Showing papers on "Magnetite published in 2017"

Oxidation of magnetite nanoparticles: impact on surface and crystal properties

Abstract: Iron oxide nanoparticles are of great scientific interest due to their huge versatility of applications. The oxidation process of magnetite to maghemite is difficult to monitor as both iron oxide polymorphs possess connatural chemical properties. Especially the surface composition and reactivity of these nanosystems, which are most relevant for interactions with their environment, are not completely understood. Here, the oxidation of magnetite is investigated under mild and harsh conditions in order to understand the oxidation behaviour and the chemical stability of transition forms. Therefore, the oxidation process, is investigated with Raman, Mossbauer and X-ray photoelectron spectroscopy as well as X-ray diffraction and magnetometry. The multi-analytical approach allows new insights into surface composition and rearrangement according to respective different depth profiles. For both conditions investigated, the ferrous iron components are oxidised prior to structural changes in the Fe–O vibrations and crystal structure. The process starts from the outer layers and is acid catalysed. Oxidation leads to a decrease of magnetisation which still remains higher than 54 emu g−1. The charge and surface reactivity can be affected by the different oxidation methods and the irreversible adsorption of acid molecules. Biocompatibility and catalytic properties of iron oxide nanoparticles open doors to future applications.

Composition Tunable Manganese Ferrite Nanoparticles for Optimized T2 Contrast Ability

Abstract: Manganese-doped magnetite nanoparticles as magnetic resonance imaging (MRI) contrast agents have been well developed in recent years due to their higher saturation magnetization and stronger transverse (T2) contrast ability compared to parent magnetite. However, the underlying role that manganese doping plays in altering the contrast ability of magnetite is still not thoroughly understood. Herein, we investigate the effects of manganese doping on changes of ferrite crystal structures, magnetic properties, and contrast abilities. We developed a successful one-pot synthesis of uniform manganese-doped magnetite (MnxFe3–xO4) nanoparticles with different manganese contents (x = 0–1.06). The saturation magnetization and T2 contrast ability of ferrite nanoparticles increase along with rising manganese proportion, peak when the doping level of MnxFe3–xO4 reaches x = 0.43, and decrease dramatically as the manganese percentage continues to augment. At high manganese doping level, the manganese ferrite nanoparticles...

On the ‘centre of gravity’ method for measuring the composition of magnetite/maghemite mixtures, or the stoichiometry of magnetite-maghemite solid solutions, via 57Fe Mössbauer spectroscopy

Abstract: We evaluate the application of 57Fe Mossbauer spectroscopy to the determination of the composition of magnetite (Fe3O4)/maghemite (γ-Fe2O3) mixtures and the stoichiometry of magnetite-maghemite solid solutions In particular, we consider a recently proposed model-independent method which does not rely on a priori assumptions regarding the nature of the sample, other than that it is free of other Fe-containing phases In it a single parameter, δRT—the ‘centre of gravity’, or area weighted mean isomer shift at room temperature, T = 295 ± 5 K—is extracted by curve-fitting a sample’s Mossbauer spectrum, and is correlated to the sample’s composition or stoichiometry We present data on highpurity magnetite and maghemite powders, and mixtures thereof, as well as comparison literature data from nanoparticulate mixtures and solid solutions, to show that a linear correlation exists between δRT and the numerical proportion of Fe atoms in the magnetite environment: α = Femagnetite/Fetotal = − ( ) δ δ RT o /m, where δo = 03206 ± 00022mm s−1 and m = 02135 ± 00076mm s−1 We also present equations to relate α to the weight percentage w of magnetite in mixed phases, and the magnetite stoichiometry x = Fe2+/Fe3+ in solid solutions The analytical method is generally applicable, but is most accurate when the absorption profiles are sharp; in some samples this may require spectra to be recorded at reduced temperatures We consider such cases and provide equations to relate δ ( ) T to the corresponding α value

Microwave-assisted ammonia decomposition reaction over iron incorporated mesoporous carbon catalysts

Abstract: Microwave-assisted ammonia decomposition reaction was investigated to produce COx free hydrogen, for fuel cell applications. Iron incorporated mesoporous carbon catalysts were prepared at different metal loadings, following an impregnation procedure. Mesoporous carbon acted as the catalyst support, as well as the microwave receptor. Complete conversion of ammonia was achieved at 450 °C over the catalyst having 7.7 wt% Fe, when the reaction was carried out in the microwave reactor system, using pure ammonia (GHSV of 36000 ml/h gcat). However, in the case of using the conventionally heated reactor, complete conversion of ammonia was achieved only at 600 °C. Iron oxides, namely maghemite (γ-Fe2O3), magnetite (Fe3O4) and hematite (α-Fe2O3) simultaneously appeared in the structure of the synthesized catalysts, after their calcination at 450 °C, under pure N2 flow. Iron oxides present in the calcined catalytic materials then were reduced to metallic iron at 500 °C. Formation of iron carbide crystals was observed in the structure of spent catalysts that were used in microwave reactor system, while metallic iron crystals were still present in the catalysts that were tested in conventionally heated system.

