Reactive Iron Oxide Research Articles

Magnetic confinement-enabled membrane reactor for enhanced removal of wide-spectrum contaminants in water: Proof of concept, synergistic decontamination mechanisms, and sustained treatment performance

Membrane chemical reactors (MCRs) have demonstrated a great potential for simultaneous removal of wide-spectrum pollutants in advanced water treatment. However, current catalyst (re)loading and catalytic reactivity limitations obstruct their practical applications. Herein, as a proof-of-concept, we report a hollow fiber membrane chemical reactor (HF-MCR) with high and sustainable catalytic reactivity, enabled by novel magnetic confinement engineering of the catalysts. Namely, the zerovalent iron (ZVI) nanocatalysts were spatially dispersed and confined to nearly parallel magnetic induction lines, forming forest-like microwire arrays in the membrane lumen. Such arrays exhibited ultrahigh hydrodynamic stability. The HF-MCR integrated sequential membrane separation and Fenton-like catalysis, thus being capable of high and synergistic wide-spectrum decontamination. The membrane separation process completely removed large nanoplastics (NPs) via size exclusion, and thus the subsequent Fenton-like catalysis process enhanced removal efficiency of otherwise permeated bisphenol A (BPA) and phosphate (P) by in situ generated reactive oxygen species (primarily 1O2) and iron (oxyhydr)oxides, respectively. Furthermore, highly dispersed ZVI arrays and their continuous surface depassivation driven by magnetic gradient and hydrodynamic forces conferred abundant accessible catalytic sites (i.e., Fe0 and FeII) to stimulate Fenton-like catalysis. The consequent enhancement of BPA and P removal kinetics was 3–765 and 49–492 folds those in conventional (flow-through or batch) systems, respectively. Periodic ZVI reloading ensured sustained decontamination performance of the HF-MCR. This is the first demonstration of the magnetic confinement engineering that enables efficient and unlimited catalyst (re)loading and sustainable catalytic reactivity in the MCR for water treatment, which is beyond the reach of current approaches.

Martian and lunar sulfur concrete mechanical and chemical properties considering regolith ingredients and sublimation

• Sulfur sublimation rate at vacuum is 8000 times lower at −60 °C than at 20 °C. • Sulfur concrete permeability decreases strongly with higher sulfur content. • Sulfur concrete porosity does not change much with varying sulfur content. • The presence of alumina in sulphur concrete results in higher porosity. • Alumina and iron oxide together in sulphur concrete leads to lower porosity. Space constructions require unique materials suitable for the harsh conditions in space. Sulfur concrete (SC) is one of the most promising materials for such applications. However, the behavior of sulfur concrete in a vacuum, microgravity, and different temperature conditions and its chemical changes has not been well established yet. This study investigated the effect of vacuum, sulfur content, and compaction (trowel effect) on mechanical and hydro-mechanical properties. Moreover, this study assesses the impact of chemical reactions of aluminum and iron oxides with molten sulfur on the sulfur concrete’s mechanical properties via X-ray diffraction analysis (XRD), scanning electron microscopy (SEM-EDS), and Fourier-transform infrared spectroscopy (FTIR). The predictions from this research show that the sublimation rate under vacuum conditions at very low temperature (−60 °C) can be reduced nearly 8000 times compared with that at 20 °C according to the vapor pressure-sublimation rate relationship. In addition, mechanical experimental tests show that a slight increase in sulfur content dramatically reduces the permeability of sulfur concrete. On the other hand, the porosity is not much affected by the sulfur content. Moreover, sample compaction leads to a significant reduction and increment of the hydro-mechanical and mechanical strength, respectively, indicating the trowel's positive effect. Chemically, sulfur dioxide reacts with alumina and iron oxide via chemical adsorption, and then reaction with molecular oxygen produces S O 4 2 - confirmed by SEM-EDS, XRD, and FTIR results. The presence of alumina increases the sample porosity, although, together with iron oxide, another mineral, namely A l 2 F e 2 S O 4 6 ( H 2 O ) 12 . 6 H 2 O (Aluminocoquimbite), is formed resulting in a lower porosity. To conclude, we have shed light on sulfur concrete properties and preparation processes in unconventional environments and we have shown how different unknown parameters affect the mechanical and chemical properties of the sulfur binder.

