Green rust

Green rust is a generic name for various green crystalline chemical compounds containing iron(II) and iron(III) cations, the hydroxide () anion, and another anion such as carbonate (), chloride (), or sulfate (), in a layered double hydroxide (LDH) structure. The most studied varieties are the following:

* carbonate green rust – GR ():
)₁₂sup>2+ · �2sup>2−;
* chloride green rust – GR ():
)₈sup>+ · �''n''sup>−;
* sulfate green rust – GR ():
)₁₂sup>2+ · �2sup>2−.

Other varieties reported in the literature are bromide , fluoride , iodide , nitrate , and selenate .

Green rust was first recognized as a corrosion crust on iron and steel surfaces. It occurs in nature as the mineral fougerite.

Structure

The crystal structure of green rust can be understood as the result of inserting the foreign anions and water molecules between brucite-like layers of iron(II) hydroxide, . The latter has an hexagonal crystal structure, with layer sequence AcBAcB... , where A and B are planes of hydroxide ions, and c those of (iron(II), ferrous) cations. In green rust, some cations get oxidized to (iron(III), ferric). Each triple layer AcB, which is electrically neutral in the hydroxide, becomes positively charged. The anions then intercalate between those triple layers and restore the electroneutrality.

There are two basic structures of green rust, "type 1" and "type 2". Type 1 is exemplified by the chloride and carbonate varieties. It has a rhombohedral crystal structure similar to that of pyroaurite (). The layers are stacked in the sequence AcBiBaCjCbAkA ...; where A, B, and C represent planes, a, b, and c are layers of mixed and cations, and i, j, and k are layers of the intercalated anions and water molecules. The ''c'' crystallographic parameter is 22.5–22.8 Å for the carbonate, and about 24 Å for the chloride.

Type 2 green rust is exemplified by the sulfate variety. It has an hexagonal crystal structure as minerals of the sjogrenite () group, with layers probably stacked in the sequence AcBiAbCjA...

Chemical properties

In oxidizing environment, green rust generally turns into oxyhydroxides, namely α- (goethite) and γ- ( lepidocrocite).

Oxidation of the carbonate variety can be retarded by wetting the material with hydroxyl-containing organic compounds such as glycerol or glucose, even though they do not penetrate the structure. Some variety of green rust is stabilized also by an atmosphere with high partial pressure.

Sulfate green rust has been shown to reduce nitrate and nitrite in solution to ammonium , with concurrent oxidation of to . Depending on the cations in the solution, the nitrate anions replaced the sulfate in the intercalation layer, before the reduction. It was conjectured that green rust may be formed in the reducing alkaline conditions below the surface of marine sediments and may be connected to the disappearance of oxidized species like nitrate in that environment.

Suspensions of carbonate green rust and orange γ- in water react over a few days producing a black precipitate of magnetite .

Occurrence

Iron and steel corrosion

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. 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 ()₂.

Reducing conditions in soils

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. Green rust has been conjectured to be present in the form of the mineral fougerite ().

Biologically mediated formation

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 with the oxidation of organic matter. This process has been conjectured to occur in soil solutions and aquifers.

In one experiment, a 160 m M suspension of orange lepidocrocite γ- in a solution containing formate (), incubated for 3 days with a culture of ''Shewanella putrefaciens'', turned dark green due to the conversion of the hydroxide to GR(), in the form of hexagonal platelets with diameter ~7 Î¼m. In this process, the formate was oxidized to bicarbonate which provided the carbonate anions for the formation of green rust. The active bacteria were necessary for the formation of green rust.

Laboratory preparation

Air oxidation methods

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, sulfate, or carbonate.

The result is a suspension of ferrous hydroxide () in a solution of the third anion. This suspension is oxidized by stirring under air, or bubbling air through it. Since the product is very prone to oxidation, it is necessary to monitor the process and exclude oxygen once the desired ratio of and is achieved.

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.

For example, carbonate green rust can be prepared by mixing solutions of iron(II) sulfate and sodium hydroxide; then adding sufficient amount of sodium carbonate solution, followed by the air oxidation step.

Sulfate green rust can be obtained by mixing solutions of ·4 and NaOH to precipitate then immediately adding sodium sulfate and proceeding to the air oxidation step.

A more direct method combines a solution of iron(II) sulfate with NaOH, and proceeding to the oxidizing step. The suspension must have a slight excess of (in the ratio of 0.5833 for each ) for green rust to form; however, too much of it will produce instead an insoluble basic iron sulfate, iron(II) sulfate hydroxide . The production of green rust is lower as temperature increases.

Stoichiometric Fe(II)/Fe(III) methods

An alternate preparation of carbonate green rust first produces a suspension of iron(III) hydroxide in an iron(II) chloride solution, and bubbles carbon dioxide through it.

