Showing papers on "Magnetite published in 2016"

Magnetite pollution nanoparticles in the human brain.

Abstract: Biologically formed nanoparticles of the strongly magnetic mineral, magnetite, were first detected in the human brain over 20 y ago [Kirschvink JL, Kobayashi-Kirschvink A, Woodford BJ (1992) Proc Natl Acad Sci USA 89(16):7683-7687]. Magnetite can have potentially large impacts on the brain due to its unique combination of redox activity, surface charge, and strongly magnetic behavior. We used magnetic analyses and electron microscopy to identify the abundant presence in the brain of magnetite nanoparticles that are consistent with high-temperature formation, suggesting, therefore, an external, not internal, source. Comprising a separate nanoparticle population from the euhedral particles ascribed to endogenous sources, these brain magnetites are often found with other transition metal nanoparticles, and they display rounded crystal morphologies and fused surface textures, reflecting crystallization upon cooling from an initially heated, iron-bearing source material. Such high-temperature magnetite nanospheres are ubiquitous and abundant in airborne particulate matter pollution. They arise as combustion-derived, iron-rich particles, often associated with other transition metal particles, which condense and/or oxidize upon airborne release. Those magnetite pollutant particles which are <∼200 nm in diameter can enter the brain directly via the olfactory bulb. Their presence proves that externally sourced iron-bearing nanoparticles, rather than their soluble compounds, can be transported directly into the brain, where they may pose hazard to human health.

Monodisperse magnetite nanoparticles with nearly ideal saturation magnetization

Abstract: We present a scalable thermolysis and high temperature oxidation procedure for synthesizing monodisperse magnetite nanoparticles with saturation magnetization of up to 80 emu g−1 (412 kA m−1), 92% of bulk magnetite. Diameters in the 15–30 nm size range are produced from iron oleate via the thermolysis method at 324 °C and varying oleic acid ratios for size control (6.7–7.6 equivalents per Fe). The influence of the iron oleate synthesis procedure on the quality of resulting nanoparticles is examined and the structure of the iron oleate is proposed to have a triironoxonium core [Fe3O+] based on magnetic susceptibility measurements. The thermolysis method is shown to initially give wustite nanoparticles, which are oxidized in situ at 318 °C using 1% oxygen in argon to form highly magnetic magnetite nanoparticles. The use of 1% oxygen offers broad application as a safe and efficient reagent for the high temperature oxidation of nanoparticles. Special consideration to the reproducibility of nanoparticle diameter and monodispersity has uncovered critical factors. Additionally, the reduction of Fe(III) to Fe(II) is shown to occur during the heat up stage of thermolysis, beginning at less than 180 °C and being complete by 320 °C. Evidence for the reduction occurring by the oxidative decarboxylation of oleic acid is presented. Decomposition of the remaining oleic acid is shown to occur by a ketonization reaction producing oleone. The nucleation event and growth of particles is examined by TEM. Comparison of the solvents 1-octadecene and octadecane are presented demonstrating the effect on the reduction of Fe(III) during heat up, the large difference in particle size, and effects on the oxidation rate of iron oxide nanoparticles. Determination of Fe(II) content in magnetic iron oxide nanoparticles by titration is presented.

Iron-rich melts, magmatic magnetite, and superheated hydrothermal systems: The El Laco deposit, Chile

Abstract: We propose an integrated model that explains the magmatic and hydrothermal features of the unique El Laco iron deposit that is located in a Pliocene–Pleistocene volcano of the Chilean Andes. (Sub)volcanic crystallization of an iron-rich melt as massive magnetite promoted the exsolution of a small volume of a hydrosaline melt and of large amounts of vapor that led to the formation of an alkali-calcic hydrothermal assemblage replacing the host andesite; this assemblage is capped and overprinted by a large zone of acid-sulfate steam-heated alteration forming as a whole a protracted shallow-level magmatic-hydrothermal system. Oxygen isotopic data for the massive magnetite (δ18O: 4.3‰–5.0‰) and the alkali-calcic altered rock (diopside δ18O: 7.2‰–8.7‰; magnetite δ18O: 4.4‰–6.7‰) suggest that these rocks are genetically related to the host andesite (δ18O: 7.4‰–9.6‰). The estimated temperature of the mineral assemblage (>∼900 °C) may be the highest recorded in hydrothermal systems, is attributed to exsolution of fluids from the crystallizing iron-rich melts, and is considered unlikely to reflect the convection of surficial fluids.

