to hydrous aluminum, iron, and manganese oxides, which are ubiquitous in soils and sediments as both discrete particles and coatings and exhibit a high affinity for these metals. Mechanistic models are required for accurately assessing risks to populations and in the long-term management of contaminated soils and sediments. This research demonstrates intraparticle diffusion is the rate-limiting step in the sorption of Cd and Zn to microporous oxides. Furthermore, as much as 90% of the total sorption sites on the oxides reside on the micropore walls. Because long-term experiments require a lengthy period of time, predictive methods would be useful for determining surface diffusivities. Theoretically, surface diffusivities can be predicted from site activation theory, which is based on the random walk model where atoms or molecules vibrate at localized s...

Diagenetic Hematite and Manganese Oxides and Fault-Related Fluid Flow in Jurassic Sandstones, Southeastern Utah

Abstract: A variety of diagenetic hematite and manganese oxide deposits occur within well-exposed Jurassic eolian and related deposits of southeastern Utah. Hematite concretions (millimeters to tens of meters in size) and strata-bound layers occur in the permeable Navajo, Page, and Entrada sandstones. Localized manganese oxide deposits without significant iron oxide occur in the overlying rocks covering the Summerville-Tidwell interval. Field, lab, and numerical modeling studies indicate the diagenetic deposits are related to the Moab fault. Fluid inclusion studies show salinities of fault fluids range from 0 to 19.7 NaCl equivalent weight percent. The d18O (SMOW) and d13C (PDB) values of cements and veins range from 7 to 27o/oo and -12 to +5o/oo, respectively. The d87Sr (SMOW) values of these cements and veins range from 0.210 to 2.977o/oo, values substantially more radiogenic than Pennsylvanian seawater. Saline brines formed from solution of Pennsylvanian salts by meteoric water and are interpreted to have flowed up the Moab fault and outward into adjacent permeable rocks. These brines are reducing from interaction with hydrocarbon, methane, organic acids, or hydrogen sulfide, and thus remove iron, manganese, and 87Sr, and bleach the sandstones near the fault. The isotopic evidence suggests multiple episodes of fluid flow up the Moab fault system. When saline, reduced brines mixed with shallow oxygenated groundwater, iron and manganese oxides were precipitated as cements to form concretions and tabular deposits in the porous sandstones. Multiple episodes of iron oxide mineralization and concretionary geometries are evident and can be explained as the result of permeability heterogeneities in the host rock, presence of favorable nucleii for precipitation, a self-organization process, or the influence of microbes.

Pressure Acid Leaching of Nickel Laterites: A Review

Abstract: A review of the literature over the past 30 years on the processing of nickel laterites by high temperature acid leaching has been carried out to provide a better understanding of the mineralogy, leaching process chemistry and effect of operating conditions on nickel recovery, residue properties and scaling. Particular attention is paid to the leaching experience of the commercial Moa Bay plant and to the recently reported testwork and flowsheets associated with the three Western Australia lalerite plants that will be operating in 1999. It is shown that laterites can vary significantly in their mineralogy according to location, climate and depth, and that the main host minerals for nickel and cobalt can be either goethite (iron oxide) or nontronite (clay) or manganese oxides. The mechanism of leaching involves acid dissolution of the host mineral lattice followed by hydrolysis and precipitation (transformation) of a variety of insoluble oxides and sulphates of iron, aluminium and silica under the...

Preparation and gas-sensing properties of CuFe2O4 at reduced temperature

Abstract: Sol-gel and co-precipitation were used for preparation of copper ferrite. Pure CuFe2O4 was not obtained until 800°C by a sol-gel process, which might be due to the loose contact between copper and iron atoms and the possible low diffusion rate. However, nanocrystalline CuFe2O4 was prepared by a co-precipitation method. The tetragonal CuFe2O4 phase was formed at 90°C. The decomposition of amorphous hydroxides in the dried precipitate continued until 450°C, according to a TG-DTA analysis. Conductance–temperature measurements of the sensor made of CuFe2O4 indirectly indicated that the amount of absorbed oxygen was small, as reflected by the relatively low response. The CuFe2O4 sensors showed a good response to alcohol, which is promising for further development of this type of sensor. The difference in response for various tested gases might be attributed to absorption of reducing gases and reaction between these gases and the absorbed oxygen. It is supposed that the gas-sensing properties of this material might be improved by the addition of catalysts.

