Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely used by humans, e.g., as iron ores, pigments, catalysts, in thermite (see the diagram) and hemoglobin. Common rust is a form of iron(III) oxide. Iron oxides are widely used as inexpensive, durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172.
- Oxide of FeII
- Mixed oxides of FeII and FeIII
- Oxide of FeIII
Several species of bacteria, including Shewanella oneidensis, Geobacter sulfurreducens and Geobacter metallireducens, metabolically utilize solid iron oxides as a terminal electron acceptor, reducing Fe(III) oxides to Fe(II) containing oxides.
Methanogenesis replacement by iron oxide reductionEdit
Under conditions favoring iron reduction, the process of iron oxide reduction can replace at least 80% of methane production occurring by methanogenesis. This phenomenon occurs in a nitrogen-containing (N2) environment with low sulfate concentrations. Methanogenesis, an Archaean driven process, is typically the predominate form of carbon mineralization in sediments at the bottom of the ocean. Methanogenesis completes the decomposition of organic matter to methane (CH4). The specific electron donor for iron oxide reduction in this situation is still under debate, but the two potential candidates include either Titanium (III) or compounds present in yeast. The predicted reactions with Titanium (III) serving as the electron donor and phenazine-1-carboxylate (PCA) serving as an electron shuttle is as follows:
- Ti(III)-cit + CO2 + 8H+ → CH4 + 2H2O + Ti(IV) + cit ΔE = –240 + 300 mV
- Ti(III)-cit + PCA (oxidized) → PCA (reduced) + Ti(IV) + cit ΔE = –116 + 300 mV
- PCA (reduced) + Fe(OH)3 → Fe2+ + PCA (oxidized) ΔE = –50 + 116 mV 
- Note: cit = citrate.
Titanium (III) is oxidized to Titanium (IV) while PCA is reduced. The reduced form of PCA can then reduce the iron hydroxide (Fe(OH)3).
Hydroxyl radical formationEdit
On the other hand when airborne, iron oxides have been shown to harm the lung tissues of living organisms by the formation of hydroxyl radicals, leading to the creation of alkyl radicals. The following reactions occur when Fe2O3 and FeO, hereafter represented as Fe3+ and Fe2+ respectively, iron oxide particulates accumulate in the lungs.
The formation of the superoxide anion (O2• –) is catalyzed by a transmembrane enzyme called NADPH oxidase. The enzyme facilitates the transport of an electron across the plasma membrane from cytosolic NADPH to extracellular oxygen (O2) to produce O2• –. NADPH and FAD are bound to cytoplasmic binding sites on the enzyme. Two electrons from NADPH are transported to FAD which reduces it to FADH2. Then, one electron moves to one of two heme groups in the enzyme within the plane of the membrane. The second electron pushes the first electron to the second heme group so that it can associate with the first heme group. For the transfer to occur, the second heme must be bound to extracellular oxygen which is the acceptor of the electron. This enzyme can also be located within the membranes of intracellular organelles allowing the formation of O2• – to occur within organelles.
The formation of hydrogen peroxide (H
2) can occur spontaneously when the environment has a lower pH especially at pH 7.4. The enzyme superoxide dismutase can also catalyze this reaction. Once H
2 has been synthesized, it can diffuse through membranes to travel within and outside the cell due to its nonpolar nature.
Fe2+ is oxidized to Fe3+ when it donates an electron to H2O2, thus, reducing H2O2 and forming a hydroxyl radical (HO•) in the process. H2O2 can then reduce Fe3+ to Fe2+ by donating an electron to it to create O2• –. O2• – can then be used to make more H2O2 by the process previously shown perpetuating the cycle, or it can react with H2O2 to form more hydroxyl radicals. Hydroxyl radicals have been shown to increase cellular oxidative stress and attack cell membranes as well as the cell genomes.
- HO• + RH → R• + H2O 
The HO• radical produced from the above reactions with iron can abstract a hydrogen atom (H) from molecules containing an R-H bond where the R is a group attached to the rest of the molecule, in this case H, at a carbon (C).
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