Coupled iron oxidation—reduction, effects

Roden EE, Leonardo MR, Ferris FG (2002) Immobilization of strontium during iron biomineralization coupled to dissimilatory hydrous ferric oxide reduction. Geochim Cosmochim Acta 66 2823-2839 Roe JE, Anbar AD, Barling J (2003) Nonbiological fractionation of Ee isotopes evidence of an equilibrium isotope effect. Chem Geol 195 69-85... 

While the effect on oxidation-reduction potentials of substituents on phenanthroline ligands is regular, studies of the oxidation of [Os(bipy)(terpy)X] + species, where X is an alkyl-substituted pyridine molecule (111), do not show a linear dependence of E° on the pA of X. These results have been explained in terms of the Baker-Nathan effect. However, taken in conjunction with the entropy data of Kratochvil and Knoeck (439) for substituted iron complexes, an explanation involving changes in solvation with substituents seems preferable. The potentials of various Os(II)/Os(III) couples (111) and Ru(II)/Ru(III) couples (220) have been used to study the effect of the overall charge on the... 

It is well established that iron reduction is coupled to soluble phosphorous release in soils dominated by iron redox couples (Figure 10.32) (see Chapter 9 for details). Although phosphorous itself is not normally involved in redox reactions, it does undergo reactions that have a pronounced effect on its reactivity. Most of this change in the reactivity of phosphorous in wetland soils and aquatic sediments is associated with the oxidation-reduction of iron and manganese. The reduction of ferric phosphate compounds results in the release of phosphorous, a major solubility mechanism in wetlands and aquatic systems. 

In natural waters occur not one but several oxidation-reduction reactions. These reactions are associated with the presence of several elements, which are capable of changing their charge, and run in parallel. For this reason, total oxidation potential of the solution is defined by the nature and concentration of all redox-couples. Components which noticeably affect the solution s oxidation-reduction potential are called electroactive. Elements whose concentration and form of existence actually control solution s oxidation are culled potential-setting. In natural waters these are usually O, S, C, N and Fe. The medium whose oxidation potential value almost does not change with the addition of oxidizers or reducers is called redox-buffers. The redox-buffer may be associated with composition of the water itself, of its host rocks or with the effect of atmosphere. In the subsurface redox-buffers are associated, as a rule, with the content of iron, sulphur or manganese. Stably high Eh value in the surface and ground waters is caused by the inexhaustible source of in the atmosphere. 

The electrochemical effects of slowly and erratically thickening oxide films on iron cathodes are, of course, eliminated when the film is destroyed by reductive dissolution and the iron is maintained in the film-free condition. Such conditions are obtained when iron is coupled to uncontrolled magnesium anodes in high-conductivity electrolytes and when iron is coupled to aluminium in high-conductivity solutions of pH less than 4-0 or more than 12 0 . In these cases, the primary cathodic reaction (after reduction of the oxide film) is the evolution of hydrogen. 

This iron-ate complex 19 is also able to catalyze the reduction of 4-nitroanisole to 4-methoxyaniline or Ullmann-type biaryl couplings of bis(2-bromophenyl) methylamines 31 at room temperature. In contrast, the corresponding bis(2-chlor-ophenyl)methylamines proved to be unreactive under these conditions. A shift to the dianion-type electron transfer(ET)-reagent [Me4Fe]Li2 afforded the biaryl as well with the dichloro substrates at room temperature, while the dibromo substrates proved to be reactive even at —78°C under these reaction conditions. This effect is attributed to the more negative oxidation potential of dianion-type [Me4Fe]Li2. 

In aspect of its toxicity, any pathway leading to abatement of chromate(VI) pollution arouse a vivid interest. One of such pathways seems to be created by cooperations between the iron and chromium photocatalytic cycles, which were reported as effectively converting chromate(Vl) into Cr(III) species. Photochemical coupling reactions between polycarboxylate Fe(III) complexes and chromate(Vl) were studied and strong collaboration between both photocatalysts was demonstrated, which was significantly affected by the oxygen concentration (16,17,95,261). On the other hand, chromium(Vl) reduction pho-toinduced by iron(lll) nitrilotriacetate accompanied by nta degradation was found to be independent of the O2 concentration, whereas the oxidation state of the chromium product depended on the pH (257). 

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