Disentangling the size-dependent redox reactivity of iron oxides using thermodynamic relationships

Nanoparticles often exhibit size-dependent redox reactivities, with smaller particles being more reactive in some cases, while less reactive in others. Predicting trends between redox reaction rates and particle sizes is often complicated because a particle's dimensions can simultaneously influence its aggregation state, reactive surface area, and thermodynamic properties. Here, we tested the hypothesis that interfacial redox reaction rates for nanoparticles with different sizes can be described with a single linear free-energy relationship (LFER) if size-dependent reactive surface areas and thermodynamic properties are properly considered. We tested this hypothesis using a well-known interfacial redox reaction: the reduction of nitrobenzene to aniline by iron-oxide-bound Fe2+. We measured the reduction potential (EH) values of nano-particulate hematite suspensions containing aqueous Fe2+ using mediated potentiometry and characterized the size and aggregation states of hematite samples at circumneutral pH values. We used the measured EH values to calculate surface energies and reactive surface areas using thermodynamic relationships. Nitrobenzene reduction rates were lower for smaller particles, despite their larger surface areas, due to their higher surface energies. When differences in surface areas and thermodynamic properties were considered, nitrobenzene reduction kinetics for all particle sizes was described with a LFER. Our results demonstrate that when Fe2+ serves as a reductant, an antagonistic effect exists, with smaller particles having larger reactive surface areas but also more positive reduction potentials. When Fe3+ serves as an oxidant, however, these two effects work in concert, which likely explains past discrepancies regarding how iron oxide particle sizes influence redox reaction rates.

Numerical study of post-combustion characteristic in a smelting reduction furnace

Abstract For a smelting reduction furnace, the main role of the post-combustion operation in the upper space is to supply the energy required by the endothermic reduction reaction of iron oxide occurring in the melt bath, and thus the post-combustion behavior is important in the respect of reducing the coal consumption. In this paper, based on a developed 3D model composed of the Realizable k–ε model and DO radiation model, the post-combustion characteristics in a smelting reduction furnace was studied by numerical simulation, and the effects of top lance height, top-blown hot air flowrate and nozzle inclination angle of top lance on the flow and temperature fields, CO content distribution in the upper space were investigated. The results show that with the increase of top lance height, the length and temperature of combustion flame in the upper space increase, which is beneficial to expanding the post-combustion zone and improving the post-combustion of CO gas. And as the top-blown hot air flowrate increases, the position of the maximum temperature in the upper space gradually moves down and the CO content in gas mixtures decreases, which is conducive to the heat transport back to the melt bath. In addition, a proper velocity and temperature distribution in the upper space and distribution of CO content is obtained under the nozzle inclination angle range from 10° to 15°, but a larger nozzle inclination angle reduces the velocity and temperature in the central zone.

Gd, La co-doped CeO2 as an active support for Fe2O3 to enhance hydrogen generation via chemical looping water gas shift

Support materials are indispensable to promote the durability of iron oxides for chemical looping applications. However, the dilution effect of supports on the active phase would lead to decreased bulk oxygen conduction, thus leading to compromised activity. Here, we propose several Gd3+, La3+ and Nd3+ doped CeO2 as active supports for iron oxides and investigate the support effect to improve hydrogen generation via chemical looping water gas shift. The characterizations show that the dopants improve the oxygen vacancy concentration in the CeO2 lattice and Fe2O3/Ce0.8Gd0.1La0.1O2-δ exhibits the most oxygen vacancy concentration among all the oxygen carriers. Pulse reactions of oxygen carriers show that an abundance of oxygen vacancy concentration can promote the lattice oxygen transfer in bulk, thus contributing to improved redox reactions. The high oxygen conductivity mitigates the dilution effect on the active phase. Therefore, Fe2O3/Ce0.8Gd0.1La0.1O2-δ shows the highest hydrogen yield (∼9.49 mmol−1.g−1) and hydrogen generation rate (∼0.632 mmol.g−1.min−1) with only a slight decrease at 650 °C over 100 cycles. Overall, this work highlights the influence of support properties on the redox reactivity of iron oxides for chemical looping applications.

