Synthetic Magnetite Research Articles

Application of Single-Particle ICP-MS to Determine the Mass Distribution and Number Concentrations of Environmental Nanoparticles and Colloids

Analyzing the elemental compositions and size distributions of nanoparticles, colloids, and their aggregates in environmental samples represents a key task in understanding contaminant, substrate, and nutrient cycling. Single-particle ICP-MS (spICP-MS) is a high-throughput method that is capable of providing the elemental mass of thousands of particles within minutes. The challenge, however, lies in data analysis and interpretation, especially for complex environmental samples. Here we present successful applications of spICP-MS for environmental samples. We first analyzed the homoaggregation behavior of synthetic microplastic and magnetite (abiogenic and biogenic) nanoparticles. The measured distribution of aggregate mass was described as a function of the number of primary particles/aggregate (Npp). In tandem with dynamic light scattering data, differences in aggregates’ compactness (primary particles per nanometer) between samples can be determined. Second, we showed how sequential elemental analysis allows evaluation of the mobility of a toxic arsenic metalloid and its inferred association with colloidal Fe(III) (oxyhydr)oxides. Finally, we investigated the composition of heterogeneous iron–carbon-rich colloidal flocs, highlighting distinct colloidal Fe and C distributions and C/Fe ratios between samples from different permafrost thawing stages. On the basis of our results, we provide guidelines for successful sample preparation and promising future spICP-MS opportunities and applications with environmental samples.

Beneficiation of low-grade, goethite-rich iron ore using microwave-assisted magnetizing roasting

Microwave-assisted reduction roasting of a goethite-rich, reject iron ore waste stream (−2 mm) was used to produce a high-grade concentrate. Reduction roast experiments were conducted at 370 °C, 450 °C, 600 °C and 1000 °C under gas atmospheres of 30:70 and 40:60 CO/CO2, with a soak time of 20 min. Goethite was converted to hematite above 370 °C under both gas mixtures while at the higher roasting temperatures, increasing amounts of magnetite formed. Roasting conditions for the best conversion of goethite to synthetic magnetite were 600 °C in a gas atmosphere of 40:60 CO/CO2, with a soak time of 20 min. Laboratory-based magnetic separations in a Davis tube indicated that a blast furnace grade (+62 wt% Fe) pellet concentrate could be produced with an acceptable iron recovery of > 88 wt%. Under both gas atmospheres, a higher reduction temperature of 1000 °C achieved a greater conversion of goethite to magnetite but resulted in over-reduction and the generation of wüstite, fayalite and Fe-rich spinel phases with different magnetic susceptibilities that are expected to make subsequent beneficiation difficult. Further processing to optimize the microwave-assisted magnetizing roast and the magnetic separation conditions can be expected to maximize the efficiency of upgrading the iron content in low grade goethite-rich iron ores.

Electromagnetic shielding response of magnetite elastomeric composites: Source and filler content dependence

The structural and electromagnetic shielding properties of NBR rubber filled with magnetite (Fe3O4) were studied. Rubber composite materials with ore magnetite and synthetic magnetite have been produced separately in order to observe its differences. Samples have been fabricated using a rubber mill and different magnetite ratios. Also, carbon black was added in the same ratio in all samples, not only to improve the mechanical properties of the composites, but also to enhance its dielectric permittivity and reduce its electric resistivity. This kind of composites are used for the manufacture of electronic elements and especially for the production of electromagnetic shielding materials in both passive defense and civil construction applications; buildings, vehicles or airplanes. FESEM-FIB micrographs of the fillers showed the differences in size and shape of the fillers, which influenced the distribution of the particles inside the matrix and therefore the final properties of the composite. Elemental distribution imaging of the composites exhibited the formation of both synthetic magnetite and carbon black aggregates, these structures affect the samples resistivity and the electromagnetic losses due to the conductivity at high frequencies. Electromagnetic shielding properties of the composites were measured in the X-band (8−12 GHz) and K-band of frequency (18–26.5 GHz), all samples showed attenuations greater than −26 dB in both frequency bands. Peaks of maximum attenuation and wider bandwidths were found in the K-band due to the electronic polarization that increased the permittivity of the samples at this high frequency band. The magnetite source (mineral or artificial), its ratio in the rubber matrix and the composite thickness determined the electromagnetic properties of the composite materials.

