Hematite, dissolution rate

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... 

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). 

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 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,...

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. 

Fig. 12.22 Relationship between the dissolution rate per unit surface area in Na-dithionite/citrate/bicarbonate at 25 °C and the Al substitution of 28 synthetic goethites (upper) and 24 synthetic hematites (lower) (Torrent et al., 1987, with permission).

The dissolution rate of goethite by sulfide was found to increase with surface area and proton concentration. Pyzik and Sommer (21) suggested that HS" is the reactive species that reduces surface ferric iron after exchanging versus OH . A subsequent protonation of surface ferrous hydroxide would lead to dissolution of a surface layer. Elemental sulfur was the prominent oxidation product polysulfides and thiosulfate were found to a lower extent. The dissolution rate R (in moles per square meter per second) of hematite by sulfide was demonstrated to be proportional to the surface concentration of the surface complexes >FeHS and >FeS (22). 

Figure 6 gives the rate of the reductive dissolution of a-Fe203 (hematite) by H2S. The reaction mechanism (27) implies that, in line with the scheme given in equations 14a-l4c, surface complexes of =FeS and of =FeSH are formed and then undergo electron transfer. The dissolution rate, R (mol/m2 per hour), is given by... 

Figure 6. Experimental dissolution rate (mol/m2 per hour) as a function of surface speciation (eq 17). Insert dissolution rates (mol/m2 per hour) for hematite, goethite, lepidocrocite, and magnetite as a function of the free energy (kj/mol of electrons) of the reduction reactions...

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...

X 10" M s in toluene. The reaction between Fe(II)aq and Cr(VI) is nearly instantaneous, but when the only source of Fe(II) is in hematite or biotite, the initial rate of Cr(VI) reduction is dependent on the rate of mineral dissolution. These dissolution rates can be increased by low pH or by the addition of anions that complex Fe(II). ... 

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. 

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