Reductive Dissolution Of Magnetite Research Articles

Evaluation of reaction spontaneity for acidic and reductive dissolutions of corrosion metal oxides using HyBRID chemical decontamination

Complete sets of reaction mechanisms are proposed in the acidic and reductive dissolution of magnetite, nickel ferrite, and chromite using the HyBRID (Hydrazine Based Reductive metal Ion Decontamination) process for the decontamination of a primary coolant system of nuclear power plant. Hydrazine participated in the reaction pathway of reducing ferric ions to ferrous ions and simultaneously regenerating oxidized cupric ions into cuprous ions. The data of the heat capacity, the heat of formation, the entropy of formation, and the Gibbs energy of formation for all chemical species and ions were collected from the HSC Chemistry 9 database. The enthalpy, entropy, and Gibbs energy changes of reactions were calculated in the temperature range of 298.15–373.15 K for individual reactions. The degree of spontaneity decreased with the increase of the temperature. The reaction spontaneity was significantly enhanced by addition of hydrazine and slightly increased by further addition of copper sulfate.

Reductive dissolution of magnetite and jarosite by Acidiphilium cryptum JF-5

Nowadays there is an increasing interest for applying iron reducing bacteria to hydrometallurgical processes. Although most isolated bacteria grow at circumneutral pH, Acidiphilium cryptum is able to reduce iron oxides in acidic environments and promote its dissolution.In this study, the reductive dissolution of magnetite and jarosite by A. cryptum JF-5 was tested in two culture media, over a period of 38days with 1% pulp density. In a medium without oxalic acid, bacteria were crucial to increase the magnetite and jarosite dissolution from 0.2 and 0.3% to 20.4 and 2.5%, respectively. In a medium containing oxalic acid, the abiotic dissolution of magnetite and jarosite (1.6 and 2.0%, respectively) was clearly surpassed in the inoculated tests (to 26.9 and 10.4%, respectively).The pH control was essential to attain a significant degree of magnetite dissolution. A stoichiometric ratio between sulfuric acid consumed and iron dissolved was obtained for both solids tested. Finally, magnetic stirring tests pointed out that the dissolution process was kinetically controlled by the solid surface.

Influence of seawater exchanges across the Bab-el-Mandeb Strait on sedimentation in the Southern Red Sea during the last 60 ka

[1] The location of core MD92-1008 close to the southern edge of the Red Sea is ideal to study the evolution of seawater exchanges with the Northern Indian Ocean through the Bab-el-Mandeb narrow strait. The present study was aimed at documenting the paleoceanographic evolution of this area over the past 60 ka using high-resolution magnetic, sedimentological, and geochemical indicators. Two modes of variability dominate the records: (i) long-term, glacial-interglacial variations and (ii) rapid, millennial scale variability (Dansgaard-Oeschger type) during the last glacial period. Changes in magnetic concentration were documented from natural and laboratory magnetizations after proper normalization. They are inversely correlated to the total organic carbon (TOC) content, pointing out the key role played by reductive dissolution of magnetite on the evolution of magnetic concentration. Based on the temporal evolution of TOC, CaCO3, and planktonic δ13C, we suggest that past changes in the organic matter content (i) were closely linked to glacio-eustatic variations which modulated long-term seawater exchanges and nutrient supplies to the Red Sea through the narrow Bab-el-Mandeb Strait and (ii) reflect millennial-scale changes in productivity driven by monsoon wind intensity which controlled the amount of nutrient-rich intermediate waters upwelled in the Gulf of Aden during the summer season and advected into the Southern Red Sea. The sea level rise following the onset of deglaciation generated a rapid flush of detrital material that was accumulated on the continental margins and on the previously emerged zones of the Bab-el-Mandeb area.