Experimental study on oxidation of synthetic and natural magnetites monitored by magnetic measurements

Magnetite is one of the most important iron oxides due to its relatively widespread natural occurrence, simple synthesis methods, and unique magnetic properties. In addition, when the size of its particles decreases to nanoscale, magnetite acquires new characteristics, becoming useable for environmental remediation, biomedicine, data storage, and catalysis. However, the successful use of magnetite nanoparticles in these areas strongly depends on its chemical stability and magnetic properties under various conditions. In this article, the experimental oxidation of various types of magnetite in air was studied using thermomagnetic, thermogravimetric, and X-ray diffraction analyses, along with magnetometry. The results showed that natural and synthetic micro-sized magnetites oxidized to form hematite at 200 °C, while the formation of other phases was not detected. The complete conversion of magnetite to hematite was achieved after heating at 600–700 °C for 60 min. In contrast, synthetic nano-sized magnetites oxidized via the formation of maghemite-magnetite solid solution, revealing an increase in the Curie temperature and only a slight decrease in the mass magnetization at heating to 500 °C. Above 500 °C, the magnetization decreased due to the transformation of maghemite to hematite or, possibly, amorphous iron oxide. Also, the formation of an intermediate phase with Curie temperatures of 325–425 °C was observed at a later stage of the oxidation of synthetic nano-sized magnetite. This can be explained by maghemite phase associated with small residual clusters that have not yet undergone conversion. The oxidation of magnesiomagnetite with 9 wt% of MgO proceeded differently due to the stabilizing effect of Mg ions. A gradual decrease in Curie temperature from 508 to 315 °C and magnetization from 65 to 25 A m2 kg−1 indicate the oxidation of Fe2+ to Fe3+ and cation redistribution. The sample after heating at 900 °C for 60 min was represented by the spinel phase with magnetic properties close to magnesioferrite, while hematite formation was not detected. The phase transformations of magnetite with different grain sizes and composition can be used for the synthesis of iron oxide nanoparticles (IONs) with tunable magnetic properties and improved stability.

Investigating the ferric ion binding site of magnetite biomineralisation protein Mms6

The biomineralization protein Mms6 has been shown to be a major player in the formation of magnetic nanoparticles both within the magnetosomes of magnetotactic bacteria and as an additive in synthetic magnetite precipitation assays. Previous studies have highlighted the ferric iron binding capability of the protein and this activity is thought to be crucial to its mineralizing properties. To understand how this protein binds ferric ions we have prepared a series of single amino acid substitutions within the C-terminal binding region of Mms6 and have used a ferric binding assay to probe the binding site at the level of individual residues which has pinpointed the key residues of E44, E50 and R55 involved in Mms6 ferric binding. No aspartic residues bound ferric ions. A nanoplasmonic sensing experiment was used to investigate the unstable EER44, 50,55AAA triple mutant in comparison to native Mms6. This suggests a difference in interaction with iron ions between the two and potential changes to the surface precipitation of iron oxide when the pH is increased. All-atom simulations suggest that disruptive mutations do not fundamentally alter the conformational preferences of the ferric binding region. Instead, disruption of these residues appears to impede a sequence-specific motif in the C-terminus critical to ferric ion binding.

The economic potential of the African iron-ore tailings: synthesis of magnetite for the removal of trace metals in groundwater—a review

Africa has the second largest ultimately recoverable resources (URR) of gigaton (Gt) iron (Fe) ore in the world. South Africa is among the countries in the world with high-quality iron ore deposits. The most significant sources of Fe ore in the African continent are hosted by the banded iron ore formation of Precambrian age with high-quality hematite (Fe2O3) bodies. Synthetic magnetite (Fe3O4) nanoparticles (NPs), in recent years, have emerged as novel magnetic NPs with structurally distinct properties because of intensive research efforts by many researchers. They exhibited almost the same chemical and physical properties with naturally occurring Fe3O4, and they have been successfully adopted in removing trace metals from groundwater. For a better understanding of the development of suitable techniques for their application in contaminated groundwater, this paper emphasizes the synthetic methodology, surface modification, coating with porous materials/inorganic clays, functionalized composites, properties, and its ex situ groundwater remediation application. Magnetic separation is the most suitable and cost-effective method for the groundwater remediation of trace metals for both high and lower concentrations.