Investigation of magnetite nanoparticles stability in air by thermal analysis and FTIR spectroscopy

Abstract: Magnetic iron oxides were prepared by precipitation of Fe(II) hydroxide using different precipitation agents: ammonia, benzylamine and sodium hydroxide, followed by oxidation with the oxygen dissolved in water. Thermal analysis, coupled with FTIR spectroscopy, has evidenced the formation of a mixture of magnetite and maghemite, with a higher content of magnetite in case of the powder synthesized with benzylamine. The stability of magnetite at oxidation by air during storage at room temperature and 60 °C was investigated by means of TG/DSC simultaneous thermal analysis, FTIR spectroscopy and X-ray diffractometry. Thermal analysis evidenced an exothermic process with mass gain in temperature range 100–190 °C, corresponding to magnetite oxidation process, but due to the superposition of other processes it could not offer quantitative information. FTIR spectroscopy has provided, especially through the first and second derivatives of FTIR spectra, the most valuable information regarding the evolution of magnetite to maghemite, due to their different characteristic bands. XRD technique has evidenced a slight shift of the main diffraction peaks at higher 2-theta values during the evolution of magnetite to maghemite. According to thermal analysis data, the powder synthesized with ammonia was completely oxidized after 15 days, while the other two powders, synthesized with benzylamine and sodium hydroxide, were completely oxidized after 110 days of keeping in air at room temperature. For a temperature of 60 °C, the oxidation was much faster; the oxidation process of the powder synthesized with benzylamine disappeared from TG/DSC curves after 1 day. All final powders were formed from nanoparticles with diameters up to 25 nm, with magnetic properties characteristic to nanometric maghemite.

Oxygen Vacancies Engineering of Iron Oxides Films for Solar Water Splitting

Abstract: Hematite (α-Fe2O3) can be considered as one of the top candidates to act as photoanode in the framework of clean hydrogen production through solar water splitting. The O:Fe ratio, that in this material plays a crucial role in the definition of its photoelectrochemical properties, has been investigated in detail. For this purpose, we examined thermal magnetite oxidation and hematite reduction as two possible routes to produce semiconducting iron oxides layers with controlled stoichiometry. We report on properties of single crystalline nanometric films elaborated by atomic oxygen plasma assisted molecular beam epitaxy as model systems to disentangle structural phase transition effects from pure stoichiometry ones. We provide new insights into the mechanisms related to hematite properties modifications and their correlation with photocurrent changes upon the presence of oxygen vacancies and phase mixing with magnetite, with respect to the vacancies concentration regimes. We show on one hand that crystallogra...

Removal of Humic Acid in Water by Rice Hull Magnetic Activated Carbon and Magnetic Separation

Abstract: We have newly developed a rice hull magnetic activated carbon (RH-MAC) and studied its adsorption property for humic acid from water and high-gradient magnetic separation (HGMS) characteristics. Rice hull was impregnated with an iron nitrate solution and heat-treated in both nitrogen and carbon dioxide atmosphere. In these processes, the rice hull changed to activated carbon that had a lot of mesopores and nanosize magnetite inside. Magnetization of RH-MAC increased with an increase in iron nitrate concentration during the process and reached 20.8 Am 2 /kg at 0.5 T. In the adsorption examination, it was able to adsorb humic acid effectively, and the removal ratio achieved 97.6% from a humic acid solution of 50 mg/L. In HGMS studies, we used the magnetic wire filter rounded on a magnet drum with 0.5-T permanent magnet and the slurry of RH-MAC concentration of 100 mg/L. Consequently, 99.4% of RH-MAC in the slurry of 2 L was captured in the magnetic filter at flow rates of 230 mL/min. These results show that this water purification system has higher removal ratio of humic acid than that for current advanced water treatments.

Mixed iron oxides as Fenton catalysts for gallic acid removal from aqueous solutions

Abstract: A study was conducted on the behavior as Fenton catalysts of three commercial mixed iron oxides: copper ferrite, magnetite, and ilmenite, using H2O2 for the degradation and mineralization of gallic acid (GA) at temperatures between 15 and 35 °C and pH of 4.3. Investigation of the activity of the catalysts was complemented by study of the metal ion leaching under reaction conditions. GA was completely mineralized with the three mixed iron oxides in the following order of catalytic activity: CuFe2O4 > Fe3O4 > FeTiO3, and the ferrites could be quickly and completely separated after the reaction by a magnetic field. According to these results, there appears to be a synergic effect between Cu and Fe ions placed in octahedral sites, and Cu ions may be the main active sites for HO radical generation. The efficiency of H2O2 utilization followed the inverse order of the catalytic activity of the mixed iron oxides, likely because the excess hydroxyl radicals generated during the reaction were quenched by hydrogen peroxide, yielding the less reactive hydroperoxide radicals. A very large amount of Cu and Fe ions leached from copper ferrite and decreased at higher reaction temperatures. These leached ions also acted as homogeneous Fenton catalysts in GA degradation and mineralization. A much lower amount of Fe ions was leached from magnetite and ilmenite in comparison to copper ferrite. From the perspective of long-term applications, magnetite with intermediate catalytic activity but with higher efficiency and much lower Fe ion leaching in comparison to copper ferrite may be more appropriate for GA removal.

