Iron Oxide Coatings Research Articles

Reactions between ferric oxyhydroxide mineral coatings and a dimethoxyhydroquinone: A source of hydroxyl radicals

Quinones are organic molecules that facilitate electron-transfer reactions in terrestrial environments. The reduced forms, hydroquinones, are powerful reductants that can trigger non-enzymatic radical-based decomposition of organic matter and contaminants by simultaneous reduction of iron and oxygen. Iron oxides often occur as coatings on other minerals, thus our study investigated the reactions between the ferric oxyhydroxide (FeO(OH)) surface coatings on gibbsite (Al(OH)3) and 2,6-dimethoxy-1,4-hydroquinone (2,6-DMHQ). The main aim was to investigate the oxidation of 2,6-DMHQ and the generation ∙OH in the presence of O2 at low Fe concentrations in a novel setup that allows local structural characterization. The heterogeneous redox reactions between 2,6-DMHQ and the FeO(OH) coatings were studied at pH 5.0 as a function of the amount of Fe present on the gibbsite surfaces, including the effect of aging of the FeO(OH) coatings. The results showed that reactions between 2,6-DMHQ and FeO(OH) coated gibbsite under ambient conditions can generate substantial amounts of ·OH, comparable with amounts generated on pure ferrihydrite surfaces. The ·OH is the product of two sequential reactions: hydroquinone oxidation by O2 and degradation of the formed H2O2. The calculated rate constant of the former reaction is the same regardless of amount of FeO(OH) coating suggesting a surface catalytic process where 2,6-DMHQ is oxidized by O2 resulting in formation of H2O2. Subsequently, the observed induction period, the low Fe2+ (aq) concentrations in solution and the dependency of FeO(OH) coating amount influencing ·OH formation suggest that the pathway for ∙OH is through H2O2 decomposition by the surface sites on the FeO(OH) coating. Overall, this study shows that co-existence of oxygen, FeO(OH) and organic reductants, possibly secreted by soil microorganisms, creates favorable conditions for generation of ·OH contributing to decomposition of organic matter and organic pollutants in soil environments.

MiGIS: micromorphological soil and sediment thin section analysis using an open-source GIS and machine learning approach

Abstract. ​​​​​​​Micromorphological analysis using a petrographic microscope is one of the conventional methods to characterise microfacies in rocks (sediments) and soils. This analysis of the composition and structure observed in thin sections (TSs) yields seminal, but primarily qualitative, insights into their formation. In this context, the following question arises: how can micromorphological features be measured, classified, and particularly quantified to enable comparisons beyond the micro scale? With the Micromorphological Geographic Information System (MiGIS), we have developed a Python-based toolbox for the open-source software QGIS 3, which offers a straightforward solution to digitally analyse micromorphological features in TSs. By using a flatbed scanner and (polarisation) film, high-resolution red–green–blue (RGB) images can be captured in transmitted light (TL), cross-polarised light (XPL), and reflected light (RL) mode. Merging these images in a multi-RGB raster, feature-specific image information (e.g. light refraction properties of minerals) can be combined in one data set. This provides the basis for image classification with MiGIS. The MiGIS classification module uses the random forest algorithm and facilitates a semi-supervised (based on training areas) classification of the feature-specific colour values (multi-RGB signatures). The resulting classification map shows the spatial distribution of thin section features and enables the quantification of groundmass, pore space, minerals, or pedofeatures, such nodules being dominated by iron oxide and clay coatings. We demonstrate the advantages and limitations of the method using TSs from a loess–palaeosol sequence in Rheindahlen (Germany), which was previously studied using conventional micromorphological techniques. Given the high colour variance within the feature classes, MiGIS appears well-suited for these samples, enabling the generation of accurate TS feature maps. Nevertheless, the classification accuracy can vary due to the TS quality and the academic training level, in micromorphology and in terms of the classification process, when creating the training data. However, MiGIS offers the advantage of quantifying micromorphological features and analysing their spatial distribution for entire TSs. This facilitates reproducibility, visualisation of spatial relationships, and statistical comparisons of composition among distinct samples (e.g. related sediment layers).

