Green rust is a generic name for various green crystalline chemical compounds containing iron(II) and iron(III) cations, the

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...

() anion, and another anion such as carbonate (),

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...

(), or

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

(), in a

layered double hydroxide Layered double hydroxides (LDH) are a class of ionic solids characterized by a layered structure with the generic layer sequence cB Z AcBsub>''n'', where c represents layers of metal cations, A and B are layers of hydroxide () anions, and Z are ...

structure. The most studied varieties are * 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 A bromide ion is the negatively charged form (Br−) of the element bromine, a member of the halogens group on the periodic table. Most bromides are colorless. Bromides have many practical roles, being found in anticonvulsants, flame-retardant ...

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ty ...

iodide An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine defic ...

nitrate Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolubl ...

, and

selenate The selenate ion is . Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures. Unlike sulfate, selenate is a somewhat good oxidizer; it can be reduced to selenite o ...

.P Refait, L Simon, J-M R Génin. Reduction of SeO₄²⁻ Anions and Anoxic Formation of Iron(II)−Iron(III) Hydroxy-Selenate Green Rust. ''Environ. Sci. Technol.'', 2000, 34 (5), pp 819–825 Green rust was first recognized as a

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

crust on iron and

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant t ...

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 Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg( OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and ...

-like layers of

iron(II) hydroxide Iron(II) hydroxide or ferrous hydroxide is an inorganic compound with the formula Fe(OH)2. It is produced when iron(II) salts, from a compound such as iron(II) sulfate, are treated with hydroxide ions. Iron(II) hydroxide is a white solid, but even ...

, ()₂. The latter has an hexagonal structure, with layer sequence AcBAcB... , where A and B are planes of

ions, and c those of (

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in fr ...

(II), ferrous)

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s. In the 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 neutrality. 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 In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a three-dimensional figure with six faces which are rhombi. It is a special case of a parallelepiped where all edges are the same length. It can be us ...

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 a hexagonal crystal structure, with layers probably stacked in the sequence AcBiAbCjA...

Chemical properties

In oxidizing environment, green rust generally turns into

oxyhydroxide Iron(III) oxide-hydroxide or ferric oxyhydroxideA. L. Mackay (1960): "β-Ferric Oxyhydroxide". ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 32, issue 250, pages 545-557. is the chemical compound of iron, oxygen, ...

s, namely α- (

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

) and γ- (

lepidocrocite Lepidocrocite (γ-FeO(OH)), also called esmeraldite or hydrohematite, is an iron oxide-hydroxide mineral. Lepidocrocite has an orthorhombic crystal structure, a hardness of 5, specific gravity of 4, a submetallic luster and a yellow-brown strea ...

). Oxidation of the carbonate variety can be retarded by wetting the material with

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy g ...

-containing compounds such as

glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

, even though they do not penetrate the structure. Some variety of green rust is stabilized also by an atmosphere with high

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...

. Sulfate green rust has been shown to reduce nitrate and

nitrite The nitrite ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also re ...

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 will react over a few days produce a black precipitate of

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...

Occurrence

Iron and steel corrosion

Green rust compounds were identified in green corrosion crusts that form on iron and steel surfaces, in alternating

aerobic Aerobic means "requiring air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Aerobic respiration, the aerobic process of cellu ...

and

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

conditions, by water containing anions such as chloride, sulfate, carbonate, or

bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemic ...

. They are believed to be intermediates in the oxidative corrosion of iron to form iron(III) oxyhydroxides (ordinary brown

rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...

). The green rust may be formed either directly from metallic iron or from

()₂ .

Soil

On the basis of Mössbauer spectroscopic analysis, green rust minerals are suspected to occur as minerals in certain bluish-green soils that are formed in alternating

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

conditions, and turn ochre once exposed to air. The 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 a byproducts of bioreduction of ferric oxyhydroxides by dissimilatory iron-reducing bacteria, such as ''

Shewanella putrefaciens ''Shewanella putrefaciens'' is a Gram-negative pleomorphic bacterium. It has been isolated from marine environments, as well as from anaerobic sandstone in the Morrison Formation in New Mexico. ''S. putrefaciens'' is also a facultative anaerobe ...

'', 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 Formate (IUPAC name: methanoate) is the conjugate base of formic acid. Formate is an anion () or its derivatives such as ester of formic acid. The salts and esters are generally colorless.Werner Reutemann and Heinz Kieczka "Formic Acid" in ''U ...

(), incubated for 3 days with a culture of ''S. putrefaciens'', turned dark green due to the conversion of the hydroxide to GR(), in the form of hexagonal platelets with diameter ~7 µm. In this process, the formate was oxidized to bicarbonate which provided the carbonate anions for the formation of the green rust. The live bacteria were shown to be necessary for the formation of the green rust.

Laboratory preparation

Air oxidation methods

Green rust compounds can be synthesized at ordinary 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 ()₂ in a solution of the third anion. This suspension is oxidized by stirring in 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 Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alka ...

to form the ferrous hydroxide suspension. Then the

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable is ...

salt of the third anion is added, and the suspension is oxidized by stirring in 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 to precipitate ()₂ then immediately adding

sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 milli ...

and proceeding to the air oxidation step. A more direct method combines a solution of

iron(II) sulfate Iron(II) sulfate (British English: iron(II) sulphate) or ferrous sulfate denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7) but several values for x are know ...

with , and proceeding to the oxidizing step. The suspension must have a slight excess of (in the ratio of 0.5833 for each ) for the green rust to form; however, too much of it will produce instead an insoluble basic iron sulfate, iron(II) sulfate hydroxide ()₂·''n''. The production of green rust is reduced as temperature increases.

Stoichometric Fe(II) - Fe(III) methods

An alternate preparation of carbonate green rust first creates a suspension of iron(III) hydroxide ()₃ in an

iron(II) chloride Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point. The compound is white, but typical samples are often off-white. FeCl2 crystallizes from water ...

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 is added, all in the stoichometric 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

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