A Close-up of the Effect of Iron Oxide Type on the Interfacial Interaction between Epoxy and Carbon Steel: Combined Molecular Dynamics Simulations and Quantum Mechanics

Abstract: In the present study, the effect of different types of iron oxides, which naturally exist on steel substrate, on the interfacial interaction between an epoxy coating and a carbon steel substrate was studied at the molecular/atomic level by employing molecular dynamics (MD) simulations and quantum mechanics (QM) calculations. Three types of iron oxide, that is, ferrous oxide (FeO), ferric oxide (Fe2O3, hematite), and ferrous ferric oxide (Fe3O4, magnetite), were considered for modeling, and their binding energies were calculated and compared by altering the concentration of hydroxide groups on the surface. To probe the effect of curing agent on interfacial interactions, computations were performed for either uncured or aminoamide-cured epoxy resins. The effect of the acid–base properties of the iron oxide on the molecular bonding was theoretically investigated by imposing diverse iron hydroxide/oxide termination groups. Noticeably, MD and QM calculations confirmed rather well earlier experimental evaluatio...

Bioinspired synthesis of magnetite nanoparticles

Abstract: Magnetite (Fe3O4) is a widespread magnetic iron oxide encountered in many biological and geological systems, and also in many technological applications. The magnetic properties of magnetite crystals depend strongly on the size and shape of its crystals. Hence, engineering magnetite nanoparticles with specific shapes and sizes allows tuning their properties to specific applications in a wide variety of fields, including catalysis, magnetic storage, targeted drug delivery, cancer diagnostics and magnetic resonance imaging (MRI). However, synthesis of magnetite with a specific size, shape and a narrow crystal size distribution is notoriously difficult without using high temperatures and non-aqueous media. Nevertheless, living organisms such as chitons and magnetotactic bacteria are able to form magnetite crystals with well controlled sizes and shapes under ambient conditions and in aqueous media. In these biomineralization processes the organisms use a twofold strategy to control magnetite formation: the mineral is formed from a poorly crystalline precursor phase, and nucleation and growth are controlled through the interaction of the mineral with biomolecular templates and additives. Taking inspiration from this biological strategy is a promising route to achieve control over the kinetics of magnetite crystallization under ambient conditions and in aqueous media. In this review we first summarize the main characteristics of magnetite and what is known about the mechanisms of magnetite biomineralization. We then describe the most common routes to synthesize magnetite and subsequently will introduce recent efforts in bioinspired magnetite synthesis. We describe how the use of poorly ordered, more soluble precursors such as ferrihydrite (FeH) or white rust (Fe(OH)2) can be employed to control the solution supersaturation, setting the conditions for continued growth. Further, we show how the use of various organic additives such as proteins, peptides and polymers allows for either the promotion or inhibition of magnetite nucleation and growth processes. At last we discuss how the formation of magnetite-based organic–inorganic hybrids leads to new functional nanomaterials.

High spin-dependent tunneling magnetoresistance in magnetite powders made by arc-discharge

Abstract: We report the successful synthesis of ferrimagnetic magnetite powders made using an arc-discharge method in a partial oxygen atmosphere. X-ray and electron diffraction measurements show that the powders also contain some antiferromagnetic hematite and a small amount of FeO and Fe that has not oxidized. The Raman data show that there is a small fraction of ferrimagnetic maghemite that cannot be seen in the x-ray diffraction data. There is a wide particle size distribution where there are nanoparticles as small as 7 nm, larger faceted nanoparticles, and particles that are up to 25 μm in diameter. The saturation magnetization at high magnetic fields is ∼74% of that found in the bulk magnetite, where the lower value is due to the presence of some antiferromagnetic hematite. The temperature dependence of the saturation magnetization changes at the Verwey transition temperature, and it has a power low dependence with an exponent of 3/2 at low temperatures and 2.23 at high temperatures above the Verwey transitio...