Zeolite-magnetite composites to remove Hg2+ from water

It is reported an innovative though still exploratory study devoted to characterizing their structure and evaluating the response of zeolite-magnetite composites as adsorbing materials to remove Hg2+ from water. The zeolite material collected from a sampling site in Parral, Chile, was identified and structurally characterized as containing mordenite with crystallographic orthorhombic unit cell dimensions a = 18.060(1) A; b = 20.429(2) A and c = 7.5091(7) A. From Mossbauer data, two paramagnetic ferric iron sites occur in its structure. This zeolite, a sample of synthetic magnetite and their two-component composites were used as adsorbers to sequester Hg2+ dissolved in water. The adsorption rate was found to reach a steady state of nearly constant Hg concentrations corresponding to 26.2; 22.1; 20.6; 17.4 and 16.7 mg Hg per g adsorber, between 24 h and 72 h reaction, for the samples zeolite only, for the composites zeolite:magnetite mass ratio 10:1; 5:1; 1:1 and for the magnetite only, respectively. These corresponding maximum adsorption capacities reduced the initial concentration of Hg2+ in water from 700 mg L−1 to an equilibrium concentration of 0; 110; 150; 240 and 250 mg L−1, respectively. Although the composites showed a lower adsorption capacity of Hg than did the sole zeolite (zeol sample), the composites contained magnetic particles, which allow the adsorbing systems to be readily removed from the aqueous medium, with a magnetic field. These combined characteristic point to the remarkable potential of such materials, particularly the mag-zeol-2, to be used to remediate natural bodies of water contaminated with Hg.

Development of a modified SEDEX phosphorus speciation method for ancient rocks and modern iron-rich sediments

We report the development of a modified method for evaluating different reservoirs of sedimentary phosphorus (P) in ancient marine sedimentary rocks and in modern Fe-rich sediments. Utilising the existing SEDEX scheme for P partitioning in modern sediments, we initially demonstrate limitations in the application of the original scheme to sediments and rocks containing crystalline hematite and magnetite. We tested additional extractions for these crystalline Fe phases, using both synthetic minerals, and modern and ancient sediments. The addition of 6 h oxalate and 6 h citrate-dithionate-acetate extractions considerably enhanced the total recovery of synthetic magnetite and hematite to 88.7 ± 1.1% and 76.9 ± 3.8%, respectively. In addition, application of the 6 h oxalate extraction to synthetic P-containing magnetite recovered 93.9 ± 1.7% of the Fe present and 88.2 ± 12.8% of the co-precipitated P. Based upon these results we developed a modified SEDEX extraction scheme. The modified scheme was applied to modern Fe-rich sediments from Golfo Dulce, Costa Rica, which resulted in 16% higher Fe-bound P recovery. Application of the scheme to a variety of ancient marine rocks increased the recovery of Fe-bound P by up to 22%. We also highlight the potential for authigenic carbonate fluorapatite to convert to more crystalline apatite in ancient rocks during deep burial and metamorphism. We suggest that in such systems minimum and maximum estimates of the total reactive P pool may be calculated with and without the inclusion of crystalline P. It is noted that the application of the revised method may have important implications for understanding the cycling of P in ancient marine environments.

Evaluation of Maghemite-Rich Iron Oxide Composite Prepared from Magnetite as Adsorbent for Gold from Chloride Solution

The capacity and effectiveness of a composite magnetic iron oxide, consisting mainly of maghemite $$ (\gamma {\text{Fe}}_{2} {\text{O}}_{3} ) $$, prepared by oxidation of synthetic magnetite at 350°C have been evaluated. X-ray diffraction analysis of the synthesized product indicated an iron oxide with 44%, 33%, and 23% of maghemite, hematite (Fe2O3), and magnetite (Fe3O4), respectively. The produced composite material was used in this study as adsorbent to recover gold from chloride solution. The effects of different process parameters, such as solution pH, contact time, chloride ion concentration, and initial Au concentration, on the recovery of Au were studied. Almost all Au in solution (99.78%) was recovered at pH of about 6.5 after 24 h of contact time. The maximum Au uptake of about 11.5 µmol/g was obtained at initial Au concentration of 3.5 × 10−4 mol/L. No significant difference in Au uptake was observed upon increasing the contact time to 48 h. High chloride concentration was found to be detrimental to Au recovery. These results indicate that the prepared maghemite-rich iron oxide composite is more effective than synthetic magnetite reported in previous studies for recovery of Au from chloride solution.