Nano-Iron Oxide Coating for Enhanced Heat Transfer in Gas–Solid Fluidized Bed Systems

This study explores using iron oxide coatings on glass beads to improve heat transfer efficiency in fluidized bed reactors. Techniques such as BET surface area analysis, SEM imaging, and X-ray diffraction were used to characterize the coated beads. Results showed the successful creation of a crystalline iron layer on the beads’ surface and increased thermal conductivity, especially at elevated temperatures. The study also quantified the impact of air surface velocity and heating power on the heat transfer coefficient, revealing substantial improvements, especially at higher velocities. It was found that the heat transfer coefficient for 600 µm glass beads increases significantly from 336.4 W/m2·K to 390.3 W/m2·K when the velocity is 0.27 m/s and the heating flux is 125 W. This demonstrates the effectiveness of the iron oxide coating in improving heat transfer. The results of this study emphasize the efficacy of iron oxide coatings in augmenting heat transmission characteristics, particularly in fluidized bed reactor.

Enhancement of Phosphorus Sorption onto Peanut shell by Means of Aluminum and Iron Oxide Coatings

Phosphorus serves as a crucial nutrient for the organisms. Still, the discharge of high levels of phosphate concentrations into limited aquatic environments leads to the phenomenon known as eutrophication, which subsequently leads to the degradation of the overall water quality. In this study the capacity of the peanut shell, Al-coated and Fe-coated is determined in relating by adding 10, 20, and 30, 40 mg P L-1 as (KH2PO4) to 1.0 g of each sample and shaking for 2, 6, 12, and 24 h and at pH ranged between 4.6, 5.2, 6.3 and 7.8. At the end of each period, the suspension was filtered and analyzed which presents the concentration of equilibrium P. The maximum P adsorption of 28.9±0.07 and 50.6±0.49% was recorded for 24 h of incubation and at pH (4.6) with an Al-coated peanut shell respectively. The findings of this study demonstrated that the adsorption of phosphorus (P) increased as the duration of incubation increased. At the same time, it decreased with a decrease in the pH of the solution. Furthermore, the outcomes indicated that the Freundlich model provided the best fit for the data, based on higher R2 values ranging from 0.9867 to 0.9938, in comparison to the range of 0.9659 to 0.9904 observed for the Longmuir model. These results suggested that peanut shells coated with Al can be used to remove high amounts of P in the solution. Also, the removal resulted from the physical adsorption of P rather than a chemical reaction.

Characterizing basalt-atmosphere interactions on Venus: A review of thermodynamic and experimental results

Abstract The surface of Venus is in contact with a hot (~470 °C), high pressure (92 bars), and caustic (CO2 with S, but little H2O) atmosphere, which should cause progressive alteration of the crust in the form of sulfate and iron-oxide coatings; however, the exact rate of alteration and mineral species are not well constrained. Different experimental approaches, each with its own limitations, are currently being used to constrain mineralogy and alteration rates. One note is that no experimental approach has been able to fully replicate the necessary conditions and sustain them for a significant length of time. Furthermore, geochemical modeling studies can also constrain surface alteration mineralogy, again with different assumptions and limitations. Here, we review recent geochemical modeling and experimental studies to constrain the state of the art for alteration mineralogy, rate of alteration, open questions about the surface mineralogy of Venus, and what can be constrained before the fleet of missions arrives later this decade. Combining the new results confirms that basalt on the surface of Venus should react quickly and form coatings of sulfates and iron-oxides; however, the mineralogy and rate of alteration are dependent on physical properties of the protolith (including bulk composition, mineralogy, and crystallinity), as well as atmospheric composition, and surface temperature. Importantly, the geochemical modeling results show that the mineralogy is largely controlled by atmospheric oxygen fugacity, which is not well constrained for the near-surface environment on Venus. Therefore, alteration experiments run over a range of oxygen and sulfur fugacities are needed across a wide range of Venus analog materials with varying mineralogy and crystallinity.