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.

Electrochemistry

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

References

Ludovic Legrand, Léo Mazerolles and Annie Chaussé (2004): "The oxidation of carbonate green rust into ferric phases: Solid-state reaction or transformation via solution". Geochimica et Cosmochimica Acta, volume 68, issue 17, pages 3497—3507.

M. Abdelmoula, Ph. Refait, S. H. Drissi, J. P. Mihe, and J.-M. R. Génin (1996): "Conversion electron Mössbauer spectroscopy and X-ray diffraction studies of the formation of carbonate-containing green rust one by corrosion of metallic iron in NaHCO₃ and (NaHCO₃ + NaCl) solutions". Corrosion Science, volume 38, pages 623–633.

M. Abdelmoula, F. Trolard, G. Bourrié and J.-M. R. Génin (1998): "Evidence of the Fe(II)–Fe(III) green rust `fougerite' mineral occurrence in a hydromorphic soil and its transformation with depth". Hyperfine Interactions, volume 111, pages 231–238.

R. Allmann (1968): "The crystal structure of pyroaurite". Acta Crystallographica, series B, volume 24, pages 972–977.

J. D. Bernal, D. R. Dasgupta, and A. L. Mackay (1959): "The oxides and hydroxides of iron and their structural inter-relationships". Clay Minerals Bulletin, volume 4, pages 15–30.

Pascale M. Bonin, Wojciech Jȩdral, Marek S. Odziemkowski, and Robert W. Gillham (2000): "Electrochemical and Raman spectroscopic studies of the influence of chlorinated solvents on the corrosion behaviour of iron in borate buffer and in simulated groundwater". Corrosion Science 42, pages 1921–1939.

G. Butler and J. G. Beynon (1967): "The corrosion of mild steel in boiling salt solutions". Corrosion Science 7, pages 385–404.

J. Detournay, L. de Miranda, R. Derie, and M. Ghodsie (1975): "The region of stability of green rust II in the electrochemical potential-pH diagram in sulphate medium". Corrosion Science, volume 15, pages 295–306.

J. Detournay, R. Derie, and M. Ghodsie (1976): "Etude de l’oxydation par aération de Fe(OH)₂ en milieu chlorure". Zeitschrift für anorganische und allgemeine Chemie, volume 427, pages 265–273.

P. Refait, L. Simon, J.-M. R. Génin (2000): Reduction of SeO₄^2− Anions and Anoxic Formation of Iron(II)−Iron(III) Hydroxy-Selenate Green Rust. Environmental Science Technology, 34 (5), pp 819–825

S. H. Drissi, Ph. Refait, M. Abdelmoula, and J.-M. R. Génin (1995): "The preparation and thermodynamic properties of Fe(II)-Fe(III) hydroxide-carbonate (green rust 1); Pourbaix diagram of iron in carbonate-containing aqueous media". Corrosion Science, volume 37, pages 2025–2041.

W. Feitknecht and G. Keller (1950): "Über die dunkelgrünen Hydroxyverbindungen des Eisens". Zeitschrift für anorganische und allgemeine Chemie, volume 262, pages 61–68.

J. K. Fredrickson, J. M. Zachara, D. W. Kennedy, H. Dong, T. C. Onstott, N. Hinman, and S. M. Li (1998): "Biogenic iron mineralization accompanying the dissimilatory reduction of hydrous ferric oxide by a groundwater bacterium". Geochimica et Cosmochimica Acta, volume 62, issues 19–20, pages 3239–3257.

J. R. Gancedo, M. L. Martinez, and J. M. Oton (1983): "Formación de 'herrumbre verde' en soluciones de NH₄NO₃" ("Formation of green rust in NH₄NO₃ solutions"). Anales de Química, Série A, volume 79, pages 470–472.

J.-M. R. Génin, A. A. Olowe, B. Resiak, N. D. Benbouzid-Rollet, M. Confente and D. Prieur (1993): "Identification of sulphated green rust 2 compound produced as a result of microbially induced corrosion of steel sheet piles in harbour". In Marine Corrosion of Stainless Steels: Chlorination and Microbial Effects, European Federation Corrosion Series, The Institute of Materials, London; volume 10, pages 162–166.

J.-M. R. Génin, A. A. Olowe, Ph. Refait, and L. Simon (1996): "On the stoichiometry and Pourbaix diagram of Fe(II)-Fe(III) hydroxy-sulphate or sulphate-containing green rust 2: An electrochemical and Mössbauer spectroscopy study". Corrosion Science, volume 38, pages 1751–1762.

J.-M. R. Génin, Ph. Refait, L. Simon, and S. H. Drissi (1998): "Preparation and E_h-pH diagrams of Fe(II)-Fe(III) green rust compounds; hyperfine interaction characteristics and stoichiometry of hydroxy-chloride, -sulphate and –carbonate". Hyperfine Interactions, volume 111, pages 313–318.