Reduction and Simultaneous Removal of 99Tc and Cr by Fe(OH)2(s) Mineral Transformation

Abstract: Technetium (Tc) remains a priority remediation concern due to persistent challenges, including mobilization due to rapid reoxidation of immobilized Tc, and competing comingled contaminants, e.g., Cr(VI), that inhibit Tc(VII) reduction and incorporation into stable mineral phases. Here Fe(OH)2(s) is investigated as a comprehensive solution for overcoming these challenges, by serving as both the reductant, (Fe(II)), and the immobilization agent to form Tc-incorporated magnetite (Fe3O4). Trace metal analysis suggests removal of Tc(VII) and Cr(VI) from solution occurs simultaneously; however, complete removal and reduction of Cr(VI) is achieved earlier than the removal/reduction of comingled Tc(VII). Bulk oxidation state analysis of the final magnetite solid phase by XANES shows that the majority of Tc is Tc(IV), which is corroborated by XPS measurements. Furthermore, EXAFS results show successful, albeit partial, Tc(IV) incorporation into magnetite octahedral sites. Cr XPS analysis indicates reduction to Cr(III) and the formation of a Cr-incorporated spinel, Cr2O3, and Cr(OH)3 phases. Spinel (modeled as Fe3O4), goethite (α-FeOOH), and feroxyhyte (δ-FeOOH) are detected in all reacted final solid phase samples analyzed by XRD. Incorporation of Tc(IV) has little effect on the spinel lattice structure. Reaction of Fe(OH)2(s) in the presence of Cr(III) results in the formation of a spinel phase that is a solid solution between magnetite (Fe3O4) and chromite (FeCr2O4).

Macroporous Carbon Supported Zerovalent Iron for Remediation of Trichloroethylene

Abstract: Groundwater contamination with chlorinated hydrocarbons has become a widespread problem that threatens water quality and human health. Permeable reactive barriers (PRBs), which employ zerovalent iron, are effective for remediation; however, a need exists to reduce the economic and environmental costs associated with constructing PRBs. We present a method to produce zerovalent iron supported on macroporous carbon using only lignin and magnetite. Biochar-ZVI (BC-ZVI) produced by this method exhibits a broad pore size distribution with micrometer sized ZVI phases dispersed throughout a carbon matrix. X-ray diffraction revealed that pyrolysis at 900 °C of a 50/50 lignin–magnetite mixture resulted in almost complete reduction of magnetite to ZVI and that compression molding promotes iron reduction in pyrolysis due to mixing of starting materials. High temperature pyrolysis of lignin yields some graphite in BC-ZVI due to reduction of carbonaceous gases on iron oxides. TCE was removed from water as it passed thr...

Self‐Assembled Magnetite Mesocrystalline Films: Toward Structural Evolution from 2D to 3D Superlattices

Abstract: This study describes synthesis and detailed characterization of 2D and 3D mesocrystalline films produced by self-assembly of iron oxide (magnetite) truncated nanocubes. The orientational relations between nanocrystals within the superlattice are examined and atomistic models are introduced. In the 2D case, two distinct superstructures (i.e., translational order) of magnetite nanocubes can be observed with p4mm and c2mm layer symmetries while maintaining the same orientational order (with [100]magnetite perpendicular to the substrate). The 3D structure can be approximated by a slightly distorted face-centered cubic (fcc) superlattice. The most efficient space filling within the 3D superstructure is achieved by changing the orientational order of the nanoparticles and following the “bump-to-hollow” packing principle. Namely orientational order is determined by the shape of the nanoparticles with the following orientational relations: [001]SL||[310]magnetite, [001]SL||[301]magnetite, [001]SL||[100]magnetite. Overall the presented data provide a fundamental understanding of a mesocrystal formation mechanism and their structural evolution. Structure, composition, and magnetic properties of the synthesised nanoparticles are also characterized.

