SiO2 Precipitation Research Articles

Source of quartz cement and its impact on reservoir quality in Jurassic Shaximiao Formation in central Sichuan Basin, China

Quartz cement is an essential authigenic mineral in the tight sandstones of the Shaximiao Formation in the Sichuan Basin, China. Accurately identifying its sources and investigating its impact on reservoir quality is important for understanding diagenetic fluid evolution and pore development in these reservoirs. To address this, we conducted a systematic analysis of the silicon sources in quartz cement employing mineralogical, fluid inclusion, and geochemical data. Quartz cement is characterized by two phases of quartz overgrowth and pore-filling authigenic quartz. Homogenization temperatures of fluid inclusions range from 60 to 150 °C, indicating that quartz cementation occurred from the early diagenetic phase (B stage) to the late mesodiagenetic phase. During the early diagenetic stage, volcanic material was dissolved by meteoric water, resulting in the formation of the earliest quartz cement and smectite. At temperatures of approximately 70–100 °C, smectite illitization, accompanied by K-feldspar consumption and SiO2 precipitation. In the mesodiagenetic stage (temperature approximately 100–130 °C), organic acids led to the dissolution of minerals like feldspar and laumontite, releasing SiO2. In the deep burial stage, clastic quartz grains experienced dissolution due to high stress, providing a limited source of SiO2. Smectite alteration and the dissolution of aluminosilicate minerals represent the primary sources of silicon for quartz cementation, while contributions from volcanic material hydrolysis and pressure solution are relatively minor. The presence of chlorite films inhibits the development of quartz overgrowths, and the limited precipitation of authigenic quartz within the primary porosity has a minimal impact on reservoir quality. Consequently, sandstones rich in chlorite films represent a favorable lithology for the development of high-quality reservoirs.

Formation Mechanism of High‐MnO‐Content SiO2/SiO2–MnO–(CaO–Al2O3) Dual‐Phase Inclusions in Si–Mn‐Killed Steel

This study investigates the deformation characteristics of SiO2–MnO–(CaO–Al2O3)‐type inclusions in silicon manganese deoxidized steel from continuous casting to rolling. The results reveal that the inclusions contain 60–80% SiO2, 10–35% MnO, and a combined CaO and Al2O3 content of <20%. SiO2 precipitates inside the inclusions during the continuous casting and rolling processes, which results in the formation of pure SiO2/SiO2–MnO–(CaO–Al2O3) dual‐phase inclusions in the remaining components owing to the increase in the MnO content to 15–40%. According to the experimental results of melting rate measurement and thermodynamic calculation using FactSage8.1, the SiO2–MnO–(CaO–Al2O3) phase has a high proportion of liquid components at 1100–1360 °C and exhibits good deformation properties during the breaking down and rolling processes. When the total content of CaO, Al2O3, and MgO in the SiO2–CaO–Al2O3–MgO–MnO inclusions reaches 30–50%, SiO2 precipitation is inhibited during the continuous casting to rolling process. Through the addition of metal manganese and carbon powder during the pre‐deoxidation process, followed by alloying with ferrosilicon and metal manganese, and then the addition of acidic synthetic slag for slag formation, the proportion of SiO2/SiO2–MnO–(CaO–Al2O3) dual‐phase inclusions in the casting billet is increased from 12% to >60%.

Mechanism of nano-SiO2 internal generation for modification of cement-based materials

In order to address the issue of agglomeration of nano-SiO2 mineral powder and maximize its nano-effect, a novel method is proposed to internally generate nano-SiO2 for modifying cement-based materials. Firstly, a highly stable nano-SiO2 precursor solution (NP) is synthesized by liquid-phase method. The existence state and precipitation evolution of SiO2 in NP are then studied based on the theory of silicic acid polymerization and rheological reaction kinetics. Then, the effects of NP on cement-based materials from microstructure (morphology, composition, pore) to macroscopic mechanical behavior are studied by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and mercury porosimeter. The results show that SiO2 in NP primarily exists in the form of silicate molecules, silicate oligomers, and colloidal particles, thereby avoiding the agglomeration problem commonly associated with solid nanoparticles. When NP is mixed with cement, the original NP dispersion system is destabilized, resulting in the precipitation of SiO2 as nanoparticles and fully utilizing the nano-effect. Consequently, the cement paste exhibits a denser microstructure and significantly higher compressive strength after hardening.