A. Géhin, C. Ruby, M. Abdelmoula, O. Benali, J. Ghanbaja, Ph. Refait, and J.-M. R. Génin (2002): "Synthesis of Fe(II-III) hydroxysulfate green rust by coprecipitation". Solid State Science, volume 4, pages 61–66.

Hans C. B. Hansen (1989): "Composition, stabilization, and light absorption of Fe(II)Fe(III) hydroxy-carbonate ('green rust')". Clay Minerals, volume 24, pages 663–669.

Hans C. B. Hansen, Christian Bender Koch, Hanne Nancke-Krogh, Ole K. Borggaard and Jan Sørensen (1996): "Abiotic nitrate reduction to ammonium: Key role of green rust". Environmental Science & Technology, volume 30, pages 2053–2056.

Hans C. B. Hansen and Christian Bender Koch (1998): "Reduction of nitrate to ammonium by sulphate green rust: activation energy and reaction mechanism". Clay Minerals, volume 33, pages 87–101.

Christian Bender Koch and Hans C. B. Hansen (1997)
"Reduction of nitrate to ammonium by sulfate green rust"
In: K. Auerswald, H. Stanjek, & J. M. Bigham (Eds.), Soils and environment: soil processes from mineral to landscape scale. Advances in GeoEcology, volume 30, pages 373–393. Catena Verlag.

L. Legrand, S. Savoye, A. Chaussé, and R. Messina (2000): "Study of oxidation products formed on iron in solutions containing bicarbonate/carbonate". Electrochimica Acta, volume 46, issue 1, pages 111–117.

W. L. Lindsay (1979): "Chemical Equilibria in Soils . Wiley Interscience.

I. R. McGill, B. McEnaney, and D. C. Smith (1976): "Crystal structure of green rust formed by corrosion of cast iron". Nature, volume 259, pages 1521–1529.

A. A. Olowe and J.-M. R. Génin (1991): "The mechanism of oxidation of Fe(II) hydroxide in sulphated aqueous media: importance of the initial ratio of the reactants". Corrosion Science 32, pages 965–984.

A. A. Olowe, B. Pauron, J.-M. R. Génin (1991): "The influence of temperature on the oxidation of ferrous hydroxide in sulphated aqueous medium: Activation energies of formation of the products and hyperfine structure of magnetite" Corrosion Science, volume 32, issue 9, pages 985–1001.

G. Ona-Nguema, M. Abdelmoula, F. Jorand, O. Benali, A. Géhin, J. C. Block, and J.-M. R. Génin (2002): "Iron(II,III) hydroxycarbonate green rust formation and stabilization from lepidocrocite bioreduction". Environmental Science & Technology, volume 36, pages 16–20.

F. N. Ponnamperuma (1972): "The chemistry of submerged soils. Advances in Agronomy 24, pages 173–189.

Ph. Refait and J.-M. R. Génin (1993): "The oxidation of Fe(II) hydroxide in chloride-containing aqueous media and Pourbaix diagrams of green rust I. Corrosion Science 34, pages 797–819.

S. Savoye, L. Legrand, G. Sagon, S. Lecomte, A. Chaussé, R. Messina, and P. Toulhoat (2001): "Experimental investigations on iron corrosion products formed in bicarbonate/carbonate-containing solutions at 90 °C. Corrosion Science 43, pages 2049–2064.

U. Schwertmann and H. Fechter (1994): "The formation of green rust and its transformation to lepidocrocite. Clay Minerals, volume 29, pages 87–92.

P. P. Stampfl (1969): "Ein basisches Eisen II-III Karbonat in Rost. Corrosion Science 9, pages 185–187.

R. M. Taylor (1980): "Formation and properties of Fe(II)-Fe(III) hydroxycarbonate and its possible significance in soil formation. Clay Minerals, volume 15, pages 369–382.

R. M. Taylor (1982): "Stabilization of colour and structure in the pyroaurite compounds Fe(II)Fe(III)Al(III) hydroxycarbonates". Clay Minerals, volume 17, pages 369–372.

F. Trolard, J.-M. R. Génin, M. Abdelmoula, G. Bourrié, B. Humbert, and A. Herbillon (1997): "Identification of a green rust mineral in a reductomorphic soil by Mössbauer and Raman spectroscopies. Geochimica et Cosmochimica Acta 61, pages 1107–1111.

L. Vins, J. Subrt, V. Zapletal, and F. Hanousek (1987): "Preparation and properties of green rust type substances". Collect. Czech. Chem. Comm. volume 52, pages 93–102.

Corrosion
Hydroxide minerals
Iron compounds
Redox