Green rust is a generic name for various green
crystalline
chemical compounds containing iron(II) and iron(III) cations, the
hydroxide (OH−
) anion, and another anion such as
carbonate (CO2−
3),
chloride (Cl−
), or
sulfate (SO2−
4), in a
layered double hydroxide (LDH) structure. The most studied varieties are the following:
[1]
Other varieties reported in the literature are
bromide Br−
,
[7]
fluoride F−
,
[7]
iodide I−
,
[9]
nitrate NO−
3,
[10] and
selenate SeO2−4.
[11]
Green rust was first recognized as a corrosion crust on iron and steel surfaces. [2] It occurs in nature as the mineral fougerite. [1]
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, Fe(OH)2. The latter has an
hexagonal crystal structure, with layer sequence AcBAcB... , where A and B are planes of
hydroxide ions, and c those of Fe2+
(
iron(II), ferrous)
cations. In green rust, some Fe2+
cations get oxidized to Fe3+
(iron(III), ferric). Each triple layer AcB, which is electrically neutral in the hydroxide[
clarification needed], becomes positively charged. The anions then intercalate between those triple layers and restore the electroneutrality.
[1]
There are two basic structures of green rust, "type 1" and "type 2".
[12] Type 1 is exemplified by the chloride and carbonate varieties. It has a
rhombohedral crystal structure similar to that of
pyroaurite (Mg6Fe2(OH)16CO3·4H2O). The layers are stacked in the sequence AcBiBaCjCbAkA ...; where A, B, and C represent OH−
planes, a, b, and c are layers of mixed Fe2+
and Fe3+
cations, and i, j, and k are layers of the intercalated anions and water molecules.
[1]
[13]
[14] The c crystallographic parameter is 22.5–22.8
Å for the carbonate, and about 24 Å for the chloride.
[4]
Type 2 green rust is exemplified by the sulfate variety. It has an hexagonal crystal structure as minerals of the sjogrenite (Mg6Fe2(OH)16CO3·4H2O) group, with layers probably stacked in the sequence AcBiAbCjA... [1] [7] [13]
In
oxidizing environment, green rust generally turns into Fe3+
oxyhydroxides, namely α-FeOOH (
goethite) and γ-FeOOH (
lepidocrocite).
[13]
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.
[3] Some variety of green rust is stabilized also by an atmosphere with high CO
2
partial pressure.
[3]
[15]
Sulfate green rust has been shown to
reduce nitrate NO−
3 and
nitrite NO−
2 in
solution to
ammonium NH+
4, with concurrent
oxidation of Fe2+
to Fe3+
. 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.
[16]
[17]
[18]
Suspensions of carbonate green rust and orange γ-FeOOH in water react over a few days producing a black precipitate of
magnetite Fe
3O
4.
[19]
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 m
M 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]
Green rust is a generic name for various green
crystalline
chemical compounds containing iron(II) and iron(III) cations, the
hydroxide (OH−
) anion, and another anion such as
carbonate (CO2−
3),
chloride (Cl−
), or
sulfate (SO2−
4), in a
layered double hydroxide (LDH) structure. The most studied varieties are the following:
[1]
Other varieties reported in the literature are
bromide Br−
,
[7]
fluoride F−
,
[7]
iodide I−
,
[9]
nitrate NO−
3,
[10] and
selenate SeO2−4.
[11]
Green rust was first recognized as a corrosion crust on iron and steel surfaces. [2] It occurs in nature as the mineral fougerite. [1]
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, Fe(OH)2. The latter has an
hexagonal crystal structure, with layer sequence AcBAcB... , where A and B are planes of
hydroxide ions, and c those of Fe2+
(
iron(II), ferrous)
cations. In green rust, some Fe2+
cations get oxidized to Fe3+
(iron(III), ferric). Each triple layer AcB, which is electrically neutral in the hydroxide[
clarification needed], becomes positively charged. The anions then intercalate between those triple layers and restore the electroneutrality.
[1]
There are two basic structures of green rust, "type 1" and "type 2".
[12] Type 1 is exemplified by the chloride and carbonate varieties. It has a
rhombohedral crystal structure similar to that of
pyroaurite (Mg6Fe2(OH)16CO3·4H2O). The layers are stacked in the sequence AcBiBaCjCbAkA ...; where A, B, and C represent OH−
planes, a, b, and c are layers of mixed Fe2+
and Fe3+
cations, and i, j, and k are layers of the intercalated anions and water molecules.
[1]
[13]
[14] The c crystallographic parameter is 22.5–22.8
Å for the carbonate, and about 24 Å for the chloride.
[4]
Type 2 green rust is exemplified by the sulfate variety. It has an hexagonal crystal structure as minerals of the sjogrenite (Mg6Fe2(OH)16CO3·4H2O) group, with layers probably stacked in the sequence AcBiAbCjA... [1] [7] [13]
In
oxidizing environment, green rust generally turns into Fe3+
oxyhydroxides, namely α-FeOOH (
goethite) and γ-FeOOH (
lepidocrocite).
[13]
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.
[3] Some variety of green rust is stabilized also by an atmosphere with high CO
2
partial pressure.
[3]
[15]
Sulfate green rust has been shown to
reduce nitrate NO−
3 and
nitrite NO−
2 in
solution to
ammonium NH+
4, with concurrent
oxidation of Fe2+
to Fe3+
. 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.
[16]
[17]
[18]
Suspensions of carbonate green rust and orange γ-FeOOH in water react over a few days producing a black precipitate of
magnetite Fe
3O
4.
[19]
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 m
M 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]