1. The FeII-III oxyhydroxycarbonate fougerite mineral and green rusts in hydromorphic soils
J.-M. R. Génin et al.
Institut Jean Barriol
Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564 CNRS- Université Henri Poincaré-Nancy 1,
Département Matériaux et Structures, ESSTIN,
405 rue de Vandoeuvre, F Villers-lès-Nancy, France.
Green rusts and fougerite in the biogeochemical cycle of iron, A. Herbillon and J.-M. R. Génin Editors, C. R. Geoscience, 338 (2006)
“Gütlich, Bill, Trautwein: M��össbauer Spectroscopy and Transition Metal 2009”

2. Hydromorphic gley soil profile
The morphology of hydromorphic gley soils, first described in 1905 by G. N. Vysostskii1, remained a mystery up till recently when Mössbauer spectroscopy has been the determining tool to identify the iron containing compound that lies in a horizon formed under waterlogged conditions in an anaerobic environment, which encourages the reduction of iron compounds by microorganisms and often causes mottling of soil into a patchwork of greenish-blue-grey and rust colors. This finding is of utmost practical importance since there exists a correlation between the concentration of some pollutants and that of FeII ions that are dissolved in the water table. For instance, nitrates disappear where FeII appear in the anaerobic zone by following the water level in equilibrium with a mineral, which has been given the name of fougerite (IMA ). It occurs to be the FeII-III oxyhydroxycarbonate of formula FeII6(1-x) FeIII6x O12 H2(7-3x) CO3 where the domain of x is limited to [ ].
Originally studied for explaining the corrosion of iron-based materials, FeII-III hydroxysalts belong to the family of layered double hydroxides (LDH) and are constituted of layers, [FeII(1-x) FeIIIx (OH)2 ] x+, and interlayers, [(x/n)An-(mx/n)H2O]x-. Here, we shall consider only the case where the anion is CO32-.
1G. N. Vysostskii, Gley, Pochvovedeniye, 4 (1905)
fougerite
Ferric oxyhydroxides
Organic matter
Humus
Hydromorphic gley soil profile
Valley of the Vraine river, 10 km north of Vittel (France) 2 3 4 6
Exchangeable Mn
(mg kg-1) 1 10 20
CaCO3
(wt-%)
Nitrate- N
Exchangeable Fe 40 60
Fe(II) of total Fe
(%)
0,2
0,4
Total Organic Carbon
Depth (m)
Vibeke Ernstsen, Geological Survey of Denmark and Greenland
Depth profile analysis of a gleysol in Denmark through the redox zone between 2 and 3 meters deep. From left to right: Concentration of total organic carbon, calcium carbonate, nitrate, exchangeable iron, {[FeII] / [Fetotal]} and exchangeable Mn. Nitrates disappear when FeII appears.

3. FeII(1-y)FeIIIy(OH)2 (y/2)CO3 with 1/4< y < 1/3
The FeII-III hydroxycarbonate can be prepared by coprecipitation of a mixture of ferrous and ferric salts in the presence of carbonate ions when adding NaOH solution. Mössbauer spectra measured at 78 K demonstrate that the range of composition for x = [FeIII]/[Fetotal] is limited to [1/4, 1/3] since for x > 1/3 there exists two phases , the Green rust at x = 1/3, GR(CO32-), and another phase, a-FeOOH.
The spectrum of GR(CO32-) consists of 2 ferrous doublets D1 and D2 with large quadrupole splitting D and one ferric doublet D3 with small splitting. D3 D1 D2
78 K -4 -3 -2 -1 1 2 3 4
Velocity (mm s-1) 94 95 96 97 98 99
100
Transmittance %
(b)
x = 0.33 82 87 92
x = 0.25
(a)
D’1 S2 S1
(c)
x = 0.4
-12 -8 8 12 89 91 93
101
(d)
x = 0.5
FeII-FeIII ions coprecipitation giving for x > 1/3 a mixture of phases: GR(CO32-) and goethite x
D1 D2 D3 d D RA x
D D2 D3 D RA
x 0.4
D1+D2 D3 S1 S2 H d D RA
x 0.5
D1+D2 D3 