SEM-EDS and water chemistry characteristics at the early stages of glacier recession reveal biogeochemical coupling between proglacial sediments and meltwater

Most glaciers worldwide are undergoing climate-forced recession, but the impact of glacier changes on biogeochemical cycles is unclear. This study examines the influence of proglacial sediment weathering on meltwater chemistry at the early stages of glacier recession in the High Arctic of Svalbard. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) in combination with a wide range of geochemical analyses were used in this study. The SEM-EDS analyses of sediments collected in front of Werenskioldbreen show general degradation of pyrite and carbonate grains with age. The outer parts of pyrite grains have a gradual decrease in sulphur and gradual increase in iron oxides due to pyrite oxidation. This process was less advanced in the proglacial zone younger than 100 years compared to older sites such as the terminal moraine from the Little Ice Age. In both the proglacial zone and the terminal moraine, physical weathering of mineral grains, including formation of microcracks and microfractures, clearly enhanced pyrite oxidation. A consequence of proglacial sediment weathering is that the river chemistry is strongly affected by carbonate dissolution driven by sulphuric acid from sulphide oxidation. Also, reactive iron oxides, a product of sulphide oxidation, are mobilized in the proglacial zone. The results of this study show that proglacial weathering in the High Arctic of Svalbard is strongly coupled to river geochemistry, especially during the early stages of proglacial exposure after glacier recession.

Heterogeneous Fenton catalyst based on clay decorated with nano-sized amorphous iron oxides prevents oxidant scavenging through surface complexation

• Crystalline and amorphous Fe-oxide-clay composites were compared as Fenton catalysts. • Nano-sized amorphous iron-oxide-clay best-promoted phenanthrene oxidation. • A stable H 2 O 2 complex was formed with the amorphous iron-oxide-clay surface. • Single oxygen was primarily responsible for phenanthrene degradation. • Oxidation of phenanthrene was achieved with minimal H 2 O 2 waste. Heterogeneous Fenton catalysts based on iron-oxides are of great interest due to their low cost and high reactivity. Herein, the activity of nano-sized amorphous iron-oxide particles deposited on montmorillonite clay (MMT) was compared to crystalline hematite and magnetite-coated MMT. Hematite, magnetite, and their respective clay composites showed little catalytic activity and almost no phenanthrene (PHE) oxidation at circumneutral pH - in part, due to the decomposition of hydrogen peroxide. In contrast, the amorphous iron oxide (Fe)-MMT, showed very high catalytic activity (over 60% PHE degradation in 60 min) with only minimal consumption of H 2 O 2 (3.3%). In-depth characterization of the Fe-MMT coupled with kinetic and mechanistic experiments was performed to understand the reaction pathway and mechanism. The results suggest that initially H 2 O 2 is complexed to the nano-sized iron oxide catalytic sites on the surface, forming a stable Fe-MMT-H 2 O 2 complex that is activated primarily when the pollutant is introduced. The reactive species, ∙OH and 1 O 2 , were detected upon the diffusion of PHE to the surface - leading to oxidation and mineralization with a minimal decomposition of H 2 O 2 . Quenching experiments further revealed that 1 O 2 played an important role in the extensive mineralization of PHE. The amorphous Fe-MMT also exhibited high stability and performance in semi-continuous cycle batch experiments. Thus, this low-cost and simple Fe-MMT catalyst was found to be highly efficient towards the activation of H 2 O 2 , oxidizing PHE rapidly while reducing radical scavenging and unwanted side reactions. These findings, regarding the role of amorphous phases in the reactivity of iron-oxides, have the potential to improve the design and implementation of solid Fenton catalysts for pollutant remediation in engineered and natural systems.