Mechanism analysis of hydrochloric and acetic acids dissolving clay minerals

During acidification of the reservoir, the acid concentration will continually decrease as the reaction progresses. If the reaction time is long enough, the acid will eventually be exhausted. According to this phenomenon, the acidification of the reservoir can be regarded as the reaction between different concentrations of acid and minerals. Mineral dissolution is the common method for carbonate reservoirs to reform reservoirs and acid fracturing. In addition to carbonate minerals, carbonate reservoirs are likely to contain clay minerals, but less study has been done on acids with different concentrations and clay minerals. In this work, hydrochloric acid (HCl), acetic acid (HAC), and mixture acid (HCl + HAC) were used to dissolve chlorite, montmorillonite, kaolinite, and illite, respectively. The mechanism of acid dissolving clay minerals was analyzed by XRD, ICP-OES, and SEM. Moreover, we found that a high concentration of HCl has an anti-swelling and shrinkage effect on montmorillonite. The result shows that during the processes of HCl dissolving chlorite, montmorillonite and illite, there is SiO2 precipitate generated even at solutions’ pH of 0.5. In addition, chlorite may generate bischofite precipitate. Moreover, the effect of HCl dissolving chlorite is larger than that of other clay minerals. Also, by comparing the effects of various acids dissolving clay minerals, the effect of HAC dissolving clay minerals is lower than that of HCl, and the precipitate is not formed. The mixture acid dissolving clay minerals is better than that of single acid except for montmorillonite. For reservoirs with high chlorite content, the concentration of HCl can be appropriately reduced and the concentration of HAC can be increased; for reservoirs with high montmorillonite content, the concentration of HCl can be increased. Illite and kaolinite react weakly in HCl and hardly react with HAC.

Dissolution and reprecipitation of amorphous silica in silica Rich shales induces Non-Monotonic evolution of porosity in acidic reactive environments

Advances in sustainable subsurface energy technologies are crucial for meeting our energy and resource needs for a climate-resilient future. Novel strategies to harness subsurface shale reservoirs for recovering valuable metals and for enabling CO2 storage are influenced by the morphological and mineralogical heterogeneities of these materials. In this context, delineating the interactions of highly acidic solutions such as wet supercritical CO2 on shales with varying mineralogy is crucial to inform the stability of caprock seal for CO2 storage and enhancements in permeability for fluid transport, reactivity, and storage. The feedback chemical effects associated with the interactions of acidic solutions on the morphologies and mineralogies of shales have not been extensively investigated. These insights are crucial for assessing temporal changes in the reactivity and the fate of the fluids in subsurface environments. In this study, we investigate the effect of 1 M HCl solution on the chemistry and morphology of three different shale samples with varying carbonate, clay and silica contents. An increase in the amorphous content, from 37 % to 41.3 %, of silica-rich and carbonate/clay lean shale is noted due to reactions with an acidic solution which is attributed to the dissolution of Si-bearing phases such as clays, accompanied by SiO2 precipitation. In shales bearing high content of clays and carbonates, significant increase in the pore volumes and surface areas are noted. Non-monotonic changes in the micron-scale porosity of silica rich – carbonate/clay lean (e.g., Mowry shale) are noted using in-situ X-ray microtomography experiments. Due to the initial mobilization of silica and dissolution of carbonate/clay phases, the total porosity slightly increases from 6.7 % to 10.7 % followed by a decrease to ∼4 % caused by SiO2 reprecipitation. These findings suggest that even though silica is less reactive in acidic environments, the changes in the amorphous and crystalline content due to dissolution and reprecipitation alter the porosity and fluid flow paths.

