Iron oxides pathways

The corrosion of iron occurs particularly rapidly when an aqueous solution is present. This is because water that contains ions provides an oxidation pathway with an activation energy that is much lower than the activation energy for the direct reaction of iron with oxygen gas. As illustrated schematically in Figure 19-21. oxidation and reduction occur at different locations on the metal surface. In the absence of dissolved ions to act as charge carriers, a complete electrical circuit is missing, so the redox reaction is slow, hi contrast, when dissolved ions are present, such as in salt water and acidic water, corrosion can be quite rapid. 

Although thermodynamically favorable, reductive dissolution of Fe(III)(hydr)oxides by some metastable ligands (even those, such as oxalate, that can form surface complexes) does not occur in the absence of light. The photochemical pathway is depicted in Fig. 9.3e. In the presence of light, surface complex formation is followed by electron transfer via an excited state (indicated by ) either of the iron oxide bulk phase or of the surface complex. (Light-induced reactions will be discussed in Chapter 10.)... 

For the cracking of catechol and 3-methylcatechol in the presence of iron oxide, reaction pathways for the secondary products appeared to follow the same trend based on the assumption that the possible identities of the products we proposed above were correct. As presented in Scheme 12.1, catechol and 3-methylcatechol were oxidized to their corresponding quinones, 1,2-benzoquinone, and methylbenzoquinone, respectively. This was followed by an expulsion of CO to form cyclopentadienones and further followed by one more CO expulsion to form possibly vinyl acetylene from catechol and pentenyne from 3-methylcatechol. These products were eventually converted to the tertiary products. The formation of quinones was also observed in other studies where the oxidation of catechols was carried out. The formation of secondary products I in our study is in agreement with the previous studies (e.g., Wornet et Therefore, it is reasonable to propose the reaction pathways de-... 

Effects of Flooding and Redox Conditions onfs. I know of no published data on this. Bnt it is likely that the natnre of particle surfaces in intermittently flooded soils wonld restrict snrface mobility. For ions to diffuse freely on the surface there must be a continuous pathway of water molecules over the surface and uniform cation adsorption sites. But in intermittently flooded soils the surface typically contains discontinuous coatings of amorphous iron oxides on other clay minerals, and on flooding reduced iron is to a large extent re-precipitated as amorphons hydroxides and carbonates as discussed above, resulting in much microheterogeneity with adsorption sites with disparate cation affinities. 

Isomorphous substitution of iron oxides is important for several reasons. In the electronics industry, trace amounts (dopants) of elements such as Nb and Ge are incorporated in hematite to improve its semiconductor properties. Dopants are also added to assist the reduction of iron ores. In nature, iron oxides can act as sinks for potentially toxic M", M and M heavy metals. Investigation of the phenomenon of isomorphous substitution has also helped to establish a better understanding of the geochemical and environmental pathways followed by Al and various trace elements. Empirical relationships (e. g. Fe and V) are often found between the Fe oxide content of a weathered soil profile and the levels of various trace elements. Such relationships may indicate similarities in the geochemical behaviour of the elements and, particularly for Al/Fe, reflect the environment in which the oxides have formed (see chap. 16). 

Iron oxide dissolution can proceed by a variety of pathways, viz. protonation, com-plexation and reduction, photochemical and biological. 

From this short overview it follows that iron oxides can form by the following main pathways ... 

Laboratory studies have indicated an increasing number of further processes for which iron oxides may be used as catalysts. A sodium promoted iron oxide on a support of Si02 catalyses the gas phase oxidation (377-427 °C) by nitrous oxide, of pro-pene to propene oxide (Duma and Honicke, 2000). Ferrihydrite or akaganeite can be used to catalyse the reduction (at 55-75 °C) by hydrazine, of aromatic nitro compounds to aromatic amines (which are the starting materials for a huge range of chemicals) these Fe oxides have the potential to provide a safe and economical pathway to the production of these important organics (Lauwiner et al., 1998). 

Ramesh, S., Felner, ., KoltypinY. Gedanken, A. (2000) Reaction pathways at the iron-mi-crosperical silica interface mechanistic aspects of the formation of target iron oxide phases. Mater. Res. 15 944-950... 

Pigments with a physical protective action are chemically inert and are termed inactive or passive. An example is micaceous iron oxide [5.54], [5.55]. These lamellar pigments are packed in layers they lengthen the pathways and obstruct the penetration of ions. They improve adhesion between the substrate and the coating, absorb UV radiation, and protect the underlying binder (Fig. 68), [5.56]—[5.60]. 

Many cells, such as lymphocytes, do however lack efficient internalizing receptor systems. The development of a new and effective internalizing pathway thus appears as a very important goal. Weissleder et al. [83, 96] have tried to develop such a system by attaching a superparamagnetic iron oxide to a membrane translocating signal (MTS) peptide. Several MTSs have been described... 

Ferrous iron binds to H-chain ferritin with the release of 0.25H+ per Fe(II) ion. Iron oxidation then takes place via a combination of three pathways, with the proportions of each dependent on the amount of iron added. At low iron loading (<50 Fe per H-chain homopolymer, that is, less than required to saturate the ferroxidase centers) the dominant reaction is at the ferroxidase center (equation 4) ... 

Reactions involving mackinawite and an oxidized sulfur species have been repeatedly shown to lead to pyrite formation (e.g., Bemer, 1969 Rickard, 1969, 1975). In addition, Wilkin and Bames (1996) and Penning et al. (2000) have shown that pyrite formation is exceptionally rapid when the mackinawite is pre-oxidized (e.g., exposed briefly to air) prior to the experiment. Based partly on X-ray photoelectron and Auger spectroscopy results of pyrrhotite oxidation (Mycroft et al., 1995), Wilkin and Bames (1996) hypothesized that this oxidative exposure initiates an iron-loss pathway similar to Equation (13). In sulfidic solutions, Fe(II) oxyhydroxides, shown as a product in this reaction, would not accumulate, but instead would undergo reductive dissolution by a reaction similar to Equation (14) ... 

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