Tinaksite and Tokkoite: X-ray Powder Diffraction, Optical, and Vibrational Properties

In this study, natural tinaksite (K2Ca2NaTi[Si7O18OH]O) and tokkoite (K2Ca4[Si7O18OH](OH,F)) collected in charoite rocks of the Murun alkaline massif (Siberia, Russia) were examined by X-ray diffraction and optical and vibrational spectroscopic methods. A comparative analysis of the experimental diffraction patterns with respect to the calculated X-ray powder diffraction patterns was carried out for tinaksite and tokkoite powders. The shift in the diffraction peaks of tinaksite is explained by the smaller values of the unit cell parameters a and b as compared with those of tokkoite. A similar shift of the peaks is also observed in the Raman and infrared absorption spectra; however, this feature is explained by the difference in the chemical composition of the minerals. The shoulder in the absorption spectra at about 800 nm in tinaksite and 700 nm in tokkoite corresponds to the presence of Mn2+ and Fe3+ absorption bands, the presence of which determines the color of tinaksite and tokkoite. The luminescence band with a maximum at about 540–550 nm in the photoluminescence spectra is related to Mn2+ centers, while an additional band at about 610 nm can be associated with Ti3+ centers in tinaksite. The intensity of the Fe3+ ESR signal increases in both samples after heating, while the intensities of the bands associated with OH groups decrease in tinaksite and tokkoite. This characteristic is the result of iron oxidation and dehydrogenation reaction.

The Chemical Oxidation and Immobilization of Arsenic and Antimony in Simulated AMD in Karst Areas.

The widely distributed carbonate rock in karst areas can neutralize AMD, increase pH and promote Fe(II) oxidation, which produce reactive oxidation species (ROS) and iron (hydr)oxides. (ROS) and iron (hydr)oxides can facilitate oxidation and immobilization of As and Sb. The flow, pH and Fe(II) concentration of AMD may affect oxidation or immobilization of As/Sb via affecting of Fe(II) oxidation in AMD. This study investigated the influence of Fe(II) oxidation on the transfer and transformation of As/Sb in simulated AMD induced by carbonate. Low flow and high pH were beneficial for Fe(II) oxidation to produce ROS/Fe(IV) and iron (hydr)oxides to oxidize As(III)/Sb(III). Fe(II) concentration (100-500mg/L) had negligible influence on oxidation and sediment of As(III)/Sb(III). With increase of reaction time, difference in As(III)/Sb(III), total As/Sb concentrations between different flow distances were decreasing.

Green synthesis of graphite-based photo-Fenton nanocatalyst from waste tar via a self-reduction and solvent-free strategy

Thermochemical conversion of biomass yields large quantities of tar as a by-product, which is a potential precursor for the synthesis of renewable carbon-based functional materials. In this study, high-performance photo-Fenton catalyst of graphite‑carbon-supported iron nanoparticles was synthesized using a self-reduction and solvent-free approach. The results showed that the tar-derived catalyst had unique properties including a defect-rich graphitic structure, high surface area, and an abundant porous structure resulting from the inherent properties of biomass tar. The iron nanoparticles were highly dispersed within the prepared catalysts and were stably anchored on the carbonaceous surface by the FeC bond. The prepared nanocatalyst showed the highest decomposition constant (91.87 × 10−3 min−1) for 20 mM H2O2, and 40 mg/L RhB can be completely degraded within 2 h under catalyst dosage of 1 g/L and H2O2 addition of 20 mM. The degradation mechanism under the photo-Fenton catalyst/H2O2/light system included the heterogeneous Fenton reaction of iron nanoparticles and photo-Fenton reaction of iron oxide, and the efficient RhB degradation was mainly ascribed to the heterogeneous Fenton reaction. In addition, recycling and leaching tests demonstrated that the photo-Fenton catalyst had excellent reusability and stability, where only 7.3% catalytic reactivity was reduced after five cycles. This work provided a green, sustainable, and facile approach for synthesizing photo-Fenton catalysts by value-added utilization of tar wastes.

(NH4)3FeF6 mesocrystals grown by electric field-assisted in situ dissolution and the reaction of anodic iron oxides.