Potential interference of organic acids and ferrous iron in the interpretation of Fe and Mn indicators of reduction in soil

AbstractRemoval of manganese (Mn) and iron (Fe) oxide coatings from indicators of reduction in soil (IRIS) is considered a function of microbial reduction of these elements under sufficiently reducing conditions, but there may be situations where removal is abiotic. We examined whether abiotic removal of Mn and Fe oxide coatings may be facilitated by organic acids and whether loss of Mn can take place via chemical reduction by ferrous iron (Fe2+) in vitro. Loss of color from Mn‐IRIS in citric acid solution was highest at 1 and 10 mM regardless of pH, with no loss observed at lower concentrations. Effects of citric acid on Fe‐IRIS were also limited to 1 and 10 mM, but color loss only occurred at pH 4.5 and the rate and extent of loss were lower than for Mn‐IRIS. Color loss from Mn‐IRIS in oxalic acid was highest at 1 and 10 mM and increased with decreasing pH. Effects of oxalic acid on loss of color from Fe‐IRIS were limited to 1 and 10 mM. Loss of color from Mn‐IRIS was limited to 10 mM salicylic acid and pH 4.5, with no effect on Fe‐IRIS regardless of concentration and pH. Only the two highest concentrations of Fe2+ (50 and 100 mM) resulted in reduction of the Mn‐IRIS coating and subsequent deposition of an Fe oxide coating. Organic acids and pH have a differential effect on color loss from Mn‐ and Fe‐IRIS, with only high concentrations of organic acids at low pH possibly interfering with interpretation.

A comparative analysis of Cr(VI) reduction with Cu2O, ZnO and Fe2O3 coatings grown by PEO

Coatings on copper, zinc and stainless-steel substrates were fabricated using the plasma electrolytic oxidation (PEO) technique and their photocatalytic activity was evaluated in the reduction of Cr(VI), a highly toxic agent present in wastewater from industrial processes such as electroplating, manufacture of textile dyes, wood curing, and leather tanning. The concentration of hexavalent chromium in drinking water has been regulated to a maximum value established by national and international legislation of 0.05 ppm. The strategy of reduction to less toxic species such as Cr(III), followed by its precipitation in a basic medium, use several methods derived from chemistry, physics and biology for the treatment of water contaminated with this material. In the present work, some coatings of copper, zinc, and iron oxides were obtained over the corresponding metal sheets exposed to PEO, which were tested in a heterogeneous process of advanced oxidation with 1 ppm Cr(VI) solution under ultraviolet radiation. Thus, a reduction rate to Cr(III) close to 100 % in 60 min was obtained.

Rapid Pitting of Copper Tube from Deposits of Cathodic Manganese in Hot Water Systems: Mechanism and Effects of Water Chemistry

A large building with water frequently above 65°C experienced at least 300 leaks in a hot water recirculating line. The leaks were unusual, as they occurred along the very top of horizontal pipes that were heavily coated with iron and manganese sediment on the bottom half of the pipe. This etiology of the leaks is inconsistent with under-deposit corrosion but consistent with a previously hypothesized mechanism of chemistry-related hot water pitting. We verify this mechanism for the first time by: (1) developing a macrocell setup to test the proposed anodic and cathodic reaction and (2) determining how water chemistry can promote or inhibit this form of nonuniform corrosion. The macrocell demonstrated that copper or stainless steel coated with manganese oxides were highly cathodic relative to an anodic exposed copper surface. Pitting current densities as high as 200 µA/cm2 were found in the worst-case conditions at pH 8.2 in waters with a high ratio of sulfate to bicarbonate. High levels of silica inhibited pitting corrosion currents, but orthophosphate inhibitors did not. No iron oxide coatings tested in this work had cathodic tendencies.

Transport of double-stranded ribonucleic acids (dsRNA) and deoxyribonucleic acids (DNA) in sand and iron oxide-coated sand columns under varying solution chemistries.

Assessing ecological risks associated with the use of genetically modified RNA interference crops demands an understanding of the fate of crop-released insecticidal double-stranded RNA (dsRNA) molecules in soils. We studied the adsorption of one dsRNA and two double-stranded DNA as model nucleic acids (NAs) during transport through sand- and iron oxide-coated sand (IOCS)-filled columns over a range of solution pH and ionic compositions. Consistent with NA-sand electrostatic repulsion, we observed only slight retention of NAs in sand columns. Conversely, pronounced NA retention in IOCS columns is consistent with strong and irreversible NA adsorption involving electrostatic attraction to and inner-sphere complex formation of NAs with iron oxide coatings. Adsorption of NAs to iron oxides revealed a fast and a slow kinetic adsorption regime, possibly caused by the excluded-area effect. Adsorption of NAs to sand and IOCS increased in the presence of dissolved Mg2+ and with increasing ionic strength, reflecting cation-bridging and screening of repulsive electrostatics, respectively. The co-solute phosphate and a pre-adsorbed dissolved organic matter isolate competitively suppressed dsRNA adsorption to IOCS. Similar adsorption characteristics of dsRNA and similarly sized DNA suggest that existing information on DNA adsorption to soil particles helps in predicting adsorption and fate of dsRNA molecules in soils.

