Hematite dissolution

Bruno, J., W. Stumm, P. Wersin, and F. Brandberg (1991), The Influence of Carbonate in Mineral Dissolution. Part 1, The Thermodynamics and Kinetics of Hematite Dissolution in Bicarbonate Solution at T = 25° C", in preparation. 

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] +... 

There are only a few cases where the dissolution of an iron oxide by all three types of processes under comparable conditions has been investigated. Banwart et al. (1989) found that at pH 3, the rate of dissolution of hematite increased in the order, protonation < complexation < reduction with a factor of 350 between the extremes. A similar factor (400) was found for goethite (Zinder et al, 1986) (Fig. 12.15). Hematite dissolution processes were also compared in the pH range similar to that found in neutral environments (Fig. 12.16). Again, dissolution by simple protonation was extremely slow, whereas reduction, especially when aided by Fe complexing ligands, was particularly effective (Banwart et al, 1989). It can, thus, be concluded that reduction, particularly when assisted by protonation and complexation will be the main mechanism for Fe transport in global ecosystems. 

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron. Surface Sd. 68 459—468 Bruno, J. Sturam, J.A. Wersin, P. Brand-berg, E. (1992) On the influence of carbonate on mineral dissolutions I. The thermodynamics and kinetics of hematite dissolution in bicarbonate solutions at T = 25°C. Geo-chim. Cosmochim. Acta 56 1139—1147 Brusic.V. (1979) Ferrous passivation. In Corrosion Chemistry, 153—184 Bruun Hansen, H.C. Raben-Lange, R. Rau-lund-Rasmussen, K. Borggaard, O.K. 

Bruno, J., Stumm, W., Wersin, P. Brandberg, F. 1992. On the influence of carbonate in mineral dissolution 1. The thermodynamics and kinetics of hematite dissolution in bicarbonate solutions at T = 25 °C. Geochimica et Cosmochimica Acta, 56, 1139-1147. 

Hummel, W. 2000. Comment on On the influence of carbonate in mineral dissolution 1. The thermodynamics and kinetics of hematite dissolution in... 

A schematic of the various elementary steps involved in the surface photoredox reaction leading to hematite dissolution are shown below in Scheme 8. 

Figure 5. Relative rate of the light-induced reductive dissolution of hematite in the presence of oxalate as a function of the wavelength. Experimental conditions 0,5 gL 1 hematite initial oxalate concentration 3.3 m mol L 1 pH = 3.0 nitrogen atmosphere. The relative rate is the rate of hematite dissolution at constant incident light intensity. Under the assumption that the light intensity, absorbed by the oscillator that enables the photoredox reaction, corresponds to the incident light intensity, IAX = Iox, the relative rate equals the quantum yield, of dissolved iron(II) formation. As...

Johnsson, P.A. 1994. Hematite dissolution in natural organic acids. Ph.D. diss. Stanford Univ, Stanford, CA. 

In an example of a fixed fugacity path we model the dissolution of pyrite (FeS2) at 25 °C. We start in REACT with a hypothetical water in equilibrium with hematite (Fe203) and oxygen in the atmosphere... 

The evolved hardpans (Fig. 1b) are composed by rhythmic alternation of submillimetric goethite-rich (ochreous) and hematite-rich (red) layers. This layering is the result of a complex evolution of the pristine authigenic Fe-oxides and -oxyhydroxides during which the mineral phases are cyclically involved in transformation processes including recrystallization, dissolution and reprecipitation (Carbone etal. 2005). 

HCO 3 enhances the Dissolution Rate of Hematite. Fe(III) in natural waters is present as hydroxo complexes, especially Fe(OH) , Fe(OH)3(aq), Fe(OH)4. In addition a carbonato complex - Fe(C03)2 - is present in seawater and at the surface of solid iron(III)(hydr)oxides. Fig. 5.11 shows the dependence of the dissolution rate as a function of the hydrogen carbonato surface complex... 

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). 

The Rate of reductive Dissolution of Hematite by H2S as observed between pH 4 and 7 is given in Fig. 9.6 (dos Santos Afonso and Stumm, in preparation). The HS" is oxidized to SO. The experiments were carried out at different pH values (pH-stat) and using constant PH2s- 1.8 - 2.0 H+ ions are consumed per Fe(II) released into solution, as long as the solubility product of FeS is not exceeded, the product of the reaction is Fe2+. The reaction proceeds through the formation of inner-sphere =Fe-S. The dissolution rate, R, is given by... 

Photocatalytic Reductive Dissolution of Hematite in the Presence of Oxalate... 

In the absence of oxygen the photocatalytic reductive dissolution of hematite in the presence of oxalate occurs according to the following overall stoichiometry (Siffert and Sulzberger, 1991) ... 

In aerated suspensions no measurable (in the time-frame of typical experiments) reductive dissolution takes place and hematite acts as a photocatalyst for the oxidation of oxalate by 02 ... 

Faust and Hoffmann (1986) and Litter and Blesa (1988) who investigated the wavelength-dependence of the rate of photochemical reductive dissolution of iron(III)(hydr)oxides using hematite-bisulfite and maghemite-EDTA as model systems, respectively. 

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). 

Light-induced dissolution of hematite in the presence of oxalate at pH 3. The deaerated hematite suspension was irradiated with light that had passed a monochromator (X = 375 nm I0 = 4 W nr2). Initial oxalate concentration = 3.3 mM. 

Siffert, Ch., and B. Sulzberger (1991), "Light-Induced Dissolution of Hematite in the Presence of Oxalate A Case Study", Langmuir 7,1627-1634. 

Oxide mineralogy may influence rates of reductive dissolution in several ways. Hematite (ct-Fe203) and maghemite (y-Fe203), for example, have the same stoichiometry but contain Fe(III) in quite different coordinative environments. Fe(III) in hematite occupies trigonally-distorted octahedral sites, while Fe(III) in maghemite is found in both octahedral and tetrahedral sites (42). Differences... 

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 12. Extent of dissolution and re-precipitation between aqueous Fe(III) and hematite at 98°C calculated using Fe-enriched tracers. A. Percent Fe exchanged (F values) as calculated for the two enriched- Fe tracer experiments in parts B and C. Large diamonds reflect F values calculated from isotopic compositions of the solution. Small circles reflect F values calculated from isotopic compositions of hematite, which have larger errors due to the relatively small shifts in isotopic composition of the solid (see parts B and C). Curves show third-order rate laws that are fit to the data from the solutions. B. Tracer experiment using Fe-enriched hematite, and isotopically normal Fe(lll). C. Identical experiment as in part B, except that solution Fe(lll) is enriched in Te, and initial hematite had normal isotope compositions. Data from Skulan et al. (2002).

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