Crystalline Iron Oxide Phases Research Articles

Synthesis, characterization and magnetic properties of ε-Fe2O3 nanoparticles prepared by sol-gel method

Nanoparticles of ε-Fe2O3 in an amorphous silica matrix were prepared by the sol–gel method without using a precursor. X-ray diffraction patterns reflect a completely amorphous state for the samples even after heat-treated up to 600 °C, while at higher temperatures the formation of crystalline iron oxide phases was observed. Transmission electron microscopy analysis showed a wide size distribution of nanoparticles from 3±0.5 to 46±0.8 nm. The occurrence of ε-Fe2O3 particles depends on several factors like annealing temperature, particle size and iron concentration in initial chemicals. Based on the 57Fe Mössbauer spectra, three octahedral and one tetrahedral iron site were identified for ε-phase. The optimized conditions for the preparation of ε-Fe2O3 rich sample were confirmed and ~ 90% of the ε-Fe2O3 phase was obtained with the particle size of 15±0.6 nm. The high coercivity of ~ 1.18 T at room temperature was confirmed for our nanoparticle system.

XRD, internal field-NMR and Mössbauer spectroscopy study of composition, structure and magnetic properties of iron oxide phases in iron ores

We report the phase-composition, structure and magnetic properties of two representative samples of naturally available iron-oxide containing ores/soils collected from two different regions in Karnataka, India. Presence of elements such as Fe, Si, Al and O were identified in both the samples using energy dispersive analysis of X-rays (EDAX). X-ray diffraction results confirmed that different ceramic phases such as crystalline iron oxide phases (Fe3O4, γ-Fe2O3 and α-Fe2O3), aluminosilicates (Al2SiO5) and low-quartz (SiO2) phases constitute the soil. The presence of ferrimagnetic phases (Fe3O4, γ-Fe2O3) makes the soil respond to a permanent magnet. Separation between the magnetic and non-magnetic phases was performed using a permanent magnet. The non-magnetic part of the sample contains a high amount of α-Fe2O3 phase along with aluminosilicates and low-quartz. The magnetic phases were further characterized and quantified. The presence of Fe3O4 phase in the samples was confirmed from the Verwey transition observed by internal-field NMR spectroscopy. Our results demonstrate that internal-field NMR and Mössbauer spectroscopy are complementary tools for characterizing iron containing soils. Furthermore, we found that the soil collected from the low temperature region (Sandur) contains more amounts of ferrimagnetic oxide (Fe3O4, γ-Fe2O3) phases, whereas the high temperature region (Hospete) contains more α-Fe2O3 phase. Hence, our results confirm that the phase composition of the soil is intimately related to the local daily temperature.

Influence of iron redox cycling on organo-mineral associations in Arctic tundra soil

Arctic tundra stores large quantities of soil organic matter under varying redox conditions. As the climate warms, these carbon reservoirs are susceptible to increased rates of decomposition and release to the atmosphere as the greenhouse gases carbon dioxide (CO2) and methane (CH4). Geochemical interactions between soil organic matter and minerals influence decomposition in many environments but remain poorly understood in Arctic tundra systems and are not considered in decomposition models. The accumulation of iron (Fe) oxyhydroxides and organo-iron precipitates at redox interfaces may be particularly important for carbon cycling given that ferric iron [Fe(III)] species can enhance decomposition by serving as terminal electron acceptors in anoxic soils or inhibit microbial decomposition by binding organic molecules. Here, we examine chemical properties of solid-phase Fe and organic matter in organic and mineral horizons within the seasonally thawed active layer of Arctic tundra on the North Slope of Alaska. Spectroscopic techniques, including micro-X-ray fluorescence (μXRF) mapping, micro-X-ray absorption near-edge structure (μXANES) spectroscopy, and Fourier transform infrared spectroscopy (FTIR), were coupled with chemical sequential extractions and physical density fractionations to evaluate the spatial distribution and speciation of Fe-bearing phases and associated organic matter in soils.Organic horizons were enriched in poorly crystalline and crystalline iron oxides, and approximately 60% of total Fe stored in organic horizons was calculated to derive from upward translocation from anoxic mineral horizons. Ferrihydrite and goethite were present as coatings on mineral grains and plant debris, and in aggregates with clays and particulate organic matter. Minor amounts of ferrous iron [Fe(II)] were present in iron sulfides (i.e., pyrite and greigite) in mineral horizon soils and iron phosphates (vivianite) in organic horizons. Concentrations of organic carbon in the organic horizons (28±5wt.% C) were approximately twice the concentrations in the mineral horizons (14±2wt.% C), and organic matter was dominated by base-extractable and insoluble organics enriched in aromatic and aliphatic moieties. Conversely, water-soluble organic molecules and organics solubilized through acid-dissolution of iron oxides comprised <2% of soil organic C and were consistent with a mixture of alcohols, sugars, and small molecular weight organic acids and aromatics released through decomposition of larger molecules. Integrated over the entire depth of the active layer, soils contained 11±4kgm−2 low-density, particulate organic C and 19±6kgm−2 high-density, mineral-associated organic C, indicating that 63±19% of organic C in the active layer was associated with the mineral fraction.We conclude that organic horizons were enriched in poorly crystalline and crystalline iron oxide phases derived from upward translocation of dissolved Fe(II) and Fe(III) from mineral horizons. Precipitation of iron oxides at the redox interface has the potential to contribute to mineral protection of organic matter and increase the residence time of organic carbon in arctic soils. Our results suggest that iron oxides may inhibit organic carbon degradation by binding low-molecular-weight organic compounds, stabilizing soil aggregates, and forming thick coatings around particulate organic matter. Organic matter released through acid-dissolution of iron oxides could represent a small pool of readily-degradable organic molecules temporarily stabilized by sorption to iron oxyhydroxide surfaces. The distribution of iron in organic complexes and inorganic phases throughout the soil column constrains Fe(III) availability to anaerobic iron-reducing microorganisms that oxidize organic matter to produce CO2 and CH4 in these anoxic environments. Future predictions of carbon storage and respiration in the arctic tundra should consider such influences of mineral stabilization under changing redox conditions.

