Ferrous iron reductant oxidation-reduction potentials

Most commonly, iron is discussed as being in either the ferrous (Fe2+) or ferric (Fe3+) state. Changes between these two depend on the soil s pH and Eh (where Eh is a measure of the oxidation-reduction potential of soil) as discussed in Chapter 9. Add conditions and low Eh values tend to lead to the production of ferrous ion, while high pH and high Eh values result in the predominance of ferric ion. It should be noted that the ferrous ion is more soluble than the ferric ion and, thus, it will be more available to plants. 

The reduction potentials for various alkyl halides range from +0.5 to +1.5 V therefore, when Fe° serves as an electron donor, the reaction is thermodynamically favorable. Because three reductants are present in the treatment system (Fe°, H2, and Fe2+), three possible pathways exist. Equation (13.9) represents the oxidation of Fe° by reduction of a halogenated compound. In the second pathway, the ferrous iron behaves as a reductant, as represented in Equation (13.10). This reaction is relatively slow because the ability to reduce a pollutant by ferrous iron is dependent on the speciation ferrous ions, which is determined by the ligands present in the system. The third possible pathway, Equation (13.11), is dehalogenation by hydrogen. This reaction does not occur easily without a catalyst. In addition, if hydrogen levels become too high, corrosion is inhibited (Matheson and Tratnyek, 1994) ... 

It has been generally assumed that iron is transported across biological membranes in the ferrous form and that ferric iron would have to be reduced before it can be used by the organism. Thus, based on nutritional studies it has long been recognized that Fe(II) is1 more effectively absorbed than Fe(III), and this has been attributed to differences in the thermodynamic and kinetic stability of the complexes and chelates formed by these cations (for review, see Ref. 2). The experimental proof of a transport in the ferrous form has, however, not been given until quite recently in studies of iron transport in isolated mitochondria (23) as well as in enterobacteria (33). In rat liver mitochondria we have found that Fe(III) donated from a metabolically inert water soluble complex of sucrose interacts with the respiratory chain at the level of cytochrome c (and possibly cytochrome a) (23, 32) (Figure 1 B), which has a oxidation-reduction potential of around +250 mV (34) and is localized to the outer phase of the mitochondrial inner membrane (35). 

Ferrous iron (Fe " ) appears later than Mn in soils that have been subjected to prolonged waterlogging because, as Table 7.1 shows, the reduction potential of Fe in oxides (and probably in many other soil minerals as well) is lower than that of Mn( + 3,d-4) in Mn oxides. Since Fe +, like Mn +, is rather soluble, it can reach appreciable concentrations in poorly aerated soil solutions. The introduction of dissolved oxygen causes rapid oxidation of Fe " and precipitation of ferric hydroxide if the solution pH is much higher than 6. The rate law of oxidation of dissolved Fe is known to be... 

To leach the more acid-resistant minerals containing tetravalent uranium, steam is fed to the second tank to bring the temperature to 49 to 60°C, and sodium chlorate NaQOj is added to bring the oxidation-reduction potential e, measured relative to the calomel electrode, to from —0.47 to —0.51 V. At —0.51 V, the equilibrium ratio of ferric iron to ferrous iron in the solution is 0.52. Ferric iron catalyzes the oxidation of insoluble tetravalent uranium to the soluble hexavalent uranyl form ... 

The normal oxidation-reduction potential Eq of this redox couple in relation to the hydrogen electrode is 771 mV. The oxidation-reduction potential of still wines, even when young, is often much lower, around 500 mV. This value explains why iron is present in both ferrous and ferric forms. If all the iron in wine were in ion form, the potential would be higher. It is obvious that much of the iron is involved in complexes, and is thus more difficult to identify. 

Ferric-ion complexes are important in acid-sulfate leaching because ferric ion can be generated from fenous ion using air or oxygen in situ. The reduction of ferric iron to ferrous occurs as the ferric-ion complex diffuses through fluid-filled pores and channels in the rock matrix and encounters reactive metals or sulfides. In most instances, as already discussed, the rate of ferric ion reduction is a diffusion-limited process. The oxidation of ferrous iron to ferric in aqueous solution becomes of primary importance because of its in situ regeneration capacity under appropriate oxidation potentials. 

The sequence of reactions in which the cytochromes participate is a mechanism for transferring electrons to molecular oxygen via iron complexes that are alternately in ferric and ferrous states. The order of the transfer has been deduced from studies with inhibitors, in which the electron-transport chain is broken so that components below the break are reduced, those above are oxidized from studies with poised potentials, in which the relative degrees of oxidation and reduction define the oxidation-reduction potentials of the various components and from rapid kinetic measurements, in which the order of reduction or oxidation can be seen. These methods agree on the following sequence ... 

As discussed previously, cytochrome c is reduced by several flavo-proteins. On enzymatic reduction or reaction with hydrosulfite, ascorbic acid, or any of several other reducing reagents, the typical spectrum of reduced cytochrome c is produced. The band at 550 m/ is usually used to assay reduced cytochrome c. The reduction causes a change in the iron from the ferric to the ferrous state. The oxidation-reduction potential of the couple, ferric3rtochrome Jerrocytochrome at neutral pH values (5-8) is -H0.256 volts. ... 

For example, if the amount of ferrous iron minerals present in repository backfill and fracture minerals (represented by FeC03(s) in Fig. 1(a)) is much greater than the amount of O2 remaining after closure, then with time, all O2 will be reduced to H2O by these minerals, producing iron hydroxide in the process. This would ensure that the reducing intensity would return to values at least as low as the redox potential of the Fe(0H)3(s)/FeC03(s) couple (near —0.05 V). This is below the threshold for corrosion of either copper or uranium oxide by O2. It is also shghtly above the threshold for sulphide production by sulphate reduction (—0.2 V). The presence of ferrous minerals thus buffers the redox intensity of the repository to conditions that are favourable for repository performance. 

Reaction with reductants such as ferrous iron is associated with loss of chlorine atoms from the molecule, a reaction process termed reductive dechlorination. Figure 5 shows the redox potentials associated with stepwise removal of chlorine atoms from tetra-chloromethane during a series of reductive dechlorination reactions that convert CCI4 to CH4. Methane, unlike the chlorinated parent compound, can be relatively easily oxidized to CO2 in a separate step. Thus, a reducing environment is required to initiate the reaction sequence leading to complete biodegradation. 

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