Fe2+, oxidation

In the given example, Fe2+ oxidizes to Fe3+ (de-electronation) and Ag+ reduces to Ag (electronation), and this as a whole is then an oxidation reduction process. The examples of nonionic reducing agents are hydrogen, carbon monoxide, formaldehyde and hydrazine. Their action may representatively be described for hydrogen ... 

With increasing hydrogen-ion concentration, the rate of Fe2+ oxidation decreases this is related to the hydrolysis of Fe2+ ... 

Under normal conditions about 95—97% of Ti02 from ilmenite is solubilized. Most of the iron in the solution is in the Fe2+ oxidation state. Any Fe3+ present must be reduced to Fe2+ because iron can only be removed by crystallization in its divalent form. The reduction is usually done by adding some scrap iron during the digestion step. 

Figure 7. Electrode polarization curves for oxygen-containing solutions with a mixed potential resulting from oxygen reduction accompanied by Fe2+ oxidation. Curves are schematic but in accord with available data at significant points...

Fe,v-Fe2+ system in the presence of a trace of dissolved oxygen. The measured zero-current potential is that value where the rate of 02 reduction at the electrode surface is equal to the rate of Fe2+ oxidation rather than the value of since at the latter point simultaneous 02 reduction produces excess cathodic current. In addition, because the net reaction of Em converts Fe2+ to Fe >>+, the measured potential exhibits a slow drift. Such mixed potentials are of little worth in determining equilibrium Eh values. 

Thus the time scale for Fe2+ oxidation is about 140 s, whereas that for the formation of FeL is about 70 s.)... 

Figure 17.9 Microsomal (nonmitochondrial) hydroxylation sequence involving cytochrome 450- RH is the substrate to be hydroxylated (ox, oxidized red, reduced cyt, cytochrome). Reduced cytochrome P450 has Fe2+, oxidized cytochrome P450 has Fe3+, reduced cytochrome oxidase has FADH2/FMNH2, and oxidized cytochrome oxidase has FAD/FMN.

Abiotic oxidation of Fe2+ with 02 is extremely pH-sensitive. The reaction is rapid above pH 5, and becomes extremely slow in very acidic solution. In the presence of T. ferrooxidans, however, Fe2+ oxidation is very rapid in acidic conditions. The conditions under which T. ferrooxidans activity is optimized is shown in Figures 6.3-6.5. 

The parameters 02 and Fe3+ refer to partial pressure and concentration, respectively, kj refers to rate constants, and S denotes surface area. The exponents v are experimentally determined (Daniels and Alberty, 1975). Considering that the rate of FeS2 oxidation by 02 is slow relative to that by Fe3+, and Fe2+ oxidation by 02 is slower than the rate of FeS2 oxidation by Fe3+, the latter (Fe2+ oxidation) is the pyrite oxidation... 

Equation 7.59 reveals that Fe2+ oxidation is first order with respect to Fe2+ and 02 and second order with respect to OH" (Fig. 7.14). 

The data in Figure 7.15 demonstrate kinetics of Mn2+ oxidation using the pH-stat technique. The data show at least two major slopes. The first slope (near the origin) represents Reaction 7.64, whereas the second slope represents the autocatalytic part of the reaction (Reaction 7.65). The data demonstrate that the reaction is pH-dependent. As pH increases, the autocatalytic part of the reaction represents the mechanism by which most Mn2+ oxidizes. Similar reactions for Fe2+ are shown in Figure 7.16. Note that Fe2+ oxidizes at a much lower pH than Mn2+. 

Millero and Izaguirre (1989) examined the effect various anions have on the abiotic oxidation rate of Fe2+ at constant ionic strength (/- 1,0) and found that this effect was on the order of HCOj > Br > N03- > C104- > Cl" > SO2" > B(OH)4 (see also Fig 7.18). Strong decrease in the rate of Fe2+ oxidation due to the addition of SO2 and... 

B(OH)4- pairs was attributed to the formation of FeS04 and Fe[B(04H)4]+ pairs which they assumed were difficult to oxidize. They also reported that the oxidation of Fe2+ is first-order with respect to HCO3. This HCO3 dependence of Fe2+ oxidation could be related to the formation of an FeHCOj pair which has a faster rate of oxidation than the Fe(OH)2 pair. 

Figure 7.27. Competitive inhibition of Fe2+ oxidizing activity of SM-4 cells at concentrations of 0,25, 0.50, 0.75, and 1.0 mg mL-1. The V 1 was calculated by dividing rate V by cell concentration. The insert is a plot of the slope versus cell concentration to obtain the inhibition constant Ky or in milligrams of cells per milliliter (from Suzuki et al., 1989, with permission).

The derivation of the uncompetitive equation is given in the next boxed section. The ideal linearized plot representing Equation 7.95 is shown in Figure 7.28. Actual experimental data of the uncompetitive form is shown in Figure 7.29 which shows Fe2+ oxidation by Thiobacillus. 

Figure 12.12. Kinetics of Fe2+ oxidation under various pH values (from Stumm and Morgan, 1981, with permission).

As noted earlier, oxidation is easier in basic solution, but neutral and acid solutions of Fe2+ oxidize less rapidly with increasing acidity (even though the potential of the oxidation reaction becomes more positive). This is because Fera is actually present in the form of hydroxo complexes, except in extremely add solutions, and there may also be kinetic reasons. 

Upadhyay AK, Hooper AB, Hendrich MP (2006) NO reductase activity of the tetraheme cytochrome c-554 of Nitrosomonas europaea. J Am Chem Soc 128 4330-4337 Valkova-Valchanova MB, Chan SHP (1994) Purification and characterization of two new c-type cytochromes involved in Fe2+ oxidation from Thiobacillus ferrooxidans. FEBS Lett 288 159-162... 

To this we must add a condition representing the conservation of charge. With acid-base titrations we saw that this was most readily done by invoking a proton balance here we will likewise use an electron balance, i.e., an accounting of electrons consumed and electrons generated. In the present example, each Fe2+ oxidized to Fe3+ has released one electron, and each Ce4+ reduced to Ce3+ has accepted one. Therefore, assuming that we start with only Fe2+and Ce4+, we can write the electron balance as... 

---