Electrochemically oxidized iron (rust)

Iron oxides are chemical compounds composed of iron and oxygen. There are sixteen known iron oxides and oxyhydroxides, the best known of which is rust, a form of iron(III) oxide.^[1]

Iron oxides and oxyhydroxides are widespread in nature and play an important role in many geological and biological processes. They are used as iron ores, pigments, catalysts, and in thermite, and occur in hemoglobin. Iron oxides are inexpensive and 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.

Oxides

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⁺.

• Oxide of Fe^II
  • FeO: iron(II) oxide, wüstite
  • FeO₂:^[2]iron peroxide
• Mixed oxides of Fe^II and Fe^III
  • Fe₃O₄: Iron(II,III) oxide, magnetite
  • Fe₄O₅^[3]
  • Fe₅O₆^[4]
  • Fe₅O₇^[5]
  • Fe₂₅O₃₂^[5]
  • Fe₁₃O₁₉^[6]
• Oxide of Fe^III
  • Fe₂O₃: iron(III) oxide
    • ?-Fe₂O₃: alpha phase, hematite
    • ?-Fe₂O₃: beta phase
    • ?-Fe₂O₃: gamma phase, maghemite
    • ?-Fe₂O₃: epsilon phase

Hydroxides

• iron(II) hydroxide (Fe(OH)₂)
• iron(III) hydroxide (Fe(OH)₃), (bernalite)

Thermal expansion

Oxide-hydroxides

• goethite (?-FeOOH),
• akaganéite (?-FeOOH),
• lepidocrocite (?-FeOOH),
• feroxyhyte (?-FeOOH),
• ferrihydrite (${\displaystyle {\ce {Fe5HO8.4H2O}}}$ approx.), or ${\displaystyle {\ce {5Fe2O3.9H2O}}}$, better recast as ${\displaystyle {\ce {FeOOH.}}0.4{\ce {H2O}}}$
• high-pressure pyrite-structured FeOOH.^[8] Once dehydration is triggered, this phase may form ${\displaystyle {\ce {FeO2Hx (0<x<1)}}}$.^[9]
• schwertmannite (ideally ${\displaystyle {\ce {Fe8O8(OH)6(SO).{\mathit {n}}H2O}}}$ or ${\displaystyle {\ce {Fe^{3+}16O16(OH,SO4)}}_{\text{12-13}}\cdot {\text{10-12}}{\ce {H2O}}}$)^[10]
• green rust (${\displaystyle {\ce {Fe_{\mathit {x}}^{III}Fe_{\mathit {y}}^{II}(OH)}}_{3x+2y-z}{\ce {(A^{-})}}_{z}}$ where A^- is Cl^- or 0.5SO₄^2-)

Microbial degradation

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.^[11]

Environmental effects

Methanogenesis replacement by iron oxide reduction

Under conditions favoring iron reduction, the process of iron oxide reduction can replace at least 80% of methane production occurring by methanogenesis.^[12] 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).^[12] 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                           ?E = –240 + 300 mV

Ti(III)-cit + PCA (oxidized) -> PCA (reduced) + Ti(IV) + cit                ?E = –116 + 300 mV

PCA (reduced) + Fe(OH)₃ -> Fe²⁺ + PCA (oxidized)                         ?E = –50 + 116 mV ^[12]

• 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 formation

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.^[13]

O₂ +  e⁻ -> O₂^o –[13]

The formation of the superoxide anion (O₂^o –) 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^o –. 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^o – to occur within organelles.^[14]

2O₂^o – + 2 H⁺ -> + O₂^[13][15]

The formation of hydrogen peroxide can occur spontaneously when the environment has a lower pH especially at pH 7.4.^[15] The enzyme superoxide dismutase can also catalyze this reaction. Once has been synthesized, it can diffuse through membranes to travel within and outside the cell due to its nonpolar nature.^[14]

Fe²⁺ + -> Fe³⁺ + HO^o +  OH⁻

Fe³⁺ + H₂O₂ -> Fe²⁺ + O₂^o – + 2H⁺

H₂O₂ + O₂^o – -> HO^o +  OH⁻ + O₂^[13]

Fe²⁺ is oxidized to Fe³⁺ when it donates an electron to H₂O₂, thus, reducing H₂O₂ and forming a hydroxyl radical (HO^o) in the process. H₂O₂ can then reduce Fe³⁺ to Fe²⁺ by donating an electron to it to create O₂^o –. O₂^o – 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.^[13]

HO^o + RH -> R^o + H₂O ^[13]

The HO^o 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).^[13]

See also

• Great Oxidation Event
• Iron cycle
• Iron oxide nanoparticle
• Limonite
• List of inorganic pigments

References

External links

• Information from Nano-Oxides, Inc. on Fe₂O₃.
• The Iron One-Pot Reaction
• Iron Oxide Pigments Statistics and Information
• CDC - NIOSH Pocket Guide to Chemical Hazards