Nanostructured iron oxide thin films deposited by RF sputtering as catalysts for the heterogeneous solar photo-Fenton reaction

Nanostructured iron oxide coatings have been deposited by plasma sputtering by different approaches, with and without thermal annealing, with the aim of preparing supported catalysts for solar photo-Fenton.Depending on the experimental conditions explored (O2 content in the feed, annealing or bilayer approach) the phase composition of iron oxide changes and the surface nanostructure as well.The evaluation of methylene blue degradation in photo-Fenton reaction indicates a good performance of the investigated materials. Degradation percentage between 84 and 97% have been obtained in 180 min with a constant reaction rate as high as 18.0 × 10−3 min−1, with the best result for a bilayer sample obtained without thermal annealing. This preliminary investigation highlights that the photoactivity of the coatings is related not only to the content of α-Fe2O3, but also to the presence of developed nanostructures on the coating surface, making the material more active. The proposed method for producing supported catalyst based on photo-Fenton reaction is straightforward, and the possibility to reuse the catalyst up to ten reaction cycles is demonstrated.

Virus‐Like Iron Oxide Minerals Inspired by Magnetotactic Bacteria: Towards an Outstanding Photothermal Superhydrophobic Platform on Universal Substrates

AbstractMagnetic iron oxides, as the typical photothermal materials, possess the advantages of low cost, easy preparation, and biocompatibility, which impart great expectations in broad application prospects. However, the limited photothermal efficiency of iron oxides restricts their further use. Inspired by magnetotactic bacteria, a liquid film‐confined strategy has been developed assisted by a magnetic field for mineralization and assembly of iron oxides on the surface at room temperature. Virus‐like hierarchically micro/nanostructured iron oxides can be obtained on universal substrates which exhibit excellent photothermal performance, the highest among all iron oxide coatings and even comparable with carbon‐based materials. Theoretical simulation demonstrates the promotion of light capture by these particular structures. Moreover, by virtue of this, the surface is endowed with superhydrophobicity by a simple modification to construct a photothermal superhydrophobic platform, which is demonstrated by two challenging scenarios: high‐efficient antibacterial activity and defrosting/deicing ability controlled remotely. There is no need for harsh experimental conditions and templates, the strategy reported here is mild, environmental‐friendly and adopts trace amount of liquid (55 µL cm−2), which can provide a reference for the fabrication and application of other photothermal materials.

Effect of Iron Oxide Coatings on Thermal Conductivity of Silica Sands

Engineering properties of soils coated with naturally occurring or artificially manufactured particles can significantly differ from those of uncoated soils. This study aims at evaluating the effect of iron oxide coatings on the thermal conductivity of silica sands. Laboratory thermal needle probe tests were conducted on uncoated, goethite coated and hematite coated Ottawa 20/30 and Ottawa 100/200 sands. Test results demonstrated that the thermal conductivity (k) of all tested materials increases with increasing applied vertical stress, and the k of hematite coated sands was greater than that of uncoated sands under the similar relative density. However, the measured k of both uncoated and coated sands is a single function of porosity regardless of applied stress level, reflecting the limited effect of applied stress and iron oxide mineral coating on the k of tested materials. Simple resistances in series model was employed to explain the observed test results, and the results of this study was compared with k of previous studies on uncoated natural soils.