SiO2 migration mechanism at the joints of SiO2f/SiO2 composite brazed by bismuth glass

Joining is an indispensable process for expanding the application of ceramics and composites. Recently, glasses have been extensively explored for ceramic/composite joining owing to their unique functional needs. However, the difficulty in detecting amorphous materials and lack of enthalpy data make the interfacial reaction mechanism challenging to investigate. In this study, the interfacial reaction mechanism of joints of SiO2f/SiO2 composite-brazed bismuth glass was thoroughly explored. SiO2 was dissolved from the matrix and used throughout the brazing process. In the initial stage, silica reacts with the brazing glass to form Bi4(SiO4)3. Then, owing to the decomposition of Bi4(SiO4)3, the silicate glass replaced the bismuth glass. Finally, some precipitation of SiO2 occurred at the brazing seam owing to an entropy–enthalpy dominating mode. This study may instigate the design of brazing glasses for joining SiO2f/SiO2 composites.

Single-Side Superhydrophobicity in Si3N4-Doped and SiO2-Treated Polypropylene Nonwoven Webs with Antibacterial Activity

Meltblown (MB) nonwovens as air filter materials have played an important role in protecting people from microbe infection in the COVID-19 pandemic. As the pandemic enters the third year in this current global event, it becomes more and more beneficial to develop more functional MB nonwovens with special surface selectivity as well as antibacterial activities. In this article, an antibacterial polypropylene MB nonwoven doped with nano silicon nitride (Si3N4), one of ceramic materials, was developed. With the introduction of Si3N4, both the average diameter of the fibers and the pore diameter and porosity of the nonwovens can be tailored. Moreover, the nonwovens having a single-side moisture transportation, which would be more comfortable in use for respirators or masks, was designed by imparting a hydrophobicity gradient through the single-side superhydrophobic finishing of reactive organic/inorganic silicon coprecipitation in situ. After a nano/micro structural SiO2 precipitation on one side of the fabric surfaces, the contact angles were up to 161.7° from 141.0° originally. The nonwovens were evaluated on antibacterial activity, the result of which indicated that they had a high antibacterial activity when the dosage of Si3N4 was 0.6 wt%. The bacteriostatic rate against E. coli and S. aureus was up to over 96%. Due to the nontoxicity and excellent antibacterial activity of Si3N4, this MB nonwovens are promising as a high-efficiency air filter material, particularly during the pandemic.

Coal fly ash cleaner utilization by ferric oxide assisted roasting – leaching silica: Recycling lixiviant by seeded precipitation of leachate

Recycling lixiviant from sodium silicate solution is a key to the cleaner utilization of aluminosilicate solid wastes such as coal fly ash and coal gangue via ferric oxide assisted roasting - alkali leaching silica process. This work focuses on seeded precipitation of higher-modulus sodium silicate solution to obtain recyclable spent solution with lower-modulus. The results show that adding silica hydrate seeds can induce the decomposition of sodium silicate solution to obtain amorphous silicate hydrate and low-modulus spent solution. The SiO2 precipitation yield, precipitation rate and the spent solution modulus can respectively reach up to 108.26 g/L, 72.17 % and 0.86, under the appropriate conditions: initial modulus 3.09, total Na2O concentration 50 g/L, seed amount 350 g/L-liquor, temperature 50 ℃, and duration 6 h. Meanwhile, the average particle size of precipitation product is ~ 56.20 µm. Moreover, the structural evolution of silicates during the precipitation implies that silica hydrate mainly precipitates from high-polymerized silicates. The precipitation product was further characterized as opal-A by XRD, chemical composition, TG-DSC and FT-IR analyses. This work may be beneficial either to improving the ferric oxide assisted roasting - alkali leaching silica process for treating aluminosilicate solid wastes or to developing a new technology for preparing opaline silica hydrate.

