Form Of Fe Oxides Research Articles

Redbed formation in the redox-stratified mid-Proterozoic ocean

Deposition of iron-enriched redbeds occurred episodically on different continents throughout the mid-Proterozoic, but their origin remains unclear. Since these sediments host abundant Fe oxides, their occurrence potentially represents changes in ocean-redox state, a hypothesis having significant implications for the evolution of early aerobic eukaryotes. In the present study, we report Fe speciation and isotopes, major and trace elements, organic carbon contents, and mineralogy of the 1.6-Ga Cuizhuang and 1.4-Ga Xiamaling redbeds and intercalated non-redbeds in North China to investigate their depositional environments. As expected, the redbeds show significantly higher total Fe contents (FeT), FeT/Al, and FeHR/FeT ratios (FeHR: highly reactive Fe) than the adjacent non-redbeds (mean FeT: 5.0 % vs. 2.9 %), with 73–97 % of the FeHR in the form of Fe oxides (Feox), accounting for the difference in FeHR/FeT ratios. However, almost all other major (e.g., Al and Ti, typically associated with the detrital fraction) and trace elements show no obvious difference in concentrations between the redbeds and non-redbeds, supporting a single, shared detrital source. The FeT/Al and FeHR/FeT ratios of the redbeds are higher than their respective thresholds for anoxic conditions, whereas the non-redbeds show the opposite. We thus infer that the non-redbeds were deposited in a shallow oxic setting and the redbeds in a deep anoxic setting, separated by a pronounced Fe-redoxcline. The iron was mainly sourced from seafloor hydrothermal fluids and then partially oxidized and precipitated in shallow environments, resulting in positive Fe-isotope values (+0.24 to +0.57‰). Preservation of large amounts of Fe-(hydr)oxides within an anoxic setting can be explained through low-productivity conditions, as inferred from low TOC (mean 0.1 %) and P (mean 452 ppm) contents in both redbeds and non-redbeds. Based on this scenario, modeling indicates that the degree of fractionation of FeHR isotopes (+0.36 to +1.38‰) in the redbeds is consistent with oxidation of 66–98 % of the hydrothermally supplied Fe in waters with an average photic-zone O2 concentration of <0.1 μmol/L (equivalent to atmospheric pO2 < 0.1 % PAL), providing further evidence for extremely low O2 levels during the mid-Proterozoic.

Coupling CO2 reduction with ethane aromatization for enhancing catalytic stability of iron-modified ZSM-5

The shale gas revolution and the carbon-neutrality goal are motivating the landscape toward the synthesis of value-added chemicals or fuels from underutilized ethane with the assistance of greenhouse gas CO2. Combining ethane aromatization with CO2 reduction offers an opportunity to directly produce liquid products for facile separation, storage, and transportation. In the present work, Fe/ZSM-5 catalysts showed promise in the simultaneous CO2 reduction and ethane aromatization at atmospheric pressure and 873 K. The catalysts were further investigated using X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) measurements under in-situ conditions, indicating that most of Fe species existed in the form of Fe oxides and a portion of Fe was incorporated into the ZSM-5 framework generating Lewis acid sites. Both types of Fe species remained almost unchanged under reaction conditions, contributing to an enhanced aromatization activity of Fe/ZSM-5. The effects of CO2 and steam on the acid sites and in turn aromatization activity were also investigated by transient studies, which exhibited a reversible modification behavior. Moreover, CO2 was identified to be critical to enhance coke resistance and in turn catalyst stability. This work highlights the feasibility of using CO2 to assist the upgrading of abundant ethane from shale gas to aromatics over non-precious Fe-based zeolite catalysts.