High-surface-area, iron-oxide films prepared by atomic layer deposition on γ-Al2O3

High-surface-area iron oxides were prepared by Atomic Layer Deposition (ALD) on 130-m2/g γ-Al2O3 for use as a catalyst support. Measurements of the sample mass, surface area, and pore-size distribution as a function of the number of ferrocene-O2 ALD cycles at 623K suggested that the iron oxide grew as a dense, conformal film with a growth rate similar to 0.016-nm per cycle. While films with 20 ALD cycles (20Fe2O3-Al2O3, 0.25g Fe2O3/g Al2O3) were difficult to distinguish by HAADF STEM, EDS mapping indicated the Al2O3 was uniformly coated. Raman Spectroscopy showed the films were in the form of Fe2O3; but XRD measurements on samples with as many as 100 ALD cycles (100Fe2O3-Al2O3, 0.84g Fe2O3/g Al2O3) showed no evidence for crystalline iron-oxide phases, even after calcination at 1073K. Specific rates for the water-gas-shift (WGS) reaction on the ALD-coated samples were significantly lower than those on bulk Fe2O3. However, addition of 1wt.% Pd to Fe2O3/Al2O3 supports prepared by ALD exhibited specific rates that were much higher than that observed when 1wt.% Pd was added to Fe2O3/Al2O3 prepared by conventional impregnation of Fe salts, suggesting more uniform contact between the Pd and FeOx phases on samples prepared by ALD.

Amorphous iron phases in medium temperature leach residues and associated metal loss

Leach residues, produced during pilot and demonstration scale medium temperature (150°C) hydrometallurgical processing of sulfide ores contain amorphous/poorly crystalline and metastable nano-scale iron oxides/oxyhydroxide phases. These phases control the properties of the residue and contain a relatively high loading of the valuable (Cu, Ni) metals. Residue samples from CESL and Vale pilot and demonstration plant medium temperature processes were characterized using different techniques. Poor agreement between total iron analysis by inductively coupled plasma mass spectroscopy (ICP-MS) and quantitative X-ray powder diffraction QXRPD indicated the presence of amorphous/poorly crystalline iron oxide phases. QXRPD coupled with sequential extraction was used for quantification of the amorphous iron phases. Amorphous iron oxides/oxyhydroxides were found to be a major source of copper and nickel losses to the residue. The distribution of copper and nickel into the amorphous and crystalline iron oxide phases was determined by a two-stage sequential extraction process. Association of copper and nickel to the amorphous phases was found to be approximately 2–4 times higher than with the crystalline iron oxide phases.

Interplay of thickness and photoelectrochemical properties in nanostructured α-Fe2 O3 thin films

Nanostructured Fe2O3 thin films were grown by plasma enhanced-chemical vapor deposition (PE-CVD) from Ar/O2 plasmas for photoelectrochemical (PEC) water splitting applications. Iron oxide coatings were deposited on fluorine-doped tin oxide (FTO) substrates at 300 °C under optimized conditions, and subsequently annealed ex situ in air at 650 °C. Structural and compositional analyses confirmed the formation of pure α-Fe2O3 (hematite), free from other crystalline iron oxide phases. Controlled variations of the deposition time enabled tuning of the thickness and nano-aggregate sizes in the resulting deposits and, correspondingly, their current–voltage characteristics. A maximum photocurrent density close to 1 mA cm−2 was achieved at 1.23 V versus the reversible hydrogen electrode (RHE), without the need of any oxygen evolution catalyst or over/underlayer. The present findings revealed the key role played by the engineering of Fe2O3-based nanomaterials, resulting ultimately in a lowered carrier diffusion length, and in an optimal diffusion of tin from FTO into thinner layers. These features offer an amenable opportunity for harvesting radiant energy to trigger water photoelectrolysis and produce clean hydrogen in a carbon-neutral fashion.