Moisture-regulating microcapsule and its enhanced water vapor permeability of leather polyurethane coating

A type of porous shell moisture-regulating microcapsules (MRM), was successfully prepared by self-assembly of chitosanand graphene oxide onto the surface of paraffin, followed by chemical precipitation of SiO2). The MRM was used to enhance the water vapor permeability (WVP) of polyurethane (PU) composite coating. The composite PU coating was facilely fabricated by mixing PU emulsion with the prepared MRM and applied in leather coating. The SEM and AFM of composite PU films confirmed that the MRM in PU increased the roughness of the composite films, enhancing the water contact angle. As a result, the WVP of the composite PU coating was effectively improved by MRM and exhibited a good balance between WVP and water resistance, maintaining the transparency and mechanical properties of the films. After the composite PU coating was coated on leather surface, the leather showed an improvement of WVP with slight increase in water resistance, indicating that the composite PU coating with MRM can be a promising candidate to improve the comfort of leather.

Geochemical modeling unravels the water chemical changes along the largest Mexican river

Tropical Usumacinta River is the tenth largest in North America and Mexico's principal river. Diverse and increasing anthropogenic activities (e.g., land-use change, agriculture, urban development) occur along the river and most likely alter its water quality. We used inverse geochemical models (PHREEQC code) and chemical diagrams to determine the water chemistry in the mainstream and principal tributaries, the principal geological and anthropic causal factors, and the effects of tropical seasonality. The dominant chemical water type in the river is Ca-Mg-SO4-HCO3 in the dry season, while Ca-Mg-HCO3-SO4 in the rainy season. The water-rock interaction processes in the dry season are: 1) gypsum, halite, and sylvite precipitation, and Mg2+ to Na+ ion exchange in the middle basin; 2) calcite, kaolinite, and SiO2(a) precipitation, CO2, dolomite, gypsum, halite, sylvite, and K-feldspar dissolution, nitrification, and Ca2+ to Na+ ion exchange in the lower basin. In the rainy season, the processes are: 1) plagioclase and sylvite precipitation, calcite, dolomite, gypsum, halite, kaolinite, and K-feldspar dissolution in the middle basin; 2) kaolinite precipitation, dolomite, gypsum, halite, K-feldspar, and sylvite dissolution, and nitrification, in the lower basin. The model outcome reveals the water-rock interactions that control the chemical composition in the river are mainly the carbonate and secondly the silicate weathering in the dry season. In contrast, in the rainy season, the weathering of carbonate and silicate rocks have the same weathering proportion. At the same time, the model of the dry season reveals the influence of saline water intrusion in the lower basin. The effects caused by agricultural activities as nitrification are evident in the lower basin of the dry season. In contrast, it is not present in the rainy season most likely associated to the river flow dilutes the nitrates. The results of this research are slightly different from other tropical rivers with similar geology, with a predominance of Ca2+ and Mg2+ cations, and HCO3- and SO42- anions. This investigation is the first to present the major ions composition, chemical water types, and water-rock interactions in the tropical Usumacinta River. The methodology herein used proved to be useful to understand the chemical changes in rivers with various geological materials that could be applied in other tropical and temperate rivers.

Unravelling the effect of impurities on the methanol-to-olefins process in waste-derived zeolites ZSM-5