(NH4)3FeF6 mesocrystalline octahedrons are formed by in situ dissolution and the reaction of anodic iron oxides, followed by a non-classical crystal growth process involving electrical polarization and subsequent alignment of primary (NH4)3FeF6 particles under an external electric field. The obtained (NH4)3FeF6 can be converted to nano-micro hierarchical α-Fe2O3 octahedrons by thermal annealing.

Revealing the crystal phases of primary particles formed during the coprecipitation of iron oxides.

The mechanistic investigation of the coprecipitation formation of iron oxides has been a long-standing challenge due to the rapid reaction kinetics and high complexity of iron hydrolysis reactions. Although a few studies have suggested that the coprecipitation of iron oxide nanoparticles follows a non-classic route through inter-particle attachment, the compositions of the primary particles remain undetermined. Herein, by using a specially designed gas/liquid mixed phase fluidic reactor we controlled the reaction time from 3 s to over 5 min, and successfully identified the concentration of different intermediate phases as a function of time. We suggest that the initial Fe3+ ions are hydrolyzed under the alkaline condition to give Fe(OH)3, which then rapidly dehydrates to yield α-FeOOH. In the presence of Fe2+ ions, which could also act as the catalyst, α-FeOOH finally transforms to Fe3O4.

Association of Organic Carbon With Reactive Iron Oxides Driven by Soil pH at the Global Scale

AbstractAssociation of organic carbon (C) with iron (Fe) minerals is one important mechanism for long‐term terrestrial C storage. Yet, specific edaphic variables that directly contribute to Fe‐associated C across diverse soil types are still unclear. Through analyzing soils from the National Ecological Observatory Network (NEON) and other published data, here we show that soil pH primarily controls Fe‐associated C across the globe. Fe‐associated C in most soils ranged from 0 to 20 g kg−1 soil, with a strong increase from pH 4.2 to 3.5, but a small change in soils with pH > 4.2. A microcosm experiment further showed that raising soil pH by liming reduced the formation of Fe‐associated C in an acidic Oxisol. Together, these findings demonstrate the dominant role of soil pH in controlling the abundance of Fe‐associated C.

Iron Binding in the Ferroxidase Site of Human Mitochondrial Ferritin.

Ferritins are nanocage proteins that store iron ions in their central cavity as hydrated ferric oxide biominerals. In mammals, further the L (light) and H (heavy) chains constituting cytoplasmic maxi-ferritins, an additional type of ferritin has been identified, the mitochondrial ferritin (MTF). Human MTF (hMTF) is a functional homopolymeric H-like ferritin performing the ferroxidase activity in its ferroxidase site (FS), in which Fe(II) is oxidized to Fe(III) in the presence of dioxygen. To better investigate its ferroxidase properties, here we performed time-lapse X-ray crystallography analysis of hMTF, providing structural evidence of how iron ions interact with hMTF and of their binding to the FS. Transient iron binding sites, populating the pathway along the cage from the iron entry channel to the catalytic center, were also identified. Furthermore, our kinetic data at variable iron loads indicate that the catalytic iron oxidation reaction occurs via a diferric peroxo intermediate followed by the formation of ferric-oxo species, with significant differences with respect to human H-type ferritin.

Cross-Shore and Depth Zonations in Bacterial Diversity Are Linked to Age and Source of Dissolved Organic Matter across the Intertidal Area of a Sandy Beach

Microbial communities and dissolved organic matter (DOM) are intrinsically linked within the global carbon cycle. Demonstrating this link on a molecular level is hampered by the complexity of both counterparts. We have now investigated this connection within intertidal beach sediments, characterized by a runnel-ridge system and subterranean groundwater discharge. Using datasets generated by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and Ilumina-sequencing of 16S rRNA genes, we predicted metabolic functions and determined links between bacterial communities and DOM composition. Four bacterial clusters were defined, reflecting differences within the community compositions. Those were attributed to distinct areas, depths, or metabolic niches. Cluster I was found throughout all surface sediments, probably involved in algal-polymer degradation. In ridge and low water line samples, cluster III became prominent. Associated porewaters indicated an influence of terrestrial DOM and the release of aromatic compounds from reactive iron oxides. Cluster IV showed the highest seasonality and was associated with species previously reported from a subsurface bloom. Interestingly, Cluster II harbored several members of the candidate phyla radiation (CPR) and was related to highly degraded DOM. This may be one of the first geochemical proofs for the role of candidate phyla in the degradation of highly refractory DOM.

