Synthetic Magnetite Research Articles

Effects of Oxidation on the Magnetization of Nanoparticulate Magnetite

Synthetic nanomagnetite has been suggested as a potential reactant for the in situ treatment of contaminated groundwater. Although the application of magnetite nanoparticles for environmental remediation is promising, a full understanding of particle reactivity has been deterred by the propensity of the nanoparticles to aggregate and become colloidally unstable. Attractive magnetic interactions between particles are partially responsible for their aggregation. In this study, we characterized the magnetic behavior of magnetite by determining the saturation magnetization, coercivity, remanent magnetization, susceptibility, and blocking temperature of synthetic magnetite using a superconducting quantum interference device (SQUID). We show how these properties vary in the presence of surface-associated solutes such as tetramethylammonium (TMA(+)) and ferrous (Fe(II)) cations. More importantly, because magnetite readily reacts with O(2) to produce maghemite, we analyzed the effect of oxidation on the magnetic properties of the particles. Because maghemite has a reported magnetic saturation that is less than that of magnetite, we hypothesized that oxidation would decrease the magnitude of the magnetic attractive force between adjacent particles. The presence of TMA(+) and Fe(II) caused a change in the magnetic properties of magnetite potentially because of alterations in its crystalline order. Magnetite oxidation caused a decrease in saturation magnetization, resulting in less significant magnetic interactions between particles. Oxidation, therefore, could lead to the decreased aggregation of magnetite nanoparticles and a potential enhancement of their colloidal stability.

Effect of natural clay components on sorption of Cs, Pu and Am by the clay

Sorption of Cs, Pu and Am on natural clay of complex composition was studied to better understand the sorption mechanisms. It was found that cesium sorption to natural clay was affected by its coatings and by the ionic strength of solution. The sorption of Pu and Am on the clay was compared with that on synthetic goethite, hematite and magnetite, representing components of the clay coatings. The sorption was quantitatively interpreted using models assuming ion exchange and/or complex formation on the “layer sites” and “edge sites” of the clay and its coatings. Constants characterizing properties of the sites and sorption equilibria were determined.

Comparison of the ${^{1}{\rm H}}$ NMR Relaxation Enhancement Produced by Bacterial Magnetosomes and Synthetic Iron Oxide Nanoparticles for Potential Use as MR Molecular Probes

Iron oxide nanoparticle, named synthetic magnetite, as contrast agent has been widely used in clinical MRI. Recently, a new magnetite nanocrystal, called magnetosome, has been found in magnetotactic bacteria. Physicochemical and magnetorelaxo-metric characterization of bacteria magnetosomes and iron oxide nanoparticles are investigated. Bacterial magnetosomes have the larger mean aggregate size, better dispersion and obviously stronger ferromagnetism compared to synthetic magnetites. The samples of several concentrations of magnetic nanoparticles were analyzed using a clinical 3.0 T MR-scanner. The signal decay in the Magnetic Resonance images is found to change proportionally to the nanoparticles concentration. Two kinds of nanoparticles can be though as a negative contrast agent and show slight effects on T1, but strong effects on T2 weighted images. Notice that at the same concentration the signal attenuation of bacterial magnetite samples is more obvious than that of synthetic magnetite samples.