Mechanisms of iron oxide transformations in hydrothermal systems

Coexistence of magnetite and hematite in hydrothermal systems has often been used to constrain the redox potential of fluids, assuming that the redox equilibrium is attained among all minerals and aqueous species. However, as temperature decreases, disequilibrium mineral assemblages may occur due to the slow kinetics of reaction involving the minerals and fluids. In this study, we conducted a series of experiments in which hematite or magnetite was reacted with an acidic solution under H 2-rich hydrothermal conditions ( T = 100–250 °C, P H 2 = 0.05 – 5 MPa ) to investigate the kinetics of redox and non-redox transformations between hematite and magnetite, and the mechanisms of iron oxide transformation under hydrothermal conditions. The formation of euhedral crystals of hematite in 150 and 200 °C experiments, in which magnetite was used as the starting material, indicates that non-redox transformation of magnetite to hematite occurred within 24 h. The chemical composition of the experimental solutions was controlled by the non-redox transformation between magnetite and hematite throughout the experiments. While solution compositions were controlled by the non-redox transformation in the first 3 days in a 250 °C experiment, reductive dissolution of magnetite became important after 5 days and affected the solution chemistry. At 100 °C, the presence of maghemite was indicated in the first 7 days. Based on these results, equilibrium constants of non-redox transformation between magnetite and hematite and those of non-redox transformation between magnetite and maghemite were calculated. Our results suggest that the redox transformation of hematite to magnetite occurs in the following steps: (1) reductive dissolution of hematite to Fe (aq) 2 + and (2) non-redox transformation of hematite and Fe (aq) 2 + to magnetite.

A late Miocene–early Pliocene Antarctic deepwater record of repeated iron reduction events

In this study we present a late Miocene–early Pliocene record of sixty-four zones with prominent losses in the magnetic susceptibility signal, taken on a sediment drift (ODP Site 1095) on the Pacific continental rise of the West Antarctic Peninsula. The zones are comparable in shape and magnitude and occur commonly at glacial-to-interglacial transitions. High resolution records of organic matter, magnetic susceptibility and clay mineral composition from early Pliocene intervals demonstrate that neither dilution effects nor provenance changes of the sediments have caused the magnetic susceptibility losses. Instead, reductive dissolution of magnetite under suboxic conditions seems to be the most likely explanation. We propose that during the deglaciation exceptionally high organic fluxes in combination with weak bottom water currents and prominent sediment draping diatom ooze layers produced temporary suboxic conditions in the uppermost sediments. It is remarkable that synsedimentary suboxic conditions can be observed in one of the best ventilated open ocean regions of the World.

Relative paleointensity stack during the last 250 kyr in the northwest Pacific

We have conducted a paleomagnetic study of 10 sediment cores recovered from the northwest Pacific. The purpose is to establish a regional paleointensity stack during the last 250 kyrs (NOPAPIS‐250) for global coverage of paleointensity data as well as a regional reference for paleointensity‐assisted stratigraphy. In the studied cores, reductive dissolution of magnetite appears to have occurred mainly in glacial periods. Such horizons accompany large decreases in S‐ratio (S−0.3T = (1 − IRM−0.3T/SIRM)/2) as low as 0.90. The sediments are also intercalated by frequent volcanic ash layers. Prerequisites for reliable relative paleointensity estimation are not satisfied at these horizons. We rejected unsuitable data by quantitative criteria based on S‐ratio and saturation isothermal remanent magnetization (SIRM) and constructed the NOPAPIS‐250 curve from the remaining data. In the construction of the curve, we chose IRM as a normalizer because acquisitions of anhysteretic remanent magnetization (ARM) are considered to be influenced by magnetic interactions among magnetic grains in the northwest Pacific sediments. Relative intensity variation of the NOPAPIS‐250 gives a standard deviation that is 33% of the average, if the normal distribution is assumed. The NOPAPIS‐250 curve generally shows similar variation patterns compared with other high‐resolution records reported from other regions: many paleointensity lows are recognized in common. Six out of the nine lows can be correlated with reported excursions in literature.

Reductive dissolution of magnetite in ethylenediaminedisuccinic acid solutions

Ethylenediaminedisuccinic acid (EDDS) was used to dissolve magnetite reductively. Fe 2+ ions increased the dissolution rate, and this increase depended on the solution acidity, temperature and [EDDS]:[Fe 2+] ratios. The EDDS-Fe 2+ complex was able to dissolve magnetite film coated on an iron surface. Iron metal enhanced the dissolution of magnetite from its surface. However, metal exposed to the solution suffered more corrosion than iron free of magnetite.