Redox degrees of iron-based oxygen carriers in cyclic chemical looping combustion using thermodynamic analysis

Chemical looping combustion (CLC) provides a sustainable production of energy while inherently capturing carbon dioxide using oxygen carriers (OCs). To efficiently operate the reactor of the CLC, a thermodynamic analysis of OC is essential. This study focuses on the complete thermodynamic analysis of CLC employing iron-based OCs. In the analysis, methane as a reduction gas and air as an oxidizer along with the fuel and air reactors operated at 900 °C. Six different methane-to-hematite molar ratios (M/H) and O2-to-hematite molar ratio (O/H) on iron oxide redox reactions are evaluated. The redox degree, including reduction and oxidation degrees of OCs in the fuel and air reactors, is introduced to figure out their performance. The results reveal that M/H = 1/15 with O/H = 1 yields the highest CO2 and H2O yields. Under six O/H ratios with M/H = 1/12, the highest CO2 and H2O yields are achieved at O/H ≥ 0.18. These operation conditions are conducive to carbon capture. In most cases of reduction and oxidation reactions, Fe3O4 and Fe2O3 account for the largest shares of iron in the OCs, respectively. The results show that a decrease in oxygen input to the air reactor leads to more carbon formed in the system. An enthalpy balance indicates an overall exothermic reaction behavior. Introducing the redox degree suggests that the optimal operation conditions are at M/H = 1/12 and O/H = 0.18. This study has provided crucial information about the operation and reactivity of iron oxides with methane in the chemical looping combustion process.

Macroalgae degradation promotes microbial iron reduction via electron shuttling in coastal Antarctic sediments.

Colonization of newly ice-free areas by marine benthic organisms intensifies burial of macroalgae detritus in Potter Cove coastal surface sediments (Western Antarctic Peninsula). Thus, fresh and labile macroalgal detritus serves as primary organic matter (OM) source for microbial degradation. Here, we investigated the effects on post-depositional microbial iron reduction in Potter Cove using sediment incubations amended with pulverized macroalgal detritus as OM source, acetate as primary product of OM degradation and lepidocrocite as reactive iron oxide to mimic in situ conditions. Humic substances analogue anthraquinone-2,6-disulfonic acid (AQDS) was also added to some treatments to simulate potential for electron shuttling. Microbial iron reduction was promoted by macroalgae and further enhanced by up to 30-folds with AQDS. Notably, while acetate amendment alone did not stimulate iron reduction, adding macroalgae alone did. Acetate, formate, lactate, butyrate and propionate were detected as fermentation products from macroalgae degradation. By combining 16S rRNA gene sequencing and RNA stable isotope probing, we reconstructed the potential microbial food chain from macroalgae degraders to iron reducers. Psychromonas, Marinifilum, Moritella, and Colwellia were detected as potential fermenters of macroalgae and fermentation products such as lactate. Members of class deltaproteobacteria including Sva1033, Desulfuromonas, and Desulfuromusa together with Arcobacter (former phylum Epsilonbacteraeota, now Campylobacterota) acted as dissimilatory iron reducers. Our findings demonstrate that increasing burial of macroalgal detritus in an Antarctic fjord affected by glacier retreat intensifies early diagenetic processes such as iron reduction. Under scenarios of global warming, the active microbial populations identified above will expand their environmental function, facilitate OM remineralisation, and contribute to an increased release of iron and CO2 from sediments. Such indirect consequences of glacial retreat are often overlooked but might, on a regional scale, be relevant for the assessment of future nutrient and carbon fluxes.