Analysis on variety and characteristics of maghemite

Maghemite (γ-Fe2O3) is a very common mineral at the earth’s surface and also an important material for making music and video tapes. Maghemite is usually synthesized from magnetite under oxidizing conditions after a few hours or a few days below a temperature of 300°C. The magnetic property of thermal instability and the chemical action after heating is an important character for maghemite. That is, it will become hematite in certain proportion after being heated above 250°C. Maghemite is therefore actually unable to have its Curie temperature measured. But late using synthetic sample, maghemite was further found partially thermal stable with a measurable Curie temperature ∼645°C. During our thermally magnetic experiments for a set of synthetic magnetite, we found that extra fined grain size (pseudo single domain (PSD) and small multi-domain (MD), mainly 1–10 μm) magnetite was formed to a completely thermally stable maghemite. This maghemite can also be produced by heating the same powder up to 700°C in an oven and keeping this temperature for 10 min, then cooling it down. When the generated maghemite by these two ways is heated from room temperature to 700°C, it shows almost fully reversible, or thermally stable. We used X-ray powder diffraction and Mossbauer spectroscopy to confirm the identity of this maghemite and compared its magnetic hysteresis, high temperature magnetization, low temperature thermal demagnetization, and low temperature susceptibility with those of the original preheated magnetite. Such quickly oxidized maghemite by heating to high temperature implies some types of maghemite formed in certain natural condition can carry a thermal remnant magnetization (TRM). Four types of maghemite were characterized and discussed according to their thermal stability. Among them, partially stable and fully thermally stable maghemite after heating should possess capability of carrying TRM. There is possibly a compensation of synthetizing maghemite between heating temperature and heating duration. The thermal stability of maghemite may be affected by a few factors, such as its purity (stoichiometry), heating temperature and duration. The grain size may be one of important factors. Maghemite might be similar to magnetite, having various magnetic properties corresponding to its grain size categories such as superparamagnetic (SP), single domain (SD) and MD. Low temperature measurement for PSD fine grain of synthetic magnetite shows a phenomenon of Verwey transition “suppressed”, its fundamental causes could be that the core diameter of oxidized magnetite is actually reach or approach SD size, so that its Verwey transition is shown “suppressed”.

Strain memory of the Verwey transition

We studied the effects of stress on the Verwey transition for pure, stoichiometric, synthetic multidomain magnetite and for natural multidomain magnetite having both relatively low and high degrees of oxidation. Low‐temperature measurements of the magnetic moments were carried out after pressure release. Our results unambiguously show an increase in the Verwey transition temperature with increasing pressure that ranges from 1 K/GPa for stoichiometric magnetite to 3 K/GPa for highly oxidized magnetite for pressures up to about 5 GPa. The transition width broadens with increasing stress for stoichiometric magnetite and then becomes less broad as oxidation increases until the width is invariant with respect to stress for highly oxidized magnetite. Heat treatment to 700°C does not appreciably reverse the effect, which could make the Verwey transition suitable for use as a geobarometer in cases where high pressures (>1 GPa) are involved, such as in meteorite impact craters. A model explaining the results is put forward.

Structure and superparamagnetic behaviour of magnetite nanoparticles in cellulose beads

Superparamagnetic magnetite nanoparticles were obtained starting from a mixture of iron(II) and iron(III) solutions in a preset total iron concentration from 0.04 to 0.8 mol l −1 with ammonia at 25 and 70 °C. The regeneration of cellulose from viscose produces micrometrical spherical cellulose beads in which synthetic magnetite were embedded. The characterization of cellulose–magnetite beads by X-ray diffraction, Scanning and Transmission Electron Microscopy and magnetic measurement is reported. X-ray diffraction patterns indicate that the higher is the total iron concentration and temperature the higher is the crystal size of the magnetite obtained. Transmission Electron Microscopy studies of cellulose–magnetite beads revealed the distribution of magnetite nanoparticles inside pores of hundred nanometers. Magnetite as well as the cellulose–magnetite composites exhibit superparamagnetic characteristics. Field cooling and zero field cooling magnetic susceptibility measurements confirm the superparamagnetic behaviour and the blocking temperature for the magnetite with a mean size of 12.5 nm, which is 200 K.

The decolorization of Acid Orange II in non-homogeneous Fenton reaction catalyzed by natural vanadium–titanium magnetite

The catalytic activity of natural vanadium–titanium magnetite was investigated in the decolorization of Acid Orange II by non-homogeneous Fenton process. The natural catalysts purified by magnetic separation were characterized using X-ray diffraction (XRD), polarizing microscope, X-ray absorption fine structure (XAFS) analysis and Mössbauer spectroscopy. The obtained results show that the natural samples after magnetic separation mainly contain titanomagnetite, with a small amount of ilmenite and chlorite. Titanomagnetite is doped with vanadium, whose the valency is mainly +3 and occupies the octahedral site. Batch decolorization studies were performed to evaluate the influences of various experimental parameters like initial pH, the amount of catalyst and initial concentration of hydrogen peroxide on the decolorization efficiency of Acid Orange II. The decolorization of the dye mainly relied on degradation. The degradation efficiency was strongly dependent on pH of the medium where it increased as the pH decreased in acid range. The increase of catalyst and hydrogen peroxide could accelerate the degradation. The catalytic property of natural vanadium–titanium magnetite in the degradation of Acid Orange II was stronger than that of synthetic magnetite (Fe 3O 4). The catalytic activity of the natural samples was greatly related to the titanomagnetite content. The degradation process was dominated by heterogeneous Fenton reaction, complying with pseudo-first-order rate law. The natural catalyst has a good catalytic stability.