Schwertmannite in soil materials: Limits of detection of acidified ammonium oxalate method and differential X-ray diffraction

Schwertmannite is a secondary iron mineral, found in acid mine drainage (AMD) and acid sulfate soils (ASS), that generates acidity when it transforms to stable mineral phases. Acidity liberated during schwertmannite transformation can seriously diminish water quality and soil health. Acidified ammonium oxalate (AAO) extraction in the dark coupled with differential X-ray diffraction (DXRD) analysis is routinely used to identify and to quantify poorly crystalline iron oxide phases such as schwertmannite in AMD environments. However, management of ASS environments is largely impacted due to lack of reliable methods to identify/quantify schwertmannite in soil materials. Our study aimed to evaluate the 15min AAO extraction method to identify/quantify schwertmannite in soil materials. We extracted soil samples spiked with synthetic and natural schwertmannite (termed as natural organic rich schwertmannitic material) with acidified ammonium oxalate (AAO) for 15min. We also examined soil samples spiked with schwertmannite through the DXRD analysis under ideal conditions assuming that only schwertmannite would dissolve during the extraction. Our data show that synthetic schwertmannite dissolved partially during the 15min AAO extraction and as a result the recovered Fe content from schwertmannite-spiked soils was underestimated by ~20%. The data also show that soil materials could also influence the recovery of schwertmannite. Fe/S molar ratios of schwertmannite spiked at higher rates (2% and 5%) were closer to the expected ratios. In addition to schwertmannite, goethite and other unidentified minerals in natural organic rich schwertmannitic material also dissolved during the 15min extraction time. The DXRD analysis data show that schwertmannite in soil materials at contents >5% may be identifiable through this approach. Our findings highlight that both the 15min AAO extraction procedure and the DXRD analysis have limited applicability towards detecting schwertmannite accurately in soil materials.

A magnetic investigation along a NE–SW transect of the Yasouj Plain, southwestern Iran

The magnetic susceptibility (χ) of soils varies with the slope position due to some factors such as texture, drainage class, and land use. Limited information is available about the magnetic susceptibility properties of semi arid regions of southwestern Iran. This study attempts to link χ and Fe oxides of the soils to landforms, soil characteristics, and land use (paddy and dryland soils) on the same parent materials. Ten representative pedons were taken along a NE–SW transect in different physiographic units in the Yasouj Plain. The results indicated that χ is determined mainly by topography and land use. In most pedons, the highest concentrations of poorly crystalline Fe (Feo) were observed in the top soil with different types of land use. Aquic condition in paddy soils seems the main cause for a significant decrease in the amount of Fed in the studied soils. Maximum concentration of pedogenic Fe (Fed) was noticed in more stable landforms with non-aquic moisture regime. Paddy soils exhibited higher Feo/Fed ratio values, suggesting that they have more poorly crystalline iron oxide phases than dryland soils. Magnetic measurements showed that the χ values of paddy soils were much lower (3.8 times) than those of dryland soils. The highest value of χ was found in the pedons located on plateau and piedmont plains and the lowest value of χ belongs to those that are located on hill and river lower terraces. A positive correlation was found between the frequency dependence of χ (χfd) and χ in paddy and dryland soils. Higher values of χfd were observed at soil surface than at deeper levels, suggesting a greater proportion of ultrafine grains.

Plasmodynamic synthesis of ultradispersed iron oxides

Results of a pilot study of direct plasmochemical synthesis of ultradispersed crystalline iron oxide phases in a high-speed pulse jet of iron-containing electrodischarge plasma that effluxes into a closed volume with the air atmosphere are presented. The synthesized powder contains all main oxide phases, including magnetite Fe3O4, maghemite γ-Fe2O3, and hematite α-Fe2O3. An important feature of the method is the formation of the metastable e-Fe2O3 phase under normal conditions. The minimum average size of coherently diffracting domains (CDD) (particles) of the magnetic phases is observed when they are synthesized at air pressure of 1.0 atm. The powders have high magnetic properties comparable with those of compact iron oxides.