Domain State and Temperature Dependence of Pressure Remanent Magnetization in Synthetic Magnetite: Implications for Crustal Remagnetization

AbstractPressure remanent magnetization (PRM) is acquired when a rock is compressed in the presence of a magnetic field. This process can take place in many different environments from impact and ejection processes in space, to burial and subsequent uplifting of terrestrial rocks. In this study, we systematically study the acquisition of PRM at different pressures and temperatures, using synthetic magnetite in four different grain sizes ranging from nearly single‐domain to purely multidomain. The magnitude of the PRM acquired in a 300 μT field is, within error, independent of the domain state of the sample. We propose that the acquisition of a PRM is mainly driven by the magnetostriction of the magnetic material. We further show that compared to a thermal remanent magnetization, the acquisition of PRM in large multidomain grains can be quite efficient, and may represent a significant component of magnetization in low‐temperature–high‐pressure environments.

Organophosphonate-functionalized nanosized magnetic iron oxides as sorbents for heavy metal cations

The surface of synthetic magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles was modified with phosphonic chelating compounds, 1-hydroxyethane-1,1-di(phosphonic acid) and nitrilotris(methylenephosphonic acid). The sorption affinity of functionalized magnetic iron oxides towards Niii under acidic conditions was increased by one order of magnitude.

Model colloids to study surface – ligand interactions in nanosized Fe3O4

Using electrodispersion of metallic iron in an aqueous environment without additional reagents we create a chemically clean model system based on nanosized synthetic magnetite where the details of magnetic metal oxide – surfactant chemisorption can be studied by high fidelity magnetometry with nanometric precision. A comparative analysis of model colloidal solutions based on oleic acid and iron Fe2+ oleate indicates a significant influence of the mode of surfactant adsorption on the magnetic diameter of the particles.

High‐Temperature Deformation Behavior of Synthetic Polycrystalline Magnetite

AbstractWe performed a series of deformation experiments on synthetic magnetite aggregates to characterize the high‐temperature rheological behavior of this mineral under nominally dry and hydrous conditions. Grain growth laws for magnetite were additionally determined from a series of static annealing tests. Synthetic magnetite aggregates were formed by hot isostatic pressing of fine‐grained magnetite powder at 1,100 °C temperature and 300‐MPa confining pressure for 20 hr, resulting in polycrystalline material with a mean grain size around 40 μm and containing 2–4% porosity. Samples were subsequently deformed to axial strains of up to 10% under constant load conditions at temperatures between 900 and 1,150 °C in a triaxial deformation apparatus under 300‐MPa confining pressure at applied stresses in the range of 8–385 MPa or in a uniaxial creep rig at atmospheric pressure with stresses of 1–15 MPa. The aggregates exhibit typical power‐law creep behavior with a mean stress exponent of 3 at high stresses, indicating a dislocation creep mechanism and a transition to near‐Newtonian creep with a mean stress exponent of 1.1 at lower stresses. The presence of water in the magnetite samples resulted in significantly enhanced static grain growth and strain rates. Best‐fit flow laws to the data indicate activation energies of around 460 and 310 kJ/mol for dislocation and diffusion creep of nominally dry magnetite, respectively. Based on the experimentally determined flow laws, magnetite is predicted to be weaker than most major silicate phases in relatively dry rocks such as oceanic gabbros during high‐temperature crustal deformation.