The effect of calcium ions and sodium silicate on the adsorption of a model anionic flotation collector on magnetite studied by ATR-FTIR spectroscopy

Previous studies have shown that agglomeration of the magnetite concentrate after reverse flotation of apatite is negatively affected by the collector species adsorbed on the surface of magnetite. In this work, the effect of ionic strength, calcium ions and sodium silicate on the unwanted adsorption of a model anionic flotation collector on synthetic magnetite was studied in situ using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The amount of collector adsorbed was found to increase with increasing ionic strength at pH 8.5 providing evidence to the contribution of electrostatic forces to the adsorption of the collector. Adding sodium silicate to the system resulted in a threefold decrease in the amount of collector adsorbed compared to when no sodium silicate was added, confirming the depressing activity of sodium silicate on magnetite. Calcium ions were shown to increase the adsorption of both the collector and sodium silicate on magnetite. The depressing effect of sodium silicate on collector adsorption was completely suppressed in the presence of calcium ions under the conditions studied. Furthermore, the amount of collector adsorbed on magnetite from the silicate-collector solution increased 14 times upon addition of calcium ions suggesting that calcium ions in the process water may increase undesired adsorption of the collector on the iron oxide.

Revisiting a domain-state independent method of palaeointensity determination

The use of a domain-state independent method (the Wilson method) of acquiring absolute palaeointensities is tested using three sets of historical lavas from Mexico, Italy and Iceland. The Wilson method works by plotting continuous thermal demagnetisation curve data for the original natural remanent magnetisation against those for a laboratory-induced thermoremanence. By comparing the predicted intensities with the known fields, a new empirically derived rejection criterion is proposed that accounts for chemical alteration and improves the accuracy of the palaeointensity estimates. The Wilson palaeointensity estimates are compared with palaeointensity determinations made on sister samples using standard Thellier-type heating. Generally, the Wilson palaeointensity estimates compare favourably for samples that yielded technically correct Thellier-type determinations. However, at one locality the Wilson method did not yield any estimates due to severe chemical alteration, in contrast it was still possible to determine palaeointensity estimates from the lower temperature Thellier data. To assess the Wilson palaeointensity protocol's ability to return a correct estimate for a true multidomain system, a synthetic multidomain magnetite sample was imparted with a thermoremanent magnetisation. The Wilson protocol returned the correct palaeointensity within error, in contrast the Thellier-type protocol failed. As the Wilson method is significantly faster than Thellier-type methods, it is suggested that the Wilson method should be tried as a ‘first-approach’ in palaeointensity studies, or for samples dominated by multidomain material.

Separation, characterization and initial reaction studies of magnetite particles from Hanford sediments

Magnetic and density separation methods have been applied to composite sediment samples from the Hanford formation from sediment recovered during drilling of an uncontaminated borehole located near the 200 West Area of the Hanford Site in southeastern Washington State. This paper describes the results of using those separation methods and from the characterization and initial reactivity measurements on a highly magnetic fraction isolated from that sediment. X-ray diffraction (XRD) analysis of the highly magnetic sediment fraction indicates that this material contains predominantly magnetite (Fe 3O 4). Particle morphology observed by scanning electron microscopy (SEM) and compositions determined energy dispersive spectroscopy (EDS) are consistent with this identification. Analyses by X-ray photoelectron spectroscopy (XPS) indicates that there is a thin coating on the particles that are likely a type of aluminosilicate. This highly magnetic fraction of material is not reactive with indigo carmine, an organic redox probe molecule that was shown to readily react with synthetic magnetite. Because of the limited amounts of material readily available, initial tests have been conducted that demonstrate the ability to complete U(VI) sorption on individual particles (nominally ∼100 μm in size) of the isolated sediment and to remove and mount these individual particles for analysis of the concentration and chemical state of the sorbed U species using small area XPS.