Effects of iron oxide coatings on the mobility and retardation of U(VI) in water saturated media

Iron oxide coatings occur commonly in the environment and affect significantly the fate of heavy metals. However, U(VI) transport in water saturated iron oxide-coating porous media has been seldom investigated. In this study, the adsorption and retardation of U(VI) on iron oxide-coated sand were comparatively investigated over a wide range of experimental conditions by batch and column experiments combined with modelling. The presence of iron oxide coatings significantly enhanced U(VI) adsorption which could be described by a two-site surface complexation model. The breakthrough curves of U(VI) in iron oxide coated quartz sand column show a two-plateau profile, which could not be directly reproduced by the commonly used two-site kinetic model. More sites for U(VI) have to be considered, implying that the heterogeneity of surface sites might be more significant in the non-equilibrium transport system than that in the equilibrium adsorption system. In addition, the presence of salicylic acid (SA) facilitated the transport of U(VI) at acidic pH, due to the competition between surface complexation of U(VI) and the complexation of U(VI) with SA in aqueous phase. Overall, this study provides a comprehensive understanding of the migration behavior of U(VI) in water saturated iron oxide-coated sand system.

A combined micro-Raman, X-ray absorption and magnetic study to follow the glycerol-assisted growth of epsilon-iron oxide sol-gel coatings

Epsilon iron oxide (ε-Fe2O3) coatings on Si(100) substrates are obtained by an easy one-pot sol-gel recipe assisted by glycerol in an acid medium. Glycerol, given its small dimensions, enables the formation of ε-Fe2O3 nanoparticles with a size of a few nanometers and the highest purity is reached in coatings after a densification treatment at 960 °C. The structural and compositional evolution up to 1200 °C is studied by confocal Raman microscopy and X-ray absorption spectroscopy techniques, correlating the existing magnetic properties. We report a novel characterization method, which allows monitoring the evolution of the precursor micelles as well as the intermediate and final phases formed. Furthermore, the inherent industrial technology transfer of the sol-gel process is also demonstrated with the ε-Fe2O3 polymorph, impelling its application in the coatings form.

Effects of Environmental Fe Concentrations on Formation and Evolution of Allophane in Al‐Si‐Fe Systems: Implications for Both Earth and Mars

AbstractAllophane, a common component on Earth and a probable constituent of the amorphous component on Mars, is closely associated with Fe in the form of structural Fe and/or iron (oxyhydr)oxide coatings. However, until now, the formation and evolution of allophane as products of environmental Fe concentrations have rarely been studied. We investigated allophane precipitation from gels with different Fe/(Fe + Al) molar ratios (n, 0 ≤ n ≤ 1.0). X‐ray diffraction patterns and Fourier transform infrared spectra of the products showed that allophane was nearly the only product at n ≤ 0.2 and that the crystallinity of Fe‐rich allophane decreased with increasing n. Combined with the results of transmission electron microscopy and Mössbauer spectroscopy, Fe was found to be mainly incorporated into the gibbsite‐like sheet of allophane, forming clusters; the highest Fe‐for‐Al substitution content was roughly estimated to be 20 mol.%. As n increased further, the formation of allophane was increasingly suppressed, and the Fe phases began to separate from the Al‐Si phases, resulting in mixtures of incipient allophane and incipient akaganeite and finally akaganeite. The near‐infrared spectroscopic data (1.2–2.6 μm) of the products showed incapability in identifying poorly ordered minerals in Al‐Si‐Fe systems, while the features in the range of 0.4–1.2 μm were powerful for studying iron occurrence in the products. These findings not only offer insights into the formation, evolution and geological role of allophane in Al‐Si‐Fe systems on Earth but also help constrain the paleoenvironment of locations where allophane and akaganeite co‐occur on Mars.

Characterization of iron oxide coatings prepared by MOCVD method from Fe(CO)5

This work dedicated the study of iron oxide deposition by metalorganic chemical vapour deposition (MOCVD) in hot-walls reactor with Fe(CO)5 in Ar-O2 atmosphere under pressure about 70 Pa. Phase and structure studied by x-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction(RHEED). This analysis also gives information for some common factors of MOCVD decomposition of Fe(CO)5. Topology of the coatings studied with atomic force microscopy (AFM) and scanning electron microscopy (SEM). Optical properties of the coatings studied with spectrophotometer. There was identificated that coatings which was made by MOCVD on Si 〈1 0 0〉 substrate has a cluster structure which size increase with deposition time and supposed Volmer-Weber growth mode.