Conductive carbon black/magnetite hybrid fillers in microwave absorbing composites based on natural rubber

Abstract: The study presents the microwave properties of natural rubber based composites in the 1–12 GHz frequency range as determined by the conductive carbon black/magnetite hybrid fillers obtained by impregnation at different ratios between carbon and magnetite phase. It has been established that, the changes in the investigated properties exhibit a resonance character in the 3–9 GHz range and are caused by alternations both of frequency and the ratio between the hybrid phases. The real and imaginary parts of permittivity and permeability, dielectric and magnetic loss angle tangent and conductivity have been also determined. The composite comprising a hybrid filler whose carbon:magnetite phase ratio is 90:10 has turned to be the most successful. The good microwave characteristics of the composite are related to the dielectric and magnetic losses, as well as to the achieved optimal permittivity of the composite at the given ratio of the carbon and magnetite phase. The percent content of the conductive filler has a significant impact upon EMI SE of the composites. The introduction of Fe3O4 into the hybrid filler has a crucial role for decreasing the reflection by the composites surface.

Lead (Pb2+) adsorption by monodispersed magnetite nanoparticles: surface analysis and effects of solution chemistry.

Abstract: Monodispersed magnetite nanoparticles [Fe 3 O 4 -NPs] were synthesized by chemical co-precipitation using ferrous and ferric chloride as iron precursors. The composition, surface properties and morphology were investigated using X-ray powder diffraction, transmission electron microscopy, dynamic light scattering, scanning electron microscopy and Fourier transform infrared spectroscopy. Synthesized magnetite nanoparticles have zeta potential of +22.0 mV and an energy bandgap of 2.2 eV. These NPs were used for aqueous Pb 2+ removal at different initial pHs, equilibrium time, temperature and adsorbent/adsorbate concentrations. Pb 2+ adsorption was increased with rise in solution pH. Almost 100% Pb 2+ removal was achieved [at magnetite NPs dose: 0.2 g/L; Pb 2+ concentration: 50 mg/L; pH 5.0; shaking speed: 200 rpm; temperature: 25 °C and equilibrium time: 30 min]. Pseudo-first and pseudo-second order kinetic equations were applied to evaluate the kinetic data. Pseudo-second-order rate equation better fitted the data. Spent NPs were regenerated using 0.005 M HNO 3 . Exhausted magnetite NPs were successfully recovered from aqueous system using a simple magnet. These magnetite NPs can also be used for Pb 2+ removal from waters having high concentration of suspended particles.

Aggregation and sedimentation of magnetite nanoparticle clusters

Abstract: Magnetite nanoparticles are redox active constituents of subsurface and corrosive environments. In this study, we characterized the aggregation and sedimentation behavior of well characterized magnetite nanoparticle clusters using dynamic light scattering (DLS), UV-vis-NIR spectroscopy, and small angle X-ray scattering (SAXS). Both unfunctionalized (NaOH-magnetite) and tetramethylammonium hydroxide (TMAOH-magnetite) surface functionalized nanoparticle clusters were employed. TMAOH-magnetite has a slightly smaller primary nanoparticle radius as determined by TEM (4 ± 0.7 nm vs. 5 ± 0.8 for NaOH-magnetite) and a smaller initial DLS determined cluster radius (<30 nm vs. 100–200 nm for NaOH-magnetite). Interestingly, in spite of its smaller initial nanoparticle cluster size, TMAOH-magnetite undergoes sedimentation more rapidly than NaOH-magnetite. This behavior is consistent with the more rapid aggregation of the smaller TMAOH-magnetite clusters as well as their lower fractal dimension, as determined by SAXS. This study illustrates that both nanoparticle cluster size and fractal dimension should be carefully considered when considering the environmental transport and fate of highly aggregated nanoparticles.

Discrimination of biogenic and detrital magnetite through a double Verwey transition temperature

Abstract: Magnetite occurs widely in natural environments in both inorganic and biogenic forms. Discrimination of the origin of magnetite has important implications, from searching for past microbial activity to interpreting paleomagnetic and environmental magnetic records in a wide range of settings. In this study, we present rock magnetic and electron microscopic analyses of marine sediments from the continental margin of Oman. Low-temperature magnetic data reveal two distinct Verwey transition (Tv) temperatures that are associated with the presence of biogenic and inorganic magnetite. This interpretation is consistent with room temperature magnetic properties and is confirmed by electron microscopic analyses. Our study justifies the use of two distinct Tv temperatures as a diagnostic signature for discriminating inorganic and biogenic magnetite. Simple low-temperature magnetic measurements, therefore, provide a tool to recognize rapidly the origin of magnetite within natural samples. In addition, our analyses reveal progressive down-core dissolution of detrital and biogenic magnetite, but with preservation of significant amounts of fine-grained magnetite within sediments that have been subjected to severe diagenetic alteration. We demonstrate that preservation of magnetite in such environments is due to protection of fine-grained magnetite inclusions within silicate hosts. Our results, therefore, also provide new insights into diagenetic processes in marine sediments.