One of the main challenges in the next decades for the chemical industry is the valorization of low-grade industrial waste streams into valuable materials, as their increasing amount can have a major environmental impact. For example, in copper mining industry, waste flotation tailings can be valorized into solid acid catalysts, such as zeolites. Numerous studies have shown that modification of zeolites by metal ions has a positive effect in their catalytic performance. However, the presence of impurities in waste-derived materials is usually disregarded. In this work, the influence of impurities in waste-derived zeolites ZSM-5 on the physicochemical properties and the catalytic performance in methanol-to-hydrocarbons (MTH) reaction was investigated. Flotation tailings from copper industry, mainly consist of SiO2 as quartz, next to Ca, Mg and Fe species. By altering the pH in the SiO2 precipitation step, the impurities content in the waste-derived zeolites ZSM-5 could be controlled. Chemical analysis confirmed the presence of Fe, Ca and Mg in low concentrations in the waste-derived samples. UV–Vis Diffuse reflectance spectroscopy (DRS) and electron paramagnetic resonance (EPR) showed the presence of different Fe species in the waste-derived samples in the form of isolated Fe3+, oligomeric FexOy species and some extended Fe2O3-like clusters. Temperature programmed desorption (TPD) of NH3 and infrared (IR) spectroscopy measurements after CO and pyridine adsorption were used in order to probe the effect of the cations present in the acidity of the waste-derived materials. It was found that the zeolite material with a higher amount of metal impurities contained less Brønsted acid sites and more Lewis acid sites. Catalytic testing showed that the metal impurities had a positive influence on the overall catalyst stability and the yields towards ethylene and propylene were respectively 8% and 6% up. Meanwhile, operando UV–Vis DRS showed the influence of impurities on the catalytic mechanism, as its increase led to the formation of less hydrocarbon pool species. This study not only show that waste can be transformed into value-added materials, namely a zeolite-based catalyst material, but also that the impurities present in the waste can further improve the catalytic performance of the solid catalyst.

Mass transfer of chemical absorption of CO2 in a serpentine minichannel

Based on the online method, the absorption kinetics and mass transfer of CO2 in potassium carbonate solution in a serpentine minichannel are studied. Three flow patterns are observed: slug-bubbly flow, slug flow, and slug flow with precipitation of SiO2. It is observed that the length of bubble decreases linearly with the increase of the travelling distance of bubble x in slug flow, based on which the volumetric mass transfer coefficient (kLa) is calculated. The results show that the average bubble velocity vb is only a function of the gas and liquid flow rates, and it remains unchanged in the whole flow process. Based on the online method, the instantaneous mass transfer coefficient, and instantaneous pH are studied. It’s found that the instantaneous mass transfer coefficients kLa and kL increase with the mass transfer of bubbles along the flow direction in the slug flow. It is concluded that the convection in the liquid slug plays a major role in controlling the mass transfer of fast chemical absorption in a serpentine minichannel.

A Combined Pyro- and Hydrometallurgical Approach to Recycle Pyrolyzed Lithium-Ion Battery Black Mass Part 2: Lithium Recovery from Li Enriched Slag—Thermodynamic Study, Kinetic Study, and Dry Digestion

Due to the increasing demand for battery raw materials, such as cobalt, nickel, manganese, and lithium, the extraction of these metals, not only from primary, but also from secondary sources, is becoming increasingly important. Spent lithium-ion batteries (LIBs) represent a potential source of raw materials. One possible approach for an optimized recovery of valuable metals from spent LIBs is a combined pyro- and hydrometallurgical process. The generation of mixed cobalt, nickel, and copper alloy and lithium slag as intermediate products in an electric arc furnace is investigated in part 1. Hydrometallurgical recovery of lithium from the Li slag is investigated in part 2 of this article. Kinetic study has shown that the leaching of slag in H2SO4 takes place according to the 3-dimensional diffusion model and the activation energy is 22–24 kJ/mol. Leaching of the silicon from slag is causing formation of gels, which complicates filtration and further recovery of lithium from solutions. The thermodynamic study presented in the work describes the reasons for the formation of gels and the possibilities of their prevention by SiO2 precipitation. Based on these findings, the Li slag was treated by the dry digestion (DD) method followed by dissolution in water. The silicon leaching efficiency was significantly reduced from 50% in the direct leaching experiment to 5% in the DD experiment followed by dissolution, while the high leaching efficiency of lithium was maintained. The study takes into account the preparation of solutions for the future trouble-free acquisition of marketable products from solutions.

