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Electrochemically oxidized iron (rust)

Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.[1]

Iron oxides and oxide-hydroxides are wide spread 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.

Contents

OxidesEdit

 
Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3.
 
Green and reddish brown stains on a limestone core sample, respectively corresponding to oxides/hydroxides of Fe2+ and Fe3+.

HydroxidesEdit

 
Role of iron oxides

Oxide/hydroxidesEdit

  • goethite (α-FeOOH),
  • akaganéite (β-FeOOH),
  • lepidocrocite (γ-FeOOH),
  • feroxyhyte (δ-FeOOH),
  • ferrihydrite (  approx.), or  , better recast as  
  • high-pressure FeOOH
  • schwertmannite (ideally   or  )[7]
  • green rust (  where A is Cl or 0.5SO42−)

Microbial degradationEdit

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.[8]

Environmental EffectsEdit

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.[9] 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).[9] 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 [9]

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.[10]

O₂ + e⁻ → O₂•⁻[10]

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.[11]

2O₂•⁻ + 2H⁺ → H₂O₂ + O2 [10][12]

The formation of hydrogen peroxide (H₂O₂) can occur spontaneously when the environment has a lower pH especially at pH 7.4.[12] The enzyme superoxide dismutase can also catalyze this reaction. Once H₂O₂ has been synthesized, it can diffuse through membranes to travel within and outside the cell due to its nonpolar nature.[11]

Fe²⁺ + H₂O₂→ Fe³⁺ + HO• + OH⁻

Fe3+ + H2O2 → Fe2+ + O2•⁻ + 2H+

H2O2 + O2•⁻ → HO• + OH⁻ + O2 [10]

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.[10]

HO• + RH → R• + H2O [10]

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).[10]

See alsoEdit

ReferencesEdit

  1. ^ Cornell, RM; Schwertmann, U (2003). The iron oxides: structure, properties, reactions, occurrences and uses. Wiley VCH. ISBN 3-527-30274-3. 
  2. ^ Hu, Qingyang; Kim, Duck Young; Yang, Wenge; Yang, Liuxiang; Meng, Yue; Zhang, Li; Mao, Ho-Kwang (June 2016). "FeO2 and FeOOH under deep lower-mantle conditions and Earth's oxygen–hydrogen cycles". Nature. 534 (7606): 241–244. doi:10.1038/nature18018. ISSN 1476-4687. 
  3. ^ "Discovery of the recoverable high-pressure iron oxide Fe4O5". Proceedings of the National Academy of Sciences. 108 (42): 17281–17285. Oct 2011. Bibcode:2011PNAS..10817281L. doi:10.1073/pnas.1107573108. 
  4. ^ "Synthesis of Fe5O6". 
  5. ^ a b "Structural complexity of simple Fe2O3 at high pressures and temperatures". 
  6. ^ "The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions". 
  7. ^ http://www.mindat.org/min-7281.html Mindat
  8. ^ Bretschger, O.; Obraztsova, A.; Sturm, C. A.; Chang, I. S.; Gorby, Y. A.; Reed, S. B.; Culley, D. E.; Reardon, C. L.; Barua, S.; Romine, M. F.; Zhou, J.; Beliaev, A. S.; Bouhenni, R.; Saffarini, D.; Mansfeld, F.; Kim, B.-H.; Fredrickson, J. K.; Nealson, K. H. (20 July 2007). "Current Production and Metal Oxide Reduction by Shewanella oneidensis MR-1 Wild Type and Mutants". Applied and Environmental Microbiology. 73 (21): 7003–7012. doi:10.1128/AEM.01087-07. 
  9. ^ a b c Sivan, O.; Shusta, S. S.; Valentine, D. L. (2016-03-01). "Methanogens rapidly transition from methane production to iron reduction". Geobiology. 14 (2): 190–203. doi:10.1111/gbi.12172. ISSN 1472-4669. 
  10. ^ a b c d e f g Hartwig, A.; MAK Commission 2016 (July 25, 2016). "Iron oxides (inhalable fraction) [MAK Value Documentation, 2011]". The MAK Collection for Occupational Health and Safety. Wiley-VCH Verlag GmbH & Co. KGaA. 1: 1804–1869. doi:10.1002/3527600418.mb0209fste5116. 
  11. ^ a b Bedard, Karen; Krause, Karl-Heinz (2007-01-01). "The NOX Family of ROS-Generating NADPH Oxidases: Physiology and Pathophysiology". Physiological Reviews. 87 (1): 245–313. doi:10.1152/physrev.00044.2005. ISSN 0031-9333. PMID 17237347. 
  12. ^ a b Chapple, Iain L. C.; Matthews, John B. (2007-02-01). "The role of reactive oxygen and antioxidant species in periodontal tissue destruction". Periodontology 2000. 43 (1): 160–232. doi:10.1111/j.1600-0757.2006.00178.x. ISSN 1600-0757. 

External linksEdit