Fe oxides-biochar composites produced by hydrothermal carbonization and pyrolysis of biomass waste

The synthesis of biochar-metal oxide composites for removing heavy metals from aqueous solutions or remediating polluted soils has attracted increased attention. Amongst them, Fe oxide composites stand out due to possible magnetic properties and low price. Two types of Fe oxide-biochar composites were prepared by impregnation with ferric sulfate salts of biomass waste or corresponding hydrochar, followed by pyrolysis at 550 °C for 5 h. During pyrolysis, Fe increased carbonization, leading to biochar with low H/C values. Properties of Fe oxide-biochar composite greatly depend on the feedstock. The Fe content in the final Fe oxide-biochar was higher than in biochar obtained by direct pyrolysis of biomass waste. Some of Fe was in the form of FeS. Impregnation of hydrochar followed by pyrolysis lead to a Fe oxide-biochar composite with lower ash content and higher micro- and macro-porosity. In this case, the Fe was in the form of Fe oxides.

Effects of pore sizes and oxygen-containing functional groups on desulfurization activity of Fe/NAC prepared by ultrasonic-assisted impregnation

A series of Fe-loaded activated carbons treated by HNO3 (Fe/NAC) were prepared by incipient impregnation method with or without ultrasonic assistance and characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy with energy disperse spectroscope (SEM-EDS), transmission electron microscopy (TEM) and N2 adsorption/desorption. The desulfurization activities were evaluated at a fixed bed reactor under a mixed gas simulated from flue gas. The results showed that desulfurization activity from excellent to poor is as follows: Fe/NAC-60>Fe/NAC-80>Fe/NAC-30>Fe/NAC-15>Fe/NAC-0>Fe/NAC-100>NAC. Fe/NAC-60 exhibits the best desulfurization activity and has breakthrough sulfur capacity of 319mg/g and breakthrough time of 540min. The introduction of ultrasonic oscillation does not change the form of Fe oxides on activated carbon but can change the dispersion and relative contents of Fe3O4. The types of oxygen-containing functional groups have no obvious change for all samples but the texture properties show some differences when they are oscillated for different times. The fresh Fe/NAC-60 has a surface area of 1045m2/g and total pore volume of 0.961cm3/g with micropore volume of 0.437cm3/g and is larger than Fe/NAC-0 (823m2/g, 0.733 and 0.342cm3/g). After desulfurization, surface area and pore volume of all samples decrease significantly, and those of the exhausted Fe/NAC-60 decrease to 233m2/g and 0.481cm3/g, indicating that some byproducts deposit on surface to cover pores. Pore size distribution influences SO2 adsorption, and fresh Fe/NAC-60 has more pore widths centralized at about 0.7nm and 1.0⿿2.0nm and corresponds to an excellent desulfurization activity, showing that micropore is conducive to the removal of SO2.

Accuracy of Olsen P to assess plant P uptake in relation to soil properties and P forms

Efficient use of phosphorus (P) by plants in agriculture relies on accurate estimation of the phytoavailable soil P. However, poor relationships are frequently observed between P availability indices, such as Olsen P, and P uptake by plants. We therefore studied which soil properties modify the accuracy of Olsen P as a P availability index. We conducted a soil P depletion experiment in pots with 17 soils. Two samples differing widely in Olsen P were used for each soil and named “low-P” and “high-P.” Soil was mixed with siliceous sand to achieve 1 mg of Olsen P per pot, so that the ratio of non-readily available P, either inorganic P or organic P, to Olsen P in the pot was higher for low-P than for high-P samples. Results show that, in high-P samples, P uptake by cucumber is positively correlated with the affinity of the soil solid phase for P, with R 2 of 0.76. In high-P samples, P uptake by cucumber is positively correlated with total P adsorption capacity as estimated by Fe in the form of Fe oxides, with R 2 = 0.56. P uptake is positively correlated with the inorganic P released by NaOH and citrate-bicarbonate in low-P samples, with R 2 of 0.52. In low-P samples, total organic P and phosphatase activity in the rhizosphere explained 37 % of plant P uptake. In low-P samples, phosphatase activity and pH explained 50 % of plant P uptake. These findings reveal that organic P forms and hydrolytic activity by roots are explaining P availability to plants. Whereas physico-chemical processes controlling inorganic P dynamics play a major role for low organic P/Olsen P ratios.