Dissolution of magnetite coupled galvanically with iron in environmentally friendly chelant solutions

The galvanic coupling between magnetite and iron in ethylenediaminedisuccinic acid (EDDS) solutions both with and without added iron(II) ions has been studied using electrochemical methods. The galvanic coupling accelerates the corrosion of iron due to the small shift in its potential in the anodic direction. At the same time, the potential of magnetite is cathodically polarized away from the potential range where the only faradaic process—the reductive dissolution of magnetite—takes place and results in a considerable decrease in its dissolution. Magnetite dissolves faster at the galvanic potential when [EDDS] ⩾ [Fe 2+] whereas iron is affected to a much lesser extent. The ratio between the rates of dissolution of magnetite at the galvanic potential to that at its steady state potential tends to decrease at higher temperatures and at higher EDDS and Fe 2+ concentrations. This study shows that temperature plays a decisive role in the dissolution of magnetite coupled to iron. To completely remove it from the iron surface, high temperatures should be used. Corrosion inhibitors have to be considered when high losses of the base metal cannot be tolerated.

Using magnetic susceptibility to facilitate more rapid, reproducible and precise delineation of hydric soils in the midwestern USA

Standard field indicators, currently used for hydric soil delineations [USDA-NRCS, 1998. Field indicators of hydric soils in the United States, Version 4.0. In: G.W. Hurt et al. (Ed.), United States Department of Agriculture-NRCS, Fort Worth, TX], are useful, but in some cases, they can be subjective, difficult to recognize, or time consuming to assess. Magnetic susceptibility (MS) measurements, acquired rapidly in the field with a portable meter, have great potential to help soil scientists delineate and map areas of hydric soils more precisely and objectively. At five sites in Illinois (from 5 to 15 ha in area) with contrasting soil types and glacial histories, the MS values of surface soils were measured along transects, and afterwards mapped and contoured. The MS values were found to be consistently higher in well-drained soils and lower in hydric soils, reflecting anaerobic deterioration of both detrital magnetite and soil-formed ferrimagnetics. At each site, volumetric MS values were statistically compared to field indicators to determine a critical MS value for hydric soil delineation. Such critical values range between 22×10 −5 and 33×10 −5 SI in silty loessal or alluvial soils in Illinois, but are as high as 61×10 −5 SI at a site with fine sandy soil. A higher magnetite content and slower dissolution rate in sandy soils may explain the difference. Among sites with silty parent material, the lowest critical value (22×10 −5 SI) occurs in soil with low pH (4.5–5.5) since acidic conditions are less favorable to ferrimagnetic mineral neoformation and enhance magnetite dissolution. Because of their sensitivity to parent material properties and soil pH, critical MS values must be determined on a site specific basis. The MS of studied soil samples (0–5 cm depth) is mainly controlled by neoformed ultrafine ferrimagnetics and detrital magnetite concentrations, with a minor contribution from anthropogenic fly ash. Neoformed ferrimagnetics are present in all samples but, based on high χ FD% (∼5% to 10%), are most prevalent in high pH Mollisols of northeastern Illinois. Scanning electron microscope images display significantly more detrital magnetite alteration in hydric soils, substantiating that reductive dissolution of magnetite (aided by microorganisms) is a primary cause for lower MS. Fly ash comprises 8–50% of the >5 μm strongly magnetic particles and typically accounts for 5–15% of the total MS signal. The proportion of fly ash in >5 μm strongly magnetic fractions is greater in hydric soils because of lower natural magnetite contents, possibly combined with historical topsoil accumulation in lower landscapes. Magnetic fly ash particles are also more altered in low MS soils, implying that significant magnetite dissolution can occur in less than 150 years.

Electrochemical Investigation of Magnetite Reductive Dissolution in Aqueous Solutions

Abstract The electrochemical behavior of a single-crystal magnetite (Fe3O4) was investigated in perchloric acid (HClO4) at different pH values using linear sweep voltammetry. The logarithm of peak current density of the reductive dissolution of magnetite decreased linearly with the increased solution pH. The effect of applied potential on the reductive dissolution in HClO4 was studied using electrochemical impedance spectroscopy. The results were analyzed in terms of a suitable equivalent circuit. The rate of dissolution and the double-layer capacitance were not linearly related with potential. Impedance measurements were also carried out in solutions containing oxalic acid (HOOCCOOH) and various amounts of ferrous ions. Magnetite dissolves reductively in this solution. The rate of dissolution was proportional to the ferrous ion concentration. The rate of dissolution and the double-layer capacitance were higher than those observed in HClO4.