A Study on the Effect of Alumina on the Morphology and Reduction Behavior of Sinter by In Situ Observation

The effects of Al2O3 content on the morphology and reducibility of sinter were respectively investigated using confocal laser microscopy and thermogravimetric analysis at 1273 K under CO gas. To understand the effects of the sintering process, separate samples were prepared via the equilibrium and metastable reaction routes. In the equilibrium samples, the addition of Al2O3 led to the formation of the silico-ferrite of calcium and alumino phase and a decrease in the reduction rate due to the lowered reactivity of iron oxide. In contrast, in the metastable samples, the reduction rate increased after the addition of 2.5 mass% Al2O3. The addition of Al2O3 decreased the fraction of the liquid phase and increased the fraction of pores in the sample. As a result, the reduction rate is proportional to the Al2O3 content owing to the changes in the sinter morphology. In determining the reduction rate of the sinter, the influence of the microstructure on the diffusion of the reducing gas is more significant than that of the interfacial chemical reaction due to the formation of the SFCA phase. The microstructure changes of the sinter with the addition of Al2O3 and the corresponding reduction behaviors are further discussed.

Ferric iron triggers greenalite formation in simulated Archean seawater

AbstractSedimentary rock deposits provide the best records of (bio)geochemical cycles in the ancient ocean. Studies of these sedimentary archives show that greenalite, an Fe(II) silicate with low levels of Fe(III), was an early chemical precipitate from the Archean ocean. To better understand the formation of greenalite, we explored controls on iron silicate precipitation through experiments in simulated Archean seawater under exclusively ferrous conditions or supplemented with low Fe(III). Our results confirm a pH-driven process promoting the precipitation of iron-rich silicate phases, and they also reveal an important mechanism in which minor concentrations of Fe(III) promote the precipitation of well-ordered greenalite among other phases. This discovery of an Fe(III)-triggering iron silicate formation process suggests that Archean greenalite could represent signals of iron oxidation reactions, potentially mediated by life, in circumneutral ancient seawater.

Thermodynamic Study of Energy Consumption and Carbon Dioxide Emission in Ironmaking Process of the Reduction of Iron Oxides by Carbon

Carbon included in coke and coal was used as a reduction agent and fuel in blast furnace (BF) ironmaking processes, which released large quantities of carbon dioxide (CO2). Minimizing the carbon consumption and CO2 output has always the goal of ironmaking research. In this paper, the reduction reactions of iron oxides by carbon, the gasification reaction of carbon by CO2, and the coupling reactions were studied by thermodynamic functions, which were derived from isobaric specific heat capacity. The reaction enthalpy at 298 K could not represent the heat value at the other reaction temperature, so the certain temperature should be confirmed by Gibbs frees energy and gas partial pressure. Based on Hess’ law, the energy consumption of the ironmaking process by carbon was calculated in detail. The decrease in the reduction temperature of solid metal iron has been beneficial in reducing the sensible heat required. When the volume ratio of CO to CO2 in the top gas of the furnace was given as 1.1–1.5, the coupling parameters of carbon gasification were 1.06–1.28 for Fe2O3, 0.71–0.85 for Fe3O4, 0.35–0.43 for FeO, respectively. With the increase in the coupling parameters, the volume fraction of CO2 decreased, and energy consumption and CO2 output increased. The minimum energy consumption and CO2 output of liquid iron production were in the reduction reactions with only CO2 generated, which were 9.952 GJ/t and 1265.854 kg/t from Fe2O3, 9.761 GJ/t and 1226.799 kg/t from Fe3O4, 9.007 GJ/t and 1107.368 kg/t from FeO, respectively. Compared with the current energy consumption of 11.65 GJ/t hot metal (HM) and CO2 output of 1650 kg/tHM of BF, the energy consumption and CO2 of ironmaking by carbon could reach lower levels by decreasing the coupled gasification reactions, lowering the temperature needed to generate solid Fe and adjusting the iron oxides to improve the iron content in the raw material. This article provides a simplified calculation method to understand the limit of energy consumption and CO2 output of ironmaking by carbon reduction iron oxides.