Green rust

Green rust compounds were identified in green corrosion crusts that form on iron and steel surfaces, in alternating aerobic and anaerobic conditions, by water containing anions such as chloride, sulfate, carbonate, or bicarbonate.^{[2][4][8][12][13][20][21][22]} They are considered to be intermediates in the oxidative corrosion of iron to form iron(III) oxyhydroxides (ordinary brown rust). Green rust may be formed either directly from metallic iron or from iron(II) hydroxide Fe(OH)₂.^[4]

On the basis of Mössbauer spectroscopy, green rust is suspected to occur as mineral in certain bluish-green soils that are formed in alternating redox conditions, and turn ochre once exposed to air.^{[23][24][25][26]} Green rust has been conjectured to be present in the form of the mineral fougerite ([Fe2+4Fe3+2(OH)₁₂][CO₃]·3H₂O).^[5]

Hexagonal crystals of green rust (carbonate and/or sulfate) have also been obtained as byproducts of bioreduction of ferric oxyhydroxides by dissimilatory iron-reducing bacteria, such as Shewanella putrefaciens, that couple the reduction of Fe³⁺
with the oxidation of organic matter.^[27] This process has been conjectured to occur in soil solutions and aquifers.^[19]

In one experiment, a 160 mM suspension of orange lepidocrocite γ-FeOOH in a solution containing formate (HCO⁻
₂), incubated for 3 days with a culture of Shewanella putrefaciens, turned dark green due to the conversion of the hydroxide to GR(CO²⁻
₃), in the form of hexagonal platelets with diameter ~7 μm. In this process, the formate was oxidized to bicarbonate HCO⁻
₃ which provided the carbonate anions for the formation of green rust. The active bacteria were necessary for the formation of green rust.^[19]

Green rust compounds can be synthesized at ambient temperature and pressure, from solutions containing iron(II) cations, hydroxide anions, and the appropriate intercalatory anions, such as chloride,^{[6][28][29][30]} sulfate,^{[31][32][33][34]} or carbonate.^[35]

The result is a suspension of ferrous hydroxide (Fe(OH)₂) in a solution of the third anion. This suspension is oxidized by stirring under air, or bubbling air through it.^[25] Since the product is very prone to oxidation, it is necessary to monitor the process and exclude oxygen once the desired ratio of Fe²⁺
and Fe³⁺
is achieved.^[3]

One method first combines an iron(II) salt with sodium hydroxide (NaOH) to form the ferrous hydroxide suspension. Then the sodium salt of the third anion is added, and the suspension is oxidized by stirring under air.^[3][25][36]

For example, carbonate green rust can be prepared by mixing solutions of iron(II) sulfate FeSO
₄ and sodium hydroxide; then adding sufficient amount of sodium carbonate Na
₂CO
₃ solution, followed by the air oxidation step.^[36]

Sulfate green rust can be obtained by mixing solutions of FeCl
₂·4H
₂O and NaOH to precipitate Fe(OH)₂ then immediately adding sodium sulfate Na
₂SO
₄ and proceeding to the air oxidation step.^[8][34]

A more direct method combines a solution of iron(II) sulfate FeSO
₄ with NaOH, and proceeding to the oxidizing step.^[18] The suspension must have a slight excess of FeSO
₄ (in the ratio of 0.5833 Fe²⁺
for each OH⁻
) for green rust to form; however, too much of it will produce instead an insoluble basic iron sulfate, iron(II) sulfate hydroxide Fe₂(SO₄)(OH)₂·nH₂O.^[32] The production of green rust is lower as temperature increases.^[37]

An alternate preparation of carbonate green rust first produces a suspension of iron(III) hydroxide Fe(OH)₃ in an iron(II) chloride FeCl
₂ solution, and bubbles carbon dioxide through it.^[3]

In a more recent variant, solutions of both iron(II) and iron(III) salts are first mixed, then a solution of NaOH is added, all in the stoichiometric proportions of the desired green rust. No oxidation step is then necessary.^[34]

Carbonate green rust films have also been obtained from the electrochemical oxidation of iron plates.^[35]