Hematite a-Fe

A reaction that involves chemical species in more than one phase is termed heterogeneous.1 An example is the composite reaction describing the reductive dissolution of the common soil mineral hematite (a-Fe ) in the presence of visible light by oxalic acid (H2C204), a ubiquitous plant litter degradation product ... 

These concepts are illustrated in Fig. 3.10 for the reductive dissolution of hematite (a-Fe,03) in the presence of ascorbic acid at pH 3.26 In this example, Mox = Fe(III), MRed = Fe(II), and LRed = HA, where A2 is the ascorbate anion (log K = -4 for the dissociation of H2A°, but dissociation is invoked nonetheless to promote a ligand-exchange reaction). Equation 3.46 becomes... 

Bicrystalline nanowires of hematite (a-Fe,03) have been synthesized by the oxidation of pure Fe.207 Single-crystalline hexagonal u-Fe20, nanorods and nanobeits can be prepared by a simple iron-water reaction at 673 K.20 Mesoporous quasi-single crystalline... 

Morris, R.V., Fauer, H.V. Jr., Fawson, C.A., Gibson, E.K. Jr., Nace, G.A. and Stewart C., 1985. Spectral and other physicochemical properties of submicron powders of hematite (a-Fe Oj), maghemite (y-Fe Oj), magnetite (FCjO, ), goethite (a-FeOOH), and lepidocrocite (y-FeOOH). Journal of Geophysical Research, 90 3126-3144. 

Lian, J., Duan, X., Ma, J., Peng, R, Kim, T, and Zheng, W. Hematite (a-Fe Oj) with Various Morphologies Ionic Liquid-Assisted Synthesis, Formation Mechanism, and Properties. C5Va o, 3(11), 3749-3761 (2009). 

Yanyan X., Shuang Y, Guoying Z., Yaqiu S., Dongzhao G. and Yuxiu S. (2011). Uniform hematite a-Fe Oj nanoparticles Morphology, size-controlled hydrothermal synthesis and formation mechanism. Mater Lett, 65( 12), 1911 -1914. 

Figure 16, 293 K (lop) and 8 K (botiom) Mossbauer spectra of the aerosol sample the decomposition into the spectral components is indicated by the stick bars, where A stands for magnetite (Fe Oj). B for hematite (a-Fe.Oj), C for goethite (st-FeOOH). D for Fe" -, E and F for Fe -contpounds...

Iron Oxide Reds. From a chemical point of view, red iron oxides are based on the stmcture of hematite, a-Fe202, and can be prepared in various shades, from orange through pure red to violet. Different shades are controlled primarily by the oxide s particle si2e, shape, and surface properties. Production. Four methods are commercially used in the preparation of iron oxide reds two-stage calcination of FeS047H2 O precipitation from an aqueous solution thermal dehydration of yellow goethite, a-FeO(OH) and oxidation of synthetic black oxide, Fe O. ... 

Write a balanced reaction for formation of hematite from Fe. ... 

Fig. 3.16 Schematic drawing of the MIMOS II Mossbauer spectrometer. The position of the loudspeaker type velocity transducer to which both the reference and main Co/Rh sources are attached is shown. The room temperature transmission spectrum for a prototype internal reference standard shows the peaks corresponding to hematite (a-Fe203), a-Fe, and magnetite (Fe304). The internal reference standards for MIMOS II flight units are hematite, magnetite, and metallic iron. The backscatter spectrum for magnetite (from the external CCT (Compositional Calibration Target) on the rover) is also shown...

Figure 6.6 In situ XRD of an alumina-supported iron catalyst during reduction in H2 at 675 K reveals the transition of a-Fe202 (hematite) via Fe(04 (magnetite) to metallic iron as a function of time. The graph shows the degree of reduction of supported and unsupported oc-Fe203 as determined from the XRD measurements (from Jung and Thomson f 14]).

Dependence of the dissolution rate of hematite, a-Fe203 (mol m 2 h"1) on the surface complex of the HC03-Fe(III) complex (mol rrr2). 

Rate of the photochemical reductive dissolution of hematite, = d[Fe(II)]/dt, in the presence of oxalate as a function of the wavelength at constant incident light intensity (I0 = 1000 peinsteins "1 lr1). The hematite suspensions were deaerated initial oxalate concentration = 3.3 mM pH = 3. (In order to keep the rate of the thermal dissolution constant, a high enough concentration or iron(II), [Fe2+] = 0.15 mM, was added to the suspensions from the beginning. Thus, the rates correspond to dissolution rates due to the surface photoredox process). 

Since the substrate hematite contains Fe-57, a certain Mossbauer self-absorption is inevitable in the case of measurements on Co-57. But, the effect of the absorption is considered to be not important as far as the pH dependence of the spectra is concerned, since the amount of hematite was kept constant and the adsorbed divalent Co-57 was dispersed uniformly in it. 