Changes in forms and distribution pattern of soil iron oxides due to long-term cropping in the Northwest of Iran

To assess effects of long-term agricultural practices on iron (Fe) oxide forms, 12 soil pedons belonging to six soil subgroups from the cultivated soils and the adjoining uncultivated soils were described and sampled. Soil samples were analyzed for physicochemical properties, mineralogical compounds, and different status of Fe oxides including free Fe oxides (Fed), crystalline Fe oxides (Fecry), poor crystalline Fe oxides (Feo), and organic complex Fe (Fep). For most of the examined soils, long-term cropping caused a considerable drop in organic carbon (12–64 %), electrical conductivity (3–18 %), exchangeable and soluble cations (2–96 %), and calcium carbonates (20–52 %) along with a noticeable rise in the values of clay and cation exchange capacity (3–23 %) as a result of accelerated alteration by farming activities and interaction between the used irrigation water and its receiving soils. X-ray diffraction pattern revealed that the soils were similar in mineralogical compositions, consisting of Fe-bearing minerals (such as mica, pyroxene, amphibole, and feldspars), goethite, hematite, illite, chlorite, smectite, kaolinite, and mixed minerals for both cultivated and uncultivated soils but some changes were observed in peak intensity, position, and figure mainly due to increasing the number of wetting–drying periods induced by irrigation practices as well as cropping and the dynamic of Fe and K. A relative enrichment (RE) in Fe oxide forms was highlighted with cultivation which could be attributed to the variation of weathering rate, pedogenic accumulation, and daily and seasonal fluctuations in temperature and moisture. For instance, long-term continuous cultivation led to an improvement in Fed from 2.1 % (RE of 1.02) to 181 % (RE of 2.81), Fecry from 52 % (RE of 1.53) to 667 % (RE of 7.7), crystalline index from 14 % (RE of 1.14) to 134 % (RE of 2.34), Feo from 2 % (RE of 1.02) to 18 % (RE of 1.18), and Fep from 23 % (RE of 1.23) to 500 % (RE of 6) as compared to the adjacent uncultivated soils. Soil pH, organic matter, active and equivalent of calcium carbonates were found to be the major agents in controlling the dynamic, nature, and distribution of Fe oxide status as appeared in significant correlation among the physicochemical properties and the evaluated Fe oxide forms.

Arsenic removal from soil with high iron content using a natural surfactant and phosphate

An environment friendly arsenic removal technique from contaminated soil with high iron content has been studied. A natural surfactant extracted from soapnut fruit, phosphate solution and their mixture was used separately as extractants. The mixture was most effective in desorbing arsenic, attaining above 70 % efficiency in the pH range of 4–5. Desorption kinetics followed Elovich model. Micellar solubilization by soapnut and arsenic exchange mechanism by phosphate are the probable mechanisms behind arsenic desorption. Sequential extraction reveals that the mixed soapnut–phosphate system is effective in desorbing arsenic associated with amphoteric–Fe-oxide forms. No chemical change to the wash solutions was observed by Fourier transform-infrared spectra. Soil:solution ratio, surfactant and phosphate concentrations were found to affect the arsenic desorption process. Addition of phosphate boosted the performance of soapnut solution considerably. Response surface methodology approach predicted up to 80 % desorption of arsenic from soil when treated with a mixture of ≈1.5 % soapnut, ≈100 mM phosphate at a soil:solution ratio of 1:30.