Quantitative–spatial assessment of soil contamination in S. Francisco de Assis due to mining activity of the Panasqueira mine (Portugal)

Through the years, mining and beneficiation processes produces large amounts of As-rich mine wastes laid up in huge tailings and open-air impoundments (Barroca Grande and Rio tailings) that are the main source of pollution in the surrounding area once they are exposed to the weathering conditions leading to the formation of AMD and consequently to the contamination of the surrounding environments, in particularly soils. In order to investigate the environmental contamination impact on S. Francisco de Assis (village located between the two major impoundments and tailings) agricultural soils, a geochemical survey was undertaken to assess toxic metals associations, related levels and their spatial distribution, and to identify the possible contamination sources. According to the calculated contamination factor, As and Zn have a very high contamination factor giving rise to 65.4% of samples with a moderate to high pollution degree; 34.6% have been classified as nil to very low pollution degree. The contamination factor spatial distribution put in evidence the fact that As, Cd, Cu, Pb, and Zn soils contents, downstream Barroca Grande tailing, are increased when compared with the local Bk soils. The mechanical dispersion, due to erosion, is the main contamination source. The chemical extraction demonstrates that the trace metals distribution and accumulation in S. Francisco de Assis soils is related to sulfides, but also to amorphous or poorly crystalline iron oxide phases. The partitioning study allowed understanding the local chemical elements mobility and precipitation processes, giving rise to the contamination dispersion model of the study area. The wind and hydrological factors are responsible for the chemical elements transport mechanisms, the water being the main transporter medium and soils as one of the possible retention media.

Uranium contents and 235U/ 238U atom ratios in soil and earthworms in western Kosovo after the 1999 war

The uranium content and 235U/ 238U atom ratio were determined in soils and earthworms of an area of Kosovo (Djakovica garrison), heavily shelled with depleted uranium (DU) ammunition during the 1999 war. The aim of the study was to reconstruct the small-scale distribution of uranium and assess the influence of the DU added to the surface environment. The total uranium concentration and the 235U/ 238U ratio of topsoils showed great variability and were inversely correlated. The highest uranium levels (up to 31.47 mg kg −1) and lowest 235U/ 238U ratios (minimum 0.002147) were measured in topsoils collected inside, or very close to, the clusters of DU penetrator holes. Regarding the fractionation of uranium in the surface soils, the uranium concentrations in the soluble and exchangeable fractions increased as the total uranium concentration of the topsoils increased. High and rather uniform percentage contents of uranium (24–36%) were associated with the poorly crystalline iron oxide phases of soils. In the U-enriched soils the elevated levels of the element were probably due to the presence of very small, unevenly distributed oxidized DU particles. The total uranium concentration in earthworms was in the range 0.142–0.656 mg kg −1, with the highest concentrations in Lumbricus terrestris. The juveniles of all three studied species seemed to accumulate uranium more than adults, probably due to age-related differences in metabolism. The 235U/ 238U ratio in the earthworms was variable (0.005241–0.007266) and independent of both the total uranium contents in soils and the absolute uranium levels in the animals. Bioconcentration was greater at lower U concentrations in soil, probably due to an increasing rate of elimination of uranium by the earthworms as the soil contents of the element increase. The results of this study clearly indicate that DU was added to the soil of the study area. Nevertheless, the phenomenon was very limited spatially and the total uranium concentrations fell within the natural range of the element in soils. Moreover, the absolute uranium concentrations indicate that there was no contamination of the earthworm species studied.

Sorption characteristics of zinc and iron by natural zeolite and bentonite

Understanding the sorption process in natural zeolites is necessary for effective utilization of these minerals as nutrient adsorbents and consequently as controlled releases of plant nutrients. This research was undertaken to characterize the ability of five natural zeolites and bentonite minerals to adsorb and release zinc and iron. The potential for sorption of these ions were evaluated by applying the Langmuir and Freundlich equations. Zinc sorption data followed the Langmuir equation. The ability for Zn sorption were in the following order: chabazite > analcime > clinoptilolite1 > bentonite > clinoptilolite2 > phillipsite. Diethylene triamine pentaacetic acid (DTPA) extractable Zn decreased with the increase in successive extractions. All sorbed zinc was extracted by DTPA in most zeolite species after four successive extractions while only 50% was readily extractable from chabazite. Iron sorption data followed the S-type isotherm and the results were described by a Freundlich adsorption model. The iron activity ratio (Fe ox/Fe d) of sorbed Fe indicated the dominance of amorphous or poorly crystalline phases with zeolites and crystalline iron oxide phases with bentonite. The results suggest that natural zeolites, particularly chabazite and bentonite minerals, have a high potential for Zn and Fe sorption with a high capacity for slow release fertilizers.