Taxonomic and functional insights into rock varnish microbiome using shotgun metagenomics.

Rock varnish is a microbial habitat, characterised by thin (5-500 μm) and shiny coatings of iron (Fe) and manganese (Mn) oxides associated with clay minerals. This structure is well studied by geologists, and recently there have been reports about the taxonomical composition of its microbiome. In this study, we investigated the rock varnish microbiome using shotgun metagenomics together with analyses of elemental composition, lipid and small molecule biomarkers, and rock surface analyses to explore the biogeography of microbial communities and their functional features. We report taxa and encoded functions represented in metagenomes retrieved from varnish or non-varnish samples, additionally, eight nearly complete genomes have been reconstructed spanning four phyla (Acidobacteria, Actinobacteria, Chloroflexi and TM7). The functional and taxonomic analyses presented in this study provide new insights into the ecosystem dynamics and survival strategies of microbial communities inhabiting varnish and non-varnish rock surfaces.

Transport of graphene oxide in saturated quartz sand containing iron oxides

The environmental implications of graphene oxide (GO) have received much attention. Transport of GO in subsurface environment is a critical process affecting the migration and potential risks of this important class of carbonaceous nanomaterials. To date, the effects of heterogeneity in porous media, in particular, iron oxides, on GO transport are not well studied. In this study, we investigated the transport properties of GO in saturate quartz sand as affected by the presence of iron oxides, using goethite, hematite and ferrihydrite as the model iron oxide species, and applied a two-site transport model (which accounts for both attachment and straining) to fit the transport data. We found that iron oxide coating on sand surfaces markedly inhibited GO transport, mainly due to increased electrostatic attraction between particles and collectors, as the positively charged iron oxides provided favorable deposition sites for the negatively charged GO nanosheets. Additionally, increased surface roughness was likely an additional mechanism leading to the enhanced GO deposition. The extent of transport inhibition by iron oxides also depended on the morphology iron oxides, in that at the same Fe loading a larger effect was observed when iron oxides existed as the coating on sand surface than as discreet particles. The presence of iron oxide coatings (tested using goethite) could magnify the effects of cations on GO transport. Specifically, the presence of goethite facilitated the accumulation of cations on the surface of sand, and in the case of Ca2+, the binding of GO via the cation-bridging mechanism was enhanced, as goethite contained abundant surface hydroxyl groups that are strong metal-complexing moieties.

Potential Color Change Dynamics of Beneficial Use Sediments

ABSTRACT Berkowitz, J.F.; VanZomeren, C.M., and Priestas, A.M., 2018. Potential color change dynamics of beneficial use sediments. Sediment color is important in determining aesthetic and habitat suitability for beach nourishment projects; however, sediment derived from dredging operations must meet locally established color compatibility requirements (i.e. cannot be too dark). Often, potential sediment sources are close to meeting specified thresholds, and previous observations suggest that sediments may lighten over time following beach nourishment. This work seeks to characterize the degree of color change potential based on the removal of constituents affecting sediment color. Thus, a sequential chemical treatment was developed to examine color changes associated with the removal of carbonates, organic matter, and iron oxide coatings from sediments collected from eight U.S. Army Corps of Engineers dredging operations. The results show that Munsell values increased by an average of 1.0 unit (became lig...

Microbial nanowires and electroactive biofilms.

Geobacter bacteria are the only microorganisms known to produce conductive appendages or pili to electronically connect cells to extracellular electron acceptors such as iron oxide minerals and uranium. The conductive pili also promote cell-cell aggregation and the formation of electroactive biofilms. The hallmark of these electroactive biofilms is electronic heterogeneity, mediated by coordinated interactions between the conductive pili and matrix-associated cytochromes. Collectively, the matrix-associated electron carriers discharge respiratory electrons from cells in multilayered biofilms to electron-accepting surfaces such as iron oxide coatings and electrodes poised at a metabolically oxidizable potential. The presence of pilus nanowires in the electroactive biofilms also promotes the immobilization and reduction of soluble metals, even when present at toxic concentrations. This review summarizes current knowledge about the composition of the electroactive biofilm matrix and the mechanisms that allow the wired Geobacter biofilms to generate electrical currents and participate in metal redox transformations.