Competition between sodium oleate and sodium silicate for a silicate/oleate modified magnetite surface studied by in situ ATR-FTIR spectroscopy

Attenuated Total Reflection (ATR) IR spectroscopy was utilized to monitor adsorption of sodium oleate and sodium silicate onto synthetic magnetite at pH = 8.5, both individually and in a competitive manner. Oleate was adsorbed within a concentration range of 0.01–0.5 mM. It was observed that adsorption of oleate increased linearly with increasing concentration of oleate in solution up to a concentration of 0.1 mM. The infrared spectrum of oleate showed a broad single band at 1535 cm −1 assigned to the asymmetric stretching vibration of carboxylate, implying chemisorption of oleate to the magnetite surface. The kinetics of oleate adsorption followed a pseudo first-order reaction with an apparent rate constant of k 1 = 0.030 ± 0.002 min −1. Competitve adsorption of silicate and oleate was performed either by adding silicate solution to a magnetite film initially equilibrated with 0.1 mM oleate or adding oleate solution to magnetite treated with silicate solutions in the concentration range 0.1–5 mM. It was shown that silicate, within reasonable time, had only minor effect on the amount of oleate already adsorbed on magnetite. On the other hand, oleate did not efficiently compete with silicate if the latter substance was already adsorbed on the iron oxide.

Magnetite at low temperature: Resonant inelastic x-ray scattering (RIXS) at the Fe K-edge

At room temperature, magnetite (cubic inverse spinel structure, Fd-3m s.g.) shows metal-like electrical conductivity, which discontinuously drops by two orders of magnitude below ca. 120 K (Tv). This is due to a first-order phase transition, where the cubic structure is distorted to monoclinic symmetry (Cc s.g.). Models for this metal-insulator transition are still highly debated. We performed 1s3p as well 1s2p RIXS measurements well below and above Tv on a sample of synthetic magnetite powder to probe any difference present in the electronic states. Neither 1s3p nor 1s2p RIXS spectra reveal any differences between the two phases. Our observations are consistent with earlier EXAFS data and indicate no changes in the local structural geometry around Fe.

Immobilization of non-point phosphorus using stabilized magnetite nanoparticles with enhanced transportability and reactivity in soils

Laboratory batch and column experiments were conducted to investigate the immobilization of phosphorus (P) in soils using synthetic magnetite nanoparticles stabilized with sodium carboxymethyl cellulose (CMC-NP). Although CMC-stabilized magnetite particles were at the nanoscale, phosphorus removal by the nanoparticles was less than that of microparticles (MP) without the stabilizer due to the reduced P reactivity caused by the coating. The P reactivity of CMC-NP was effectively recovered when cellulase was added to degrade the coating. For subsurface non-point P pollution control for a water pond, it is possible to inject CMC-NP to form an enclosed protection wall in the surrounding soils. Non-stabilized “nanomagnetite” could not pass through the soil column under gravity because it quickly agglomerated into microparticles. The immobilized P was 30% in the control soil column, 33% when treated by non-stabilized MP, 45% when treated by CMC-NP, and 73% when treated by both CMC-NP and cellulase.

Preparation and characterization of electroless Ni–P–Fe 3O 4 composite coatings and evaluation of its high temperature oxidation behaviour

Ni–P coatings modified with synthetic magnetite were prepared by electroless technique from a Ni–P plating bath containing magnetite powder. The coatings morphology was studied by means of scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The corrosion resistance at room temperature of the Ni–P films modified with magnetite was evaluated by means of electrochemical impedance spectroscopy (EIS), where the composite films exhibited a better behaviour. The films resistance to high temperature oxidation was evaluated by cyclic oxidation tests, SEM/EDS and X-ray diffraction analysis (XRD), where the coating with iron oxides appears to be more protective.

