Magnetite, redox reactions

Few comparative studies have been made on the reductive dissolution of different mineral phases. In one such study, the order of reaction with seven organic and transition metal reductants was found to be the same hematite (a-Fe203)>magnetite (FejO,/,)>nickel ferrite (NiFe204) (43). Magnetite is an interesting case, since both Fe(III) and Fe(II) are present in the lattice prior to reaction. Evidence indicates that Fe(IIl) sites reduced to Fe(II) sites by redox reaction dissolve more quickly than Fe(II) sites originally present in the mineral lattice (6). 

Redox Reactions and Valence States. The proposed reduction of Tc and U to the tetravalent state is indirectly indicated from the distribution measurements in non-oxidizing systems (c.f. Figure 2 and Table VII). By the addition of 10-20 mg/1 of Fe (c.f. Table II) a drastic increase of the distribution coefficient was observed both for Tc and U. Minerals like magnetite and chlorite also seem to have some reducing effect even after a short contact time. 

Redox reactions are of importance in the dissolution of Fe-bearing minerals. Reductive dissolution of Fe(III)(hydr)oxides can be accomplished with many reductants especially organic and inorganic reductants, such as ascorbate, phenols, dithionite, and HS. Fe(II) in the presence of complex formers can readily dissolve Fe(III)(hydr)oxides. The Fe(II) bound in magnetite and silicate and adsorbed to oxides can reduce O2 (White, 1990 White and Yee, 1985). 

Peterson, M.L., Brown, G.E., and Parks. G.A., Direct XAFS evidence for heterogeneous redox reaction at the aqueous chromium/magnetite interface, Colloids Surf. A, 107, n, 1996. 

High levels of chelant or oxygen affect the redox tendencies of iron-oxygen reactions and permit the liberation of Fe2+ ions (corrosion) from a metal surface and their subsequent chelation, thus preventing the formation or repair of blanketing ferric oxides, hydroxides, or a passivated magnetite film. 

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

Fig. 2.33. H2Saq concentration.s as a function of temperature for hot spring fluids at midocean ridges as a function of redox. Assuming AMPC (anhydrite-magnetite-pyrite-calcite) and PPM (pyrite-pyrrhotite) buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations indicate temperatures of approximately 370-385°C. Solid star Okinawa. (Modified after Seyfried and Ding, 1995.)...

Zhao et al. prepared magnetite (FesO nanoparticles modified with electroactive Prussian Blue [44]. These modified NPs were drop-cast onto glassy-carbon electrodes. They observed the redox processes commonly observed for PB (similar to that seen in Figure 4.8), and also demonstrated that the Prussian White material produced by PB reduction at 0.2 V served as an electrocatalyst for Fi202 reduction. They also prepared LbL films in which PB NPs and glucose oxidase were alternated between PD DA layers [99]. These were demonstrated to act as electrocatalysts for Fi202 reduction. Based on the ability to sense the product of the enzymatic reaction, these structures were shown to act as glucose sensors. 

Lapteva (1958) experimentally determined the redox potential E ) in the neutralization of a mixture of Fe and Fe sulfates by an alkali—at a pH above 7 a black magnetite deposit was formed according to the reaction ... 

In the proposed two-step process, it is important to attain high efficiencies for conversion of coal to COx (CO -I- CO2) in the first-step reaction and then for conversion of CO2 to CO in the second-step reaction by an external heat input. From the thermodynamic conditions and the low cost, the redox pair of Fe304/a-Fe was one of the promising redox systems for the two-step process, but it still required the operating temperature above 1200°C[2]. It is well known that many kinds of metal ions can be incorporated into the spinel lattice structure of magnetite by replacing ferrous or ferric ions. There is the possibility that metal-substitution for Fe or Fe " in magnetite causes a phase transition to the metallic phase, which proceeds readily even at low temperatures and improves the conversion efficiencies of coal and CO2 to CO in the two-step process. 

Surface passivating layers are further discussed by White and Peterson (55) in the oxidation of magnetite by Cr and by Fendorf (29) in the reduction of pyrolusite by Co -EDTA complexes. In both cases, surface passivation removes the bulk mineral from further reaction with the solution and significantly reduces the redox capacity of the mineral. 

Mossbauer spectroscopy in solid state chemistry under in situ conditions at high temperatures and at defined oxygen partial pressures have been made by Becker (2001) for order-disorder processes and their kinetics in nickel alu-minate spinel and magnetite. The study has also been carried out for heterogeneous solid olid and solid-gas reactions which relate to the formation of cobalt aluminate spinel and to redox processes in fayalite, Fe2Si04, respectively. The diffusion of the iron cations in Fc2Si04 has also been observed by means of Mossbauer spectroscopy. 

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