Early Paleogene biosiliceous sedimentation in the Atlantic Ocean: Testing the inorganic origin hypothesis for Paleocene and Eocene chert and porcellanite

The widespread occurrence of lower Eocene chert and porcellanite has been viewed as a major paleoceanographic issue since the advent of ocean drilling, and both biotic and abiotic forcings have been proposed to explain it. We present a reconstruction of indurated siliceous sediment (ISS) and preserved biosiliceous sediment (PBS) occurrences in the Atlantic Ocean through the Paleocene and Eocene (~66 through 34 Ma). ISS and PBS distributions reveal dissimilar temporal trends, with the peak of ISS occurrences coinciding with the Early Eocene Climatic Optimum, in line with previous studies. PBS occurrences show a generally increasing trend culminating between 44 and 43 Ma. The common co-occurrence of ISS and PBS, and their coherent geographic distribution lends strong support to the biogenic origin of the precursor to the widespread Paleogene ISS, and argues against an inorganic mode of early Cenozoic chert and porcellanite precipitation. Weight per cent biogenic opal records and trends in linear sedimentation rates indicate two plausible modes of silicification: 1) silicification due to prolonged exposure of biogenic opal-rich sediments to corrosive bottom waters; and 2) silicification due to elevated pressures and temperatures caused by rapid burial of biogenic opal-rich deposits. The confinement of ISS and PBS to proximal sites along continental margins points to the reliance of siliceous sedimentation through the Paleocene and Eocene on terrestrial supply of dissolved silicon. Consistent with this, quantitative siliceous microfossil assemblage records from the Blake Nose in the NW Atlantic indicate that the nutrient-rich marginal rather than oligotrophic pelagic settings hosted the majority of siliceous plankton production through the early Paleogene.The inorganic SiO2 precipitation model is unlikely to have been the dominant mechanism responsible for ubiquitous occurrences of early Paleogene ISS. We favor the biogenic ISS precursor scenario and reconcile it with the low-productivity early Cenozoic oceans by showing that large volumes of biogenic silica were supplied to the western North Atlantic Ocean from the North American margin through the Paleocene and Eocene. Dissolution of this surplus silica was facilitated by an early southwestward flow of young, SiO2-depleted waters from the North Atlantic. All these factors contributed to ISS and PBS focusing in the western North Atlantic through the early Paleogene.

Water-rock interaction and mixing processes of complex urban groundwater flow system subject to intensive exploitation: The case of Mexico City

In complex aquifer systems subject to intensive exploitation it is important to investigate hydrogeochemical processes in the components to understand the hydrodynamics of groundwater. In this respect, understanding the hydrochemical mechanisms of water-rock interactions and mixing processes eventually leads to the development of appropriate strategies for a sustainable groundwater management. In this study, we analyze water-rock interactions processes of the so-called Anáhuac groundwater system underlying part of the Mexico Valley comprising Mexico City and its suburbs. This intensively exploited system has four flow components: 1) local flow, 2) intermediate flow, 3) cold regional flow, and 4) hot regional flow. This is studied using inverse geochemical models, which consider uncertainties of analytical data and constraints from thermodynamic stability diagrams, speciation-solubility models, and petrographic data. Three representative modeling sections were selected for the implementation of the mass-balance approach. The general conceptual model in two sections suggests that rainwater infiltrates the subsoil and begins to dissolve CO2 in the unsaturated zone; by reaching the saturated zone it reacts with silicate minerals of the host rock producing the final chemical composition of waters. On the other hand, the third section shows mixing as the main groundwater process, as well as water-rock interactions. In general, the identified processes of water-rock interaction are dissolution of CO2, dissolution of calcite, gypsum, and halite, Ca/Na ion-exchange, and weathering of silicate minerals such as biotite, muscovite, plagioclase, epidote and pyroxene, and precipitation of kaolinite, SiO2 and Fe (OH)3. Changes between local and intermediate flow components suggest dissolution of andesite rocks and precipitation of pyrite. Changes between local flow component and the cold regional component are explained by large flow trajectories. Transformations between local and hot regional components indicate mixing flows and a deep circulation influenced by the geothermal gradient. The combination of the methods used in this study can be applied in other similar geoenvironments of the world and assist local water authorities to adequately address and manage groundwater.