Synthetic and biogenic magnetite nanoparticles for tracking of stem cells and dendritic cells

Accurate delivery of cells to target organs is critical for success of cell-based therapies with stem cells or immune cells such as antigen-presenting dendritic cells (DC). Labeling with contrast agents before implantation provides a powerful means for monitoring cellular migration using magnetic resonance imaging (MRI). In this study, we investigated the uptake of fully synthesized or bacterial magnetic nanoparticles (MNPs) into hematopoietic Flt3 + stem cells and DC from mouse bone marrow. We show that (i) uptake of both synthetic and biogenic nanoparticles into cells endow magnetic activity and (ii) low numbers of MNP-loaded cells are readily detected by MRI.

Nanofabrication of two‐dimensional arrays of magnetite particles for fundamental rock magnetic studies

Magnetic measurements of samples with precisely controlled magnetic mineralogy, grain size, and interparticle spacing are needed to provide crucial experimental rock magnetic underpinning for paleomagnetic studies. We report a novel nanofabrication method for producing two‐dimensional arrays of cylindrical synthetic magnetite particles with well‐defined composition, particle size, and interparticle spacing. The samples are fabricated by writing dot arrays with electron beam lithography, transferring these patterns into sputtered Fe thin films by reactive ion etching in a CO/NH3 plasma, and oxidizing the resulting Fe particles in a controlled atmosphere to form magnetite. Scanning electron microscopy and transmission electron microscopy have been used to monitor the fabrication process and to determine the particle geometry. The particle sizes of our samples range between 100 nm and 265 nm with center‐to‐center spacings between 180 nm and 310 nm. Low‐temperature magnetic remanence data confirm the stoichiometry of the magnetite. We present magnetic hysteresis data and first‐order reversal curve diagrams for our samples and compare these with previously published data from other synthetic and natural magnetite samples. The ability to independently control particle size and interparticle spacing of magnetite grains makes our synthetic samples ideal for studying the influence of magnetostatic interactions on the paleomagnetic recording fidelity of naturally occurring magnetite in rocks.

An ATR-FTIR study of sulphate sorption on magnetite; rate of adsorption, surface speciation, and effect of calcium ions

The adsorption of sulphate on magnetite was studied in-situ using ATR-FTIR spectroscopy. Synthetic magnetite particles were deposited on a ZnSe internal reflection element and the spectra of sulphate adsorbed at pH 4–8.5 were recorded. Two different ionic strengths were used viz. 0.01 M and 0.1 M NaCl. The spectra of adsorbed sulphate on magnetite coated ZnSe were compared with the spectra of sulphate solutions at the same pH values and in contact with uncoated ZnSe. The spectrum of adsorbed sulphate at pH 4 showed three maxima at 979, 1044, and 1115 cm −1 indicating a monodentate adsorption in which the T d symmetry of SO 4 2 − is lowered to C 3v. At pH 6.5, sulphate adsorbed as an outer-sphere complex with two weak bands appearing at 1102 and 980 cm −1. Moreover, spectra of the adsorbed sulphate at pH 4 were recorded as a function of time and sulphate concentration. The equilibrium absorbance at different concentrations fitted a Langmuir type adsorption isotherm. The Langmuir affinity constant K at pH 4 was determined from the slope and intercept of the Langmuir plot to be K = 1.2344 × 10 4 M − 1 and the Gibbs free energy of adsorption Δ G ads 0 was estimated from this value to be −33.3 kJ/mol. Kinetic analysis indicated that adsorption at pH 4 is fast, whilst the desorption kinetic at the same pH is very slow. In addition, the effect of Ca ions on sulphate adsorption was also studied. It was shown that Ca ions increased the sulphate adsorption on magnetite at pH 8.5.