Mineralogy and buffer identity effects on RDX kinetics and intermediates during reaction with natural and synthetic magnetite

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is known to undergo reduction mediated by ferrous iron in the presence of minerals, including magnetite. Idealized laboratory conditions may not provide representative reaction kinetics or pathways compared to field conditions. The effects of magnetite mineral morphology, the aquifer material matrix, the presence of aqueous Fe(II), and the buffer identity on RDX reduction kinetics and intermediate formation are investigated in this work. Reactions in bicarbonate buffer were substantially slower than those performed in 3-(N-morpholino)propanesulfonic acid (MOPS) buffer, and the presence of quartz and clays in magnetite-containing aquifer material resulted in slower reaction kinetics and production of additional iron oxide phases. Buffer identity also changed the rate controlling step and reaction product distribution. Conditions as close to those expected in field systems are necessary to evaluate the reaction rates and pathways of RDX in reduced groundwater systems.

Adsorption Properties of a Magnetite Composite with Coffee Waste

Magnetic composite sorbent consisting of a synthetic magnetite and ground coffee waste is produced and characterized. The point of zero charge is determined. The presence of carbonyl and carboxyl groups on the sorbent surface is established. The sorbent’s sorption capacity toward methylene blue dye is determined. The adsorption kinetic curves are analyzed using models of diffusion and chemical kinetics. It is shown that adsorption proceeds in the mixed diffusion regime and can be described by pseudo second-order kinetics. The correlation between the adsorption isotherms and the Langmuir and Freundlich models is evaluated. The possibility of sorbent regeneration is demonstrated. It is concluded that the proposed composite magnetic sorbent has high efficiency and can be used to purify water of organic dyes.

Thermodynamic, kinetic and spectroscopic investigation of arsenite adsorption mechanism on pine cone-magnetite composite

Adsorptive removal of arsenite from wastewater often involves pre-oxidation to the less toxic arsenate followed by adsorption. Materials capable of simultaneous oxidation and adsorption could potentially improve the treatment of arsenic contaminated water. A novel magnetic composite consisting of Fenton’s treated pine cone, an agricultural waste and magnetite nanoparticles was prepared and applied to achieve simultaneous arsenite oxidation and adsorptive removal. A mechanistic investigation of the adsorption process was conducted to provide insights into the interaction of arsenite and the composite surface. X-ray diffraction peaks of the nanoparticles matched those of synthetic magnetite and X-ray photoelectron spectroscopy (XPS) indicated that Fe2+/Fe3+ ratio was in accordance with that of magnetite at 0.5. Arsenite removal efficiency by the as-synthesised composite was approximately 90% with a maximum adsorption capacity of 17 mg g−1. Kinetic studies revealed that arsenic adsorption was a multi-step process with surface adsorption and intra-particle diffusion contributing to the rate limiting step. Ion exchange was evident from the release of pre-loaded nitrate ions during arsenite adsorption. Spectroscopic evidence confirmed ion exchange through the observance of hydroxyl peak shift, reduction in intensity of carboxylate peaks and AsOFe bond formation during adsorption. Further, XPS results confirmed that there was simultaneous reduction of Fe3+ to Fe2+, oxidation of arsenite to arsenate and adsorption of both arsenite and arsenate on the composite surface through the formation of inner sphere complexes.

Anhysteretic remanent magnetization: model of grain size distribution of spherical magnetite grains

A phenomenological model based on a linear relationship between the magnetic coercivity field and the reciprocal of the grain diameter is applied to explain the anhysteretic remanent magnetization (ARM) imparted to artificial samples with different concentrations of a very well characterized magnetite powder. By analyses of scanning electron microscopy images, the spherically shaped single domain synthetic magnetite is found to follow a lognormal grain size distribution with ~86 nm of mean diameter. The proposed model, fitted to ARM measurements up to a peak alternating field of 100 mT, yields a very good agreement. The coercivity behaviour predicted by micromagnetism theory disagrees with the experimental results of this work. A likely explanation for the discrepancy is that the magnetite particles, which consist of a mixture of grains in coherent rotation and curling modes, produce similar observations as domain processes.