Investigation on the stability of copper modification of SAPO-34 catalysts in NH3-SCR reaction after hydrothermal aging

The influence of hydrothermal aging on the structural stability of Cu-modified SAPO-34 prepared by ion-exchange and impregnation methods was studied. XRD, Ar adsorption at −196 °C, solid-state NMR, UV–vis, H2-TPR, and EPR were used to probe the structural properties of the catalysts. It was found that the precipitation of crystalline AlPO4 and SiO2, the appearance of mesopores, the migration of silicon to form siliceous islands, and the formation of copper oxide crystallites occurred in aged catalysts. Furthermore, more siliceous islands and copper oxide crystallites were present in the aged samples prepared by the impregnation method than in those prepared by the ion-exchange method. Therefore, the impregnated catalysts show a much poorer structural stability than the ion-exchanged catalysts, which leads to more serious deterioration in the NH3-SCR reaction after hydrothermal aging.

Precipitation of multiple light elements to power Earth's early dynamo

Earth has had a global magnetic field for at least 3.5 billion years, but if the iron-alloy in the core has high conductivity, it is difficult to explain this duration with energy from cooling and inner-core growth alone. Precipitation of light elements (e.g., magnesium, silicon, and oxygen) from Earth's core is a potential alternative energy source to power the dynamo. We develop a new framework of coupled thermo-chemical evolution of the Earth to consider precipitation of multiple light components from the core and their interaction with the overlying mantle layer. The precipitated material accumulates in a layer at the base of the mantle which is then continuously eroded by mantle convection. We allow the precipitation of three species (MgO, FeO, and SiO2), including their interactions not considered by most previous studies. We find that MgO, SiO2, and FeO precipitation may each dominate entropy production depending on the choice of equilibrium constants and initial model states and that the three species together can explain the duration of Earth's magnetic field across a range of plausible scenarios. Over the Earth's history, we find that the core can lose ∼1–2 wt% silicon and oxygen suggesting that light precipitation is potentially an important process for the core compositional evolution and core-mantle chemical exchange. Additionally, our results show that precipitation does not, in most cases, have a systematic influence on the timing of inner-core nucleation or magnitude of the resulting paleomagnetic signal with inner-core nucleation typically around 550 Mya. However, the onset of precipitation of individual species could produce additional sharp increases in paleomagnetic intensity at various points through Earth's history besides the inner-core nucleation event.

Comparative studies on structural, magnetic and adsorptive properties of fused Fe2O3@ SiO2and rattle shapedSiO2@Fe2O3nanospheres with reversal of core-shell

Synthesis of core-shell nanocomposites is an important domain due to their applications as nanoadsorbents for heavy metal ions. In the present work, rattle type SiO2@Fe2O3 core-shell nanospheres were synthesized by surfactant assisted direct precipitation of SiO2 on Fe2O3 nanoparticles. Whereas, fused magnetic nanospheres encapsulating mesoporous silica cores i.e. Fe2O3@SiO2 were formed by sonication method. Different techniques viz. SEM-EDX, TEM, XRD, BET and VSM were used for the confirmation of structure, crystallinity, surface morphology and magnetic properties of nanospheres. Effect of pH, contact time, adsorbent dose and temperature on Cd(II) adsorption was evaluated. The experimental data was fitted using Langmuir, Freundlich and D-R isotherm models. Monolayer adsorption efficiency (qm) for Fe2O3@SiO2 (769.0 mgg−1) was higher than SiO2@Fe2O3 (370.0 mgg−1) nanospheres, signifying better adsorption of Fe2O3@SiO2. The results confirmed that the reversal of core-shell can play an important role to effectively tune the properties of Fe2O3–SiO2 nanospheres for efficient removal of heavy metal ions.