Adsorption of Free Lead (Pb2+) by Pedogenic Oxides, Ferrihydrite, and Leaf Compost

Abstract: Iron oxides and organic matter have a high capacity to adsorb Pb and concomitantly maintain a low free Pb 2+ activity in solution. It is, therefore, important to assess the adsorption capacity of naturally occurring materials and evaluate their potential to reduce toxicity. The free Pb 2+ activity was measured in the solution in equilibrium with ferrihydrite (a synthetic Fe oxide), two field-collected pedogenic amorphous oxides, and leaf compost. The experiment used a factorial design varying total Pb loading and solution pH. The results show that ferrihydrite was more efficient in lowering Pb aqueous concentration than the two pedogenic oxides. Furthermore, of the two pedogenic oxides examined, a higher Pb 2+ activity was maintained in solution at equilibrium with the most crystalline phase, which also has a lower surface area relative to the other adsorbents. Leaf compost maintained a significantly higher free Pb 2+ activity, relative to the various oxides. The experimental data could be fitted to a semi-mechanistic model predicting free Pb 2+ activity as a function of total Pb loadings and pH, with R 2 varying from 0.77 to 0.92.

NC100150 Injection, a preparation of optimized iron oxide nanoparticles for positive-contrast MR angiography.

Abstract: A preparation of monocrystalline iron oxide nanoparticles with an oxidized starch coating, currently in clinical trials (NC100150 Injection; CLARISCAN), was characterized by magnetization measurements, relaxometry, and photon correlation spectroscopy. By combining the results with a measure of iron content, one can obtain the size and magnetic attributes of the iron cores, including the relevant correlation times for outer sphere relaxation (tau(SO) and tau(D)), and information about the interaction of the organic coating with both core and solvent. The results are 6.43 nm for the iron oxide core diameter, a magnetic moment of 4.38x10(-17) erg/G, and a water-penetrable coating region of oxidized oligomeric starch fragments and entrained water molecules. The latter extends the hydrodynamic diameter to 11.9 nm and lowers the average diffusivity of solvent about 64% (which increases tau(D) accordingly). The nanoparticles show little size-polydispersity, evidenced by the lowest value of r(2)/r(1) at 20 MHz reported to date, an asset for magnetic resonance angiography.

Removal of Contaminants from Aqueous Solution by Reaction with Iron Surfaces

Abstract: Irrigation drainage and industrial wastewaters often contain elevated levels of toxic oxyanions and oxycations such as selenate, chromate, and uranyl. A potential remediation method is to react contaminated water with zero-valent iron, which transforms the mobile contaminants into immobile forms. In this work, iron foil was exposed to aqueous solutions containing the relevant ions, and the reacted surfaces were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM images collected in situ show that the protrusions on the foil surface associated with iron oxides are smoothed out by the reaction. XPS indicates that partially reduced Se(IV) and Cr(III) are adsorbed on the surface, while uranium is deposited as U(VI), i.e., without reduction. More Se and Cr are deposited when the atmospheric gases are removed from solution because of the elimination of a competing process in which dissolved O2 increases the thickness of the iron oxide overlayer to the point where...

Identification of active sites and adsorption complexes in Fe/MFI catalysts for NOx reduction

Abstract: FTIR, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) and ESR were used to identify the active sites and the adsorption complexes present in a variety of Fe/MFI samples prepared in different ways and displaying vastly different activities and selectivities in the reduction of NOx to N2 with hydrocarbons. Iron oxide particles, charged ferric oxide nano-clusters, isolated iron ions and oxygen-bridged binuclear iron ions have been identified with various degrees of reliability. In contact with appropriate gases, NO+ ions, mono- and dinitrosyl groups, nitro groups, nitrate ions and superoxide ions have been identified. Peroxide ions, though not detectable with the methods used here, were postulated by other authors on the basis of density functional calculations. The binuclear iron oxo-ions are more abundant in the best catalysts with high Fe/Al ratio that were prepared by sublimation. They are the most probable sites for NO oxidation to NO2 and its further conversion to adsorbed nitro groups and nitrate ions, steps that are crucial for NOx reduction. Superoxide and/or peroxide ions are likely involved in the NO oxidation to NO2. This process is fast when Fe/MFI is exposed to a mixture of NO+O2, but much slower if NO is chemisorbed first, before exposure to O2.

Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass

Abstract: In this work, we study borate glasses doped with a low concentration of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (composition: 0.63B2O3–0.37Li2O–0.75×10−3 Fe2O3 in mole %) were annealed at increasing temperatures Ta, starting at the glass transition temperature. A new composite resonance at gef≈2.0 arises in the spectra measured at room temperature (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a crystalline iron-containing compound. Numerical simulations assuming a lognormal particle volume distribution show that the mean particle diameter increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K. The integrated spectra intensity shows that the total number of spins in the nanoparticles increases rapidly with Ta. At lower anneal temperatures Ta, a striking increase occurs in t...

Control of bandgap of iron oxide through its encapsulation into SiO2-based mesoporous materials

Abstract: Nanoparticles of iron oxide were encapsulated into the pores of Si-MCM-41 and porous vycor glass (PVG) with pore diameters 2.1–4.5 nm. Transmission electron micrographs (TEM) observation has revealed the formation of iron oxide particles in the pores after an adequate preparation including a suspension treatment in water. Ultraviolet–visible absorption spectra (UV-VIS) of iron oxide loaded on the respective supports were greatly changed with the pore diameters. The bandgaps of iron oxide, evaluated from the absorption edges of the UV-VIS spectra, were significantly widened with decrease in the pore diameters. The dependence of the bandgap on the pore diameter could well be explained by the Brus' equation when the effective mass of an electron and a hole in iron oxide was assumed to be 0.27. It follows that the bandgap was changed by the quantum size effect and could be controlled by the pore size of support.

Laboratory and field studies of colloidal iron oxide dissolution as mediated by phagotrophy and photolysis

Abstract: In a previous work, we have employed colloidal ferrihydrite impregnated with an inert radiotracer to probe the mechanistics of iron redox cycling in seawater via phagotrophic and photochemical processes. This paper reports further studies using the inert tracer technique, directed towards obtaining a more quantitative sense of the importance of phagotrophy relative to photolysis as a pathway for the production of bioavailable iron in oxygenated seawater. Our results indicate a maximal (i.e., near-surface at noon) rate of 12% per day for the photochemicallymediated dissolution of colloidal ferrihydrite. Protozoan-mediated dissolution of the same iron oxide phase proceeds at a rate ranging from 1‐6% per day, depending on grazing turnover rates. Thus, while photolysis should dominate the redox cycling of refractory iron solids in near-surface waters under bright daytime conditions, phagotrophy is likely to be a more important process overall when the entire euphotic zone is considered on a time-averaged basis.

Screening of catalysts in the oxidative dehydrogenation of ethylbenzene with carbon dioxide

Abstract: The influence of the type and concentration of transition and alkali metal oxides, type of the catalyst support as well as pretreatment method on catalytic activity and stability of catalysts in the dehydrogenation of ethylbenzene in the presence of carbon dioxide was determined. Active carbon-supported iron oxide promoted by alkali metal oxide was found to show the highest performance in the reaction. An increase in the catalyst mass was observed during time-on-stream. Additionally the specific surface area of the catalyst decreased. Temperature programmed oxidation showed the presence of carbon deposits which could only be gasified above 700°C.

Iron oxyhydroxide colloids stabilized with polysaccharides.

Abstract: Neutralization of iron salts in aqueous solutions of κ-carrageenan and cellulose sulfate results in iron oxyhydroxide–polysaccharide hybrid colloids with unusual pH stability up to pH 13 It is shown that both polysaccharides form a tight polymer layer surrounding the inorganic particles, which in the case of κ-carrageenan is cross-linked by helical domains forming a self-assembled nanoreactor The stabilized iron oxyhydroxide particles can undergo further reactions, for example, it is possible by a chemical reaction to produce stabilized magnetite particles Repetition of the loading/neutralization steps in the reaction results in hybrids with iron contents much higher than the stoichiometric balance of iron and functional groups of the polymer (greater than 100% Fe/SO4−) This combination of high iron content with a natural polysaccharide stabilizer makes these colloids interesting for a number of applications, for example, for nutritional purposes or as contrasting agents for tomography