Reductive dissolution of magnetite by solutions containing EDTA and Fe II

The dissolution of magnetite particles in solutions containing EDTA and Fe II was studied as a function of the total concentration of EDTA and Fe II; the influence of pH was also studied. The rate shows a Langmuir-type dependence on [FeY 2−] when [EDTA] 0 ⩽ [Fe II] 0. At constant [EDTA] 0, in the range where EDTA is in excess over Fe II, the kinetic order on Fe II is one; Fe II in large excess has no influence on the rate. At constant [Fe II] 0, the rate of dissolution is maximum when the ratio of [EDTA] 0 to [Fe II] 0 is close to 3. These results are interpreted in terms of fast solution and surface complexation processes followed by slow heterogeneous electron transfer from adsorbed FeY 2− to surface >Fe III centers and fast phase transfer of >Fe II. The inhibitory effect of excess EDTA results from competitive adsorption of FeY 2− and EDTA. The rate increases with decreasing pH up to pH 3.1; at this value a maximum is achieved. The pH dependence of rate is the resultant of several factors, the most important being the influence of pH on the adsorption preequilibrium and the need for adsorbed protons adjacent to the reactive site. The stoichiometry of the dissolution reaction is not constant and the ratio of protons consumed to iron released is sensitive to experimental conditions. In the fatest reactions, this ratio is appreciably lower than the limiting value corresponding to the release of unhydrolyzed Fe III species. The implications of this result are discussed.

A comparative study of the oxidative and reductive dissolution of magnetite in acidified CuSO4-acetonitrile-H2O and CuCl2?NaCl?H2O leach solutions

The oxidative and reductive dissolution of magnetite in acidified aqueous acetonitrile and acidified aqueous NaCl solutions have been investigated by electrochemical methods and correlated with the proton activity in these fundamentally different solution systems. The results are compared with the rates of magnetite dissolution in the presence of copper(II) and copper(I). Under anodic or oxidizing leach conditions magnetite forms a passive Fe2O3 film which inhibits reaction, but under cathodic or reducing leach conditions in the presence of copper(I), magnetite is believed to dissolve via (FeOH)+ intermediate and reacts up to 600 times faster. The rate of dissolution of magnetite depends on its potential and the activity of the proton. The rate follows a Butler-Volmer relationship with a Tafel slope of 120–130mV per decade and exhibits a proton reaction order between 0.85 and 1.0. After allowing for changes in proton activity there is no significant difference between the rate of dissolution of magnetite in the sulphate and chloride leach solutions and no discernable effect of the presence of acetonitrile in the mixed solvent system. Electrochemical studies and leaching studies in which the potential is controlled by the Cu(II)−Cu(I) couple are in good agreement and lead to a fundamental understanding of the optimum conditions required to leach copper selectively from a Cu−Fe3O4 calcine derived from the segregation roasting of chalcopyrite concentrates.

Acidic and reductive dissolution of magnetite in aqueous sulfuric acid: Site-binding model and experimental results

The open circuit dissolution of ionic metal oxides in mineral acids is modelled assuming that the rate is controlled by the transfer of metal ions in hydrolytic equilibrium with bulk metal ions, from the metal oxide surface to the Stern plane. The site-binding model of the double layer metal oxide/electrolyte solution is used to obtain the pH dependence of surface and Stern potentials. The nature of the active sites is discussed and their surface concentration is assumed to be proportional to suface charge σ 0. Again, the site-binding model is used to detemine the pH dependence of σ 0. It is thus shown that the rate order in c H + is essentially defined by the potential dependence of the charge transfer process, for oxides with points of zero charge near neutrality that dissolve in mildly or strongly acidic solutions. The role of surface complexation is also discussed in terms of the site-binding model and the difficulties in interpreting dissolution experiments under constant external applied potential are discussed in terms of the complexity of the semiconductor oxide/electrolyte solution interfacial region in magnetite. An experimental study of the open circuit dissolution of magnetite in sulfuric acid is presented and interpreted according to the proposed model. The reductive dissolution of magnetite is modelled by extension of the Valverde-Wagner model of oxide dissolution. Experimental results are presented to demonstrate that the reductive dissolution rate of magnetite in ferrous containing solutions is controlled by the rate of electron transfer from adsorbed Fe(II) to Fe(III) surface states of magnetite.