Interaction of FeII and Si under anoxic and reducing conditions: Structural characteristics of ferrous silicate co-precipitates

The interaction of FeII and Si is at the heart of many critical geochemical processes in diverse natural and engineered environments. The resulting FeII-silicate phases play important roles in regulating the concentrations and bioavailability of FeII and Si, as well as serve as sinks for trace and hazardous elements. Therefore, a detailed understanding of their structural characteristics and the underlying formation mechanisms may provide insights useful to predicting their reactivity and stability under different conditions. In this work, co-precipitates with different Si/FeII ratios (0.5, 1.0 and 2.0) were synthesized under anoxic and reducing conditions at different solution pH (7, 9 and 11). Thermodynamic calculations from solution chemistry data were carried out and co-precipitates were studied using X-ray diffraction (XRD), infrared (IR) spectroscopy and Fe K-edge extended X-ray absorption fine structure spectroscopy (EXAFS). Thermodynamic calculations predict the formation of phyllosilicate phases such as greenalite in all the samples. Solid characterization data, however, reveal significant structural variabilities and phase heterogeneity. Incipient phyllosilicate structures tend to be more pronounced in samples from pH 9 and 11, while at neutral pH conditions, polymeric silicate phases like amorphous SiO2 become predominant. These variabilities are possibly linked to heterogeneous formation processes arising from relative solubility differences between SiO2 and Fe(OH)2. High pH (>8) conditions favor the polymerization of Fe(OH)2 layers which likely serve as templates for layered silicate formation, while neutral pH conditions favor the precipitation of polymeric silicate phases like amorphous SiO2 to which aqueous Fe species may adsorb. In samples from Si/FeII = 0.5, relatively well developed Fe(OH)2 layers were identified from the EXAFS data. Increasing Si/FeII ratios lead to amorphous SiO2 precipitation and inhibition of Fe(OH)2 polymerization, resulting in smaller phyllosilicate domains embedded in a polymeric SiO2 matrix. The results of this work may be useful in interpreting structural variabilities of FeII-Si phases observed both in nature and in engineered environments. Such variabilities may influence subsequent phase recrystallization processes as well as reactivity towards environmentally relevant elements such as radionuclides.

Understanding the Formation and Evolution of Oxide Inclusions in Si-Deoxidized Spring Steel

To illustrate the mechanism of the formation and evolution of inclusions in Si-deoxidized spring steel, pilot trials and systematic samplings were conducted, and detailed information about the inclusion density, morphology and composition was elucidated by electron probe X-ray microanalysis. The steel samples were collected from the vacuum degassing furnace, tundish furnace and spring rods after hot rolling. Based on the relationship between the mass ratio of $$ X_{{\left( {\text{CaO}} \right)}} /X_{{\left( {{\text{SiO}}_{ 2} } \right)}} $$ and diameters of inclusions, CaO-SiO2 inclusions were found to be formed by the coalescence between SiO2 and CaO-SiO2 inclusions. Most CaO-SiO2 inclusions > 10 μm were located in the melilite areas with liquidus temperatures < 1400 °C. The Al2O3-SiO2-CaO inclusions in the steel samples from the vacuum degassing and tundish furnaces were considered to be formed by reduction of CaO-SiO2 inclusions by Al dissolved in steel. The increased number density of Al2O3-SiO2-CaO inclusions in spring rods was primarily attributed to the entrapment of Na2O-contained mould flux particles and inclusions crushed after hot rolling. Many Al2O3-SiO2-MnO and SiO2-MnO inclusions were observed in tundish furnace samples, which originated from inherent reactions between [Al], [Si], [Mn] and [O] in the molten steel. The SiO2 precipitation of Al2O3-SiO2-MnO inclusions was attributed to the solubility decrease during the solidification process with temperature drop.