Mineral chemistry of magnetite from magnetite-apatite mineralization and their host rocks: examples from Kiruna, Sweden, and El Laco, Chile

Abstract: Interpretation of the mineralizing environment of magnetite-apatite deposits remains controversial with theories that include a hydrothermal or magmatic origin or a combination of those two processes. To address this controversy, we have analyzed the trace element content of magnetite from precisely known geographic locations and geologic environments from the Precambrian magnetite-apatite ore and host rocks in Kiruna, Sweden, and the Pliocene-Holocene El Laco volcano in the Atacama desert of Chile. Magnetite samples from Kiruna have low trace element concentrations with little chemical variation between the ore, host, and related intrusive rocks. Magnetite from andesite at El Laco, and dacite from the nearby Lascar volcano, has high trace element concentrations typical of magmatic magnetite. El Laco ore magnetite have low trace element concentrations and displays growth zoning in incompatible elements (Si, Ca, and Ce), compatible elements (Mg, Al, and Mn), large-ion lithophile element (Sr), and high field strength element (Y, Nb, and Th). The El Laco ore magnetite are similar in composition to magnetite that has been previously interpreted to have crystallized from hydrothermal fluids; however, there is a significant difference in the internal zoning patterns. At El Laco, each zoned element is either enriched or depleted in the same layers, suggesting the magnetite crystallized from a volatile-rich, iron-oxide melt. In general, the compositions of magnetite from these two deposits plot in very wide fields that are not restricted to the proposed fields in published discriminant diagrams. This suggests that the use of these diagrams and genetic models based on them should be used with caution.

Synergistic effect of co-existence of hematite (α-Fe2O3) and magnetite (Fe3O4) nanoparticles on graphene sheet for dye adsorption

Abstract: Graphene oxide (GO) functionalized with hematite (α-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) nanoparticles (rGO-Fe 2 O 3 –Fe 3 O 4 ) was prepared using a facile one-step co-precipitation technique. It shows superior performance towards methylene blue (MB) adsorption for water purification, compared to GO functionalized with hematite (rGO-Fe 2 O 3 ) or magnetite (rGO-Fe 3 O 4 ) nanoparticles. It also shows better performance compared to a composite mixture of rGO-Fe 2 O 3 and rGO-Fe 3 O 4 (rGO-M). It has been postulated that the co-existence of hematite and magnetite nanoparticles on graphene sheet causes the synergistic effect towards MB adsorption. The adsorption behaviour of GO, reduced graphene oxide (rGO), rGO-Fe 2 O 3 , rGO-Fe 3 O 4 , rGO-Fe 2 O 3 –Fe 3 O 4 and rGO-M was studied. These materials were characterized using XRD, XPS, Raman spectroscopy, TGA, TEM, VSM and BET surface area analyzer. The phases present in the as-synthesized adsorbents were identified by XRD, Raman and XPS techniques. TGA studies confirmed the strong bonding between iron oxide particles and graphene sheet. TEM characterization was used for nanoparticles morphology and size distribution studies. Kinetics of MB adsorption was well described by the pseudo second order model. Langmuir adsorption isotherm better fits the equilibrium adsorption behaviour of rGO-Fe 2 O 3 –Fe 3 O 4 as compared to Freundlich isotherm and the maximum adsorption capacity was determined to be 72.8 ± 2.7 mg/g. Regeneration and reusability studies performed on rGO-Fe 2 O 3 –Fe 3 O 4 revealed that it retains more than 65% of the original adsorption capacity even after 3 cycles thus making it a potential candidate for water treatment.

High temperature EMAT design for scanning or fixed point operation on magnetite coated steel

Abstract: Bulk thickness measurements were performed at elevated temperatures on magnetite coated low carbon steel pipe and aluminium samples, using a permanent magnet electromagnetic acoustic transducer (EMAT). The design presented here exploits the non-contact nature of EMATs to allow continuous operation at elevated temperatures without physical coupling, sample preparation (in the form of oxide scale removal), or active cooling of the EMAT. A non-linear change in signal amplitude was recorded as the magnetite coated sample was heated in a furnace, whereas a steady decrease in amplitude was observed in aluminium. For a magnetite coated pipe sample, after a dwell time of 3 h, a SNR of 33.4 dB was measured at 450 °C, whilst a SNR of 33.0 dB was found at 25 °C. No significant EMAT performance loss was observed after one month of continuous exposure to 450 °C. EMAT-sample lift-off performance was investigated at elevated temperature on magnetite coated steel; a single-shot SNR of 31 dB for 3.0 mm lift-off was recorded at 450 °C, highlighting the suitability of this design for scanning or continuous fixed point inspection at high temperature.