Grain size dependence of low-temperature remanent magnetization in natural and synthetic magnetite: Experimental study

Abstract Magnetic measurements at cryogenic temperatures (<300 K) proved to be useful in paleomagnetic and rock magnetic research, stimulating continuous interest to low-temperature properties of magnetite and other magnetic minerals. Here I report new experimental results on a grain size dependence of the ratio (R LT) between a low-temperature (20 K) saturation isothermal remanent magnetization (SIRM) imparted in magnetite after cooling in a 2.5 T field (field cooling, FC) and in a zero field environment (zero field cooling, ZFC). Synthetic magnetite samples ranged in mean grain size from 0.15 to 100 μm, representing nearly single-domain (SD), pseudosingle-domain (PSD), and multidomain (MD) magnetic states. The R LT ratio monotonically increases from 0.58 to 1.12 with the decreasing mean grain size, being close to unity for PSD grains (0.15-5 μm) and smaller than unity for MD magnetite (12-100 μm). The R LT ratio of 1.27 is observed for acicular magnetite characterized by nearly SD behavior. These observations indicate that within the range of ~0.15 to ~5 μm, the low-temperature SIRM may be higher than that expected from “normal” magnetic domain wall displacement. Such a behavior can be caused by the presence of a SD-like component in the magnetization of these grains, which origin, however, is uncertain. The natural rocks containing nearly stoichiometric magnetite manifest a dependence of the R LT ratio on magnetic domain state identical to that observed from synthetic magnetites. Therefore, the comparison of FC SIRM and ZFC SIRM at very low temperatures may allow a crude estimate of magnetic domain state in some magnetite-bearing rocks, such as shallow mafic intrusions or some marine sediments.

A study of sodium silicate in aqueous solution and sorbed by synthetic magnetite using in situ ATR-FTIR spectroscopy

The sorption of sodium silicate by synthetic magnetite (Fe 3O 4) at different pH conditions (pH 7–11) and initial silicate concentrations ( 1 × 10 −3 and 10 × 10 −3 mol L −1 ) was studied using in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The analysis of infrared spectra of sodium silicate in solution as well as adsorbed on magnetite nano-particles clearly showed the evolution of different silicate species depending on pH and silica concentration. The silicate concentration studied ( 10 × 10 −3 mol L −1 ) contained polymeric or condensed silicate species at lower pH as well as monomers at high pH, as evident from infrared spectra. Condensation of monomers resulted in an increased intensity of absorptions in the high frequency part (>1050 cm −1) of the spectral region, which contains information about both silicate in solution and sorbed silicate viz. 1300 cm −1–850 cm −1. In the pH range studied, infrared spectra of sorbed silicate and sorbed silicate during desorption both indicated the presence of different types of surface complexes at the magnetite surface. The sorption mechanism proposed is in accordance with a ligand exchange reaction where both monodentate and bidentate complexes could exist at low surface loading level, the relative proportion of the complexes being due to both pH and concentration in solution. Oligomerization occurred on the magnetite surface at higher surface loading.

Catalytic Ozonation of Propanal Using Wood Fly Ash and Metal Oxide Nanoparticle Impregnated Carbon

Catalytic ozonation of propanal at ambient temperatures (23-25 degrees C) was investigated by varying propanal and ozone concentrations and catalyst type. The catalysts tested included wood fly ash (WFA), magnetically separated ash, synthetic hematite and magnetite, and metal oxide nanoparticle impregnated activated carbon and peanut hull char. A power law model independent of ozone concentration for WFA (r(w), moles g(-1) s(-1)) and magnetite (r(m)) were, respectively, r(w) = k'(w) C(R(0.89)) and r(m) = k'(m)C(R(1.55)), where kw, and k'(m) were 2.36 x 10(-6) g(-1) s(-1) (moles)(-0.11) (m3)(0.89) and 6.5 x 10(-4) g(-1) s(-1) (moles)(-0.55) (m3)(1.55), respectively (5-15 ppmv). Magnetite and hematite present in the WFA were theorized to be the primary active sites, since magnetically separated WFA had a significantly higher reaction rate (approximately 12x, mol m(-2) s(-1)) than that of WFA. X-ray diffraction analysis demonstrated a qualitative increase in magnetite and hematite in the magnetically separated ash, and synthetic magnetite and hematite had reaction rates >80x and 200x that of WFA or activated carbon (surface area basis). Supercritical deposition of hematite on/in peanut hull char successfully generated a porous, pelleted catalystfrom an agricultural residue capable of oxidizing propanal at rates 12x activated carbon and similar to commercially available catalysts (per mass basis). Water vapor significantly increased the propanal reaction rate when using wood fly ash and activated carbon.