Stimulatory effect of magnetite on the syntrophic metabolism of Geobacter co-cultures: Influences of surface coating

Magnetite-mediated direct interspecies electron transfer (DIET) can facilitate syntrophic metabolism in natural microbial communities and also promote the performance of the engineered systems based on syntrophic interactions. In this study, the stimulatory effect of bare synthetic magnetite (Mt), humic acid coated magnetite, and SiO 2 coated magnetite (Mt-SiO 2 ) on DIET in defined co-cultures of Geobacter metallireducens / Geobacter sulfurreducens were studied. Magnetite coated with Aldrich humic acid (HA) and Elliott Soil humic acid (HA ES ), respectively, were prepared, and the two kinds of humic acid influenced the ability of Mt to promote syntrophic metabolism of the co-cultures in a similar way. When weight concentration was the same, pure humic acid presented the stimulatory effect on DIET similar to bare magnetite. However, the presence of HA coating on magnetite surface caused 50% and 61%, respectively, decrease in the rates of ethanol consumption ( R e ) and succinate production ( R s ) in DIET processes. Pure HA in the same weight concentration as the HA coating in Mt-HA induced the similar metabolism rates as Mt-HA. In the Mt-HA mediated DIET, most electrons from ethanol metabolism were transferred to G. sulfurreducens selectively through the HA coating, and magnetite core hardly contributed to DIET processes. The SiO 2 coating on magnetite resulted in 81% and 89%, respectively, decreases in R e and R s , mainly because the non-conductive SiO 2 layer hindered electron transfer between magnetite core and bacteria. After eight-day incubation with the co-cultures, bare magnetite nanoparticles formed relatively larger and more compact aggregates with cells than Mt-HA and Mt-SiO 2 , due to the different surface charge between bare and coated Mt. The generation of dissolved Fe(II) and HCl-extractable Fe(II) due to microbial reduction of magnetite by G. metallireducens and vivianite formation were observed along with DIET processes in all DIET experiments. Based on these results, different pathways of electron transfer in defined co-cultures of Geobacters with bare and coated magnetite nanoparticles were proposed. The findings in this study demonstrate the significant effects of surface properties on the ability of magnetite to stimulate DIET, which needs to be considered in order to comprehensively understand the role and mechanisms of mineral-mediated DIET in natural and engineered systems.

Synthesis and magnetic properties of magnetite prepared by chemical reduction from hematite of various particle sizes

In the process of beneficiating iron ores by the magnetic separation, particles of hematite and quartz accumulate, forming tailings that occupy large areas of agricultural lands. Such man-made deposits are quite common and contain large quantities of iron oxides such as hematite. Therefore the tailings can be used as raw material for the production of magnetite. In this work, magnetite was synthesized by hematite reduction at a temperature of 500 °C in an atmosphere of carbon monoxide. The source hematite was separated initially into ten particle size categories ranging from 0.05 to 2.5 mm. The reduction was carried out with carbon monoxide, which was synthesized by reaction of air and activated carbon at 750 °C. It was determined by X-ray diffraction analysis and magnetometry that, after reduction, samples of each particle size consisted mainly of magnetite. In addition, the magnetic properties of the resulting magnetite were noted to be somewhat different from those of natural magnetite. In particular, remanence and coercivity are higher for synthetic magnetite than for natural magnetite. The saturation magnetization of transformed samples increases to 60–80 Am2/kg, while the source hematite does not have strong magnetic properties. Hematite with a particle size from 0.05 to 0.25 mm is completely converted into magnetite, but if the size of hematite particles increases to from 0.25 to 2.5 mm, incomplete conversion of hematite to magnetite is observed. The obtained results are important for the development of technology for converting hematite to magnetite, which can, in turn, be used to beneficiate iron ores.

Isothermal Oxidation of Magnetite to Hematite in Air and Cyclic Reduction/Oxidation Under Carbon Looping Combustion Conditions

In the carbon looping combustion process the oxygen carrier is regenerated through oxidation in air; this process has been simulated by the oxidation of dense synthetic magnetite for selected temperatures and times. The oxidation of magnetite in air is shown to occur through the formation of dense hematite layers on the particle surface. This dense hematite forms through lath type shear transformations or solid-state diffusion through the product layer. Cyclic reduction in CO-CO2/oxidation in air of hematite single crystals has been carried out under controlled laboratory conditions at 1173 K (900 °C). It has been shown that the initial reduction step is critical to determining the product microstructure, which consists of gas pore dendrites in the magnetite matrix with blocky hematite formed on the pore surfaces. The progressive growth of the magnetite layer with the application of subsequent cycles appears to continue until no original hematite remains, after which physical disintegration of the particles takes place.