Similar photo-induced reductive dissolution to that reported for lepidocrocite in the presence of citric acid has been observed for hematite (a-Fe203) in the presence of S(IV) oxyanions (42) (see Figure 3). As shown in the conceptual model of Faust and Hoffmann (42) in Figure 4, two major pathways may lead to the production of Fe(II)ag i) surface redox reactions, both photochemical and thermal (dark), involving Fe(III)-S(IV) surface complexes (reactions 3 and 4 in Figure 4), and ii) aqueous phase photochemical and thermal redox reactions (reactions 11 and 12 in Figure 4). However, the rate of hematite dissolution (reaction 5) limits the rate at which Fe(II)aq may be produced by aqueous phase pathways (reactions 11 and 12) by limiting the availability of Fe(III)aq for such reactions. The rate of total aqueous iron production (d[Fe(aq)]T/dt = d [Fe(III)aq] +... 

Figure 3. Fe(III) coordinative environment in Fe OH Caq.) and on a hematite (a-Fe202) surface (adapted from 48).

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). 

Figure 3. Plot of 10 1nP56/54 values versus 10 /7 (K) for (A) Fe minerals calculated from Polyakov and Mineev (2000), and for (B) aqueous Fe species calculated from Schauble et al. (2001). Temperature scale in °C shown at top. Mineral abbreviations are Py pyrite, Mt magnetite, Celad celadonite, Hem hematite, Goe goethite, Lepid lepidocrocite, 01 olivine, Sid siderite, Ank-2 ankerite (Cai iMg ,jFe ,3Mn ,i(C03)2), Ank-1 ankerite (CaiMgo,5Feo,5(C03)2).

Welch et al. (2003). This produces a temperature dependence that is similar to that calculated by Schauble et al. (2001), but is displaced to Fe(III)aq-Fe(II)aq fractionations that are approximately half that predicted by Schauble et al. (2001). This discrepancy lies outside the estimated uncertainties for the predicted and measured Fe(III)aq-Fe(II)aq fractionations (Fig. 5). Based on comparison of measured Fe(III)a,-hematite fractionations with those predicted from spectroscopic data, Skulan et al. (2002) noted that the predicted Fe(III)aq-hematite and Fe(III)jq-Fe(II)jq fractionations may be brought into closer agreement with the measured Fe(III)jq-hematite and Fe(III)aq-Fe(II)aq fractionations if the P56/54 factor for [Fe (H20)6] + from Schauble et al. (2001) is reduced by l-2%o at low temperatures, and this suggestion has been supported by recent ah inito calculations of the P5J/54 factor for [Fe (H20)4] +by Anbar et al. (2004 Fig. 5). 

Figure 7. Measured and corrected A Fe Fe(ni)-Hem.tite values ( and O, respectively) relative to average hematite precipitation rate for Experiments 5, 7, and 8 of Skulan et al. (2002). The A Fe jj(ni).Hem.tite values are defined as those measured at the termination of the experiments the corrected A Fe i,e(ni).Hem.tite values reflect the estimated correction required to remove any residual kinetic isotope fractionation that was produced early in experiments that was not completely removed hy dissolution and re-precipitation over the long term. Extrapolation of the corrected A Fe jj(ni).Hem.tite values to zero precipitation rates yields an estimate for the equilihrium Fe(III),q-hematite fractionation, A Fci,e(in).hem.tite,...

Fig. 2.11 Structure of hematite, a) Hexagonal lined, c) Arrangement of octahedra. Note their close packing of oxygens with cations distributed face-sharing, d) Ball-and-stick model. Unit cell in the octahedral interstices. Unit cell outlined. outlined, e) 03-Fe-03-Fe-03 triplets, (a, b Eggle-b) View down the c-axis showing the distribution ton et al., 1988 with permission c, d Stanjek, of Fe ions over a given oxygen layer and the hexa- unpubl. e Stanjek, 1991 with permission) gonal arrangement of octahedra. Unit cell out-...

Fig. 7.4 Top Nuclear energy levels of Fe as shifted by electrical monopole (left), or as split by electrical quadrupole (center) or by magnetic dipole interaction (right), schematized for hematite at room temperature (5 > 0 vs. a-Fe, EQ < 0, Bhf 0). Bottom Schematic Mossbauer spectra corresponding to the energy levels schematized on top (J. FriedI, unpubl.).

Fig. 12.25 Di ssolution features of hematites Upper Undissolved (a) and partly dissolved (b c) synthetic Al-hematite (AI/(Fe+AI) = 0.094 mol moT j in dithionite/citrate/bicarbonate at25 (Araki Sch A/ertmann unpubl.), Lo A/er Undissolved and partly dissolved hematite from a redoxomorphic subsoil of a typical Hapludalf on Permian mudstone, Ohio (Bigham et al., 1991, A/ith permission).

Fig. 15.8 Well ciystalline euhedral platy hematite (a), poorly crystalline spherical Si-containing hematite together with ring-like layer Fe-silicates ( ) (b), and akaganeite (c) from the Atlantis Deep, Red Sea, (Photo H.-Ch. Bartscherer) (Schwertmann etal., unpubl.)...

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