The microbial ferrous wheel: iron cycling in terrestrial, freshwater, and marine environments

The microbial ferrous wheel: iron cycling in terrestrial, freshwater, and marine environments

‘Black’ Soils in the Southern Alps: Clay Mineral Formation and Transformation, X-Ray Amorphous Al Phases and Fe Forms

AbstractMany soils in southern Switzerland have a black color, contain a large amount of soil organic matter (SOM) and seem to have some andic properties although they did not develop on volcanic parent material. We investigated three typical ‘black’ soils to determine the mechanisms of (clay) mineral formation and transformation. We measured total element pools as well as the dithionite-, pyrophosphate-and oxalate-extractable fractions (Fe, Al, Si). The clay fraction (&lt;2 µm) was analyzed using X-ray diffraction and FTIR spectroscopy. Iron speciation in the solid phase was determined by Mössbauer spectroscopy. With increasing weathering conditions, the plagioclase (albite) content decreases, trioctahedral species in the clay fraction such as biotite, chlorite or trioctahedral vermiculite either decompose or transform into a dioctahedral mineral such as dioctahedral vermiculite or hydroxy interlayered smectite (HIS). Typical weathering products were hydroxy interlayered vermiculite (HIV), HIS, interstratified minerals and kaolinite. The oxidation of Fe(II) into Fe(III) was evident and contributes to the transformation of trioctahedral mineral species into dioctahedral ones. In one soil, a large part of the Fe (up to 41%) was found in the form of Fe oxides. In the surface horizon, the poorly crystalline mineral ferrihydrite was dominant, while in the subsoil goethite prevailed. Maghemite (or maghemite/hematite mixture) was, furthermore, found in distinct concentrations down to a depth of ∼50 cm. The formation of this mineral requires high temperatures which means that a forest fire can influence soil mineralogy down to a considerable depth. The specific climatic conditions with periods of strong humidity alternating with periods of winter droughts, sporadic fire events and the relatively large content of poorly crystalline fractions of Fe and Al contributed to the stabilization of SOM.

Fractionation of Iron, Managanese, Aluminum, and Phosphorus in Selected Sandy Soils under Citrus Production

AbstractThe sorption and downward transport of trace metals and phosphate in soils are dependent, in part, on the distribution of various forms of Fe, Al, and Mn. The objectives of this study were to investigate (i) the distribution of various forms (exchangeable, organically bound, associated with Mn oxide, amorphous, and associated with crystalline Fe oxide forms) of Fe, Mn, Al, and P in the surface horizons, and (ii) the distribution of these elements in different depth horizons in six soil series in Florida representing Spodosols, Alfisols, and Entisols. Variation in concentration of total Fe with depth of soil was significantly correlated with clay content. Greater than 60 and 80% of total Fe was present in the amorphous and crystalline Fe oxide forms in soils with pH ≤ 6.5 and pH &gt; 6.5, respectively. Total concentration of Mn was lower than that of Fe or Al in all the soils, and it changed very little with depth of sampling. Manganese in the surface horizon was present predominantly as the organically bound form in the Spodosols and as organically bound, oxide, and amorphous forms in the Alfisols and Entisols. The distribution of total and oxalate‐extractable P was significantly related to the content of amorphous Al and Fe and crystalline Fe oxides in the soils. The distribution of various forms of the metals and P was significantly influenced by the soil pH and organic matter content. In the surface horizon soils, Mn and Al were present predominantly as organically bound forms at pH &lt; 6.5 or as Mn oxide and amorphous associated forms at pH &gt; 6.5.

CHEMICAL ASSOCIATION OF Cu, Zn, Mn, AND Pb IN SELECTED SANDY CITRUS SOILS

Distribution of various forms of copper, zinc, manganese and lead in the surface horizon and the distribution of total metal concentrations in different soil horizons down to 150 cm depth in six soil series (representing Spodosols, Alfisols, and Entisols) under citrus production were investigated. The metals were fractionated into the exchangeable, organically bound, associated with Mn oxide, amorphous, and associated with Fe oxide forms. Total concentrations of Cu, Zn, Mn, and Pb were greater in the Ap horizons and decreased considerably with depth. These metals were present predominantly in the organically bound forms in the Spodosols. In the Alfisols and Entisols, however, organically bound, associated with Mn oxide, and amorphous forms represented the dominant forms of these metals. The distribution of various forms of metals was influenced significantly by soil pH and organic matter content. The proportion of organically bound forms of metals was high in soils with pH <6.5; whereas associated with Mn oxide and amorphous forms were in greater proportion in soils with higher pH.