The electromagnetic wave absorbing properties of cement-based composites using natural magnetite powders as absorber

Abstract: In order to develop a low-cost electromagnetic interference shielding and especially absorbing cement materials. In the present study, the utilization of natural magnetite in cement matrix for this purpose was investigated. The dielectric, magnetic and electromagnetic wave absorbing properties of magnetite/cement composites were characterized in the frequency range of 2.6–3.95 GHz (S band). The results show that dielectric and magnetic losses of the composites were increased, and the performance of the composites in electromagnetic absorbing is greatly improved greatly by adding natural magnetite powders as low-cost absorbers. For magnetite/composites, two matching thickness in range of 1–30 mm were clearly observed. The composites with 15% magnetite show the best absorbing property. Its strongest absorbing peak with a reflection loss of −28 dB at 3.7 GHz, and its absorbing bandwidth (<−10 dB) reaches 0.8 GHz.

Various effects of magnetite on international simple glass (ISG) dissolution: implications for the long-term durability of nuclear glasses

Abstract: Understanding the effect of near-field materials, such as iron corrosion products, on the alteration of vitreous nuclear waste is essential for modeling long-term stability of these waste forms in a geological repository. This work presents experimental results for which monoliths of International Simple Glass—a six oxide borosilicate glass–, with polished and unpolished cut sides, were aged for 70 days under oxic conditions at 90 °C in a solution initially saturated in 29SiO2 at pH 7; then magnetite was added to the leaching environment. Solution and solid analyses were performed to correlate the changes in the surface features and dissolution kinetics. It was found that magnetite primarily influences the mechanically constrained surface of the non-polished sides of the monoliths, with little to no effect on the polished surfaces. This work highlights the importance of the unique chemistry within surface cracks that invokes a drastic change in alteration of glass in environments containing iron corrosion products. Immobilization in glass based hosts is the current geological method for disposal of long-lived radioactive waste from used nuclear fuels. A key factor that has to be understood is the fundamental mechanism that controls the glass dissolution in a geological repository involving complex reactions between glass and iron, and iron corrosion products. The team of Stephane Gin from DTCD SECM in France and Nathalie Wall from Washington State University in the USA, together with their co-workers, are seeking to decode the alteration of glass waste in the presence of iron corrosion products, specifically magnetite. It is determined that products such as magnetite primarily impact a mechanically constrained surface, particularly in the case of non-polished sides. Conversely, almost no influence can be observed on the polished surface. Such findings have implication for the long term durability of nuclear glasses.

Naturally-occurring iron minerals as inexpensive catalysts for CWPO

Abstract: This work explores the potential application of naturally-occurring minerals as inexpensive catalysts in heterogeneous Fenton, namely catalytic wet peroxide oxidation (CWPO). The availability, low cost and environmentally friendly character of those materials make them interesting candidates for such application. The performance of magnetite, hematite and ilmenite as CWPO catalysts has been tested under different working conditions, which include temperature (25–90 °C), H 2 O 2 dose (250–1000 mg L −1 ) and catalyst concentration (1–4 g L −1 ). The operating temperature plays a key role on the rate of H 2 O 2 decomposition so that with magnetite H 2 O 2 conversion after 4 h increased from 8 to 99% by increasing the temperature from 25 to 90 °C. Based on the reaction mechanism proposed, a kinetic model was developed which successfully described the experimental results on H 2 O 2 decomposition. The catalytic performance of the minerals tested at temperatures above the ambient was demonstrated using phenol (100 mg L −1 ) as target pollutant. Unprecedented efficiencies of H 2 O 2 consumption, higher than 80% were achieved, allowing high oxidation and mineralization, i.e. complete phenol conversion and almost 80% TOC reduction at 75 °C with a catalyst loading of 2 g L −1 and the theoretical stoichiometric amount of H 2 O 2 for complete mineralization of phenol (500 mg L −1 ). Magnetite is particularly attractive, since it showed the highest activity and can be easily separated from the liquid phase given its magnetic properties. All the minerals tested suffered low iron leaching and magnetite and hematite showed a good reusability upon three consecutive runs. However, in this case long-term durability is not a crucial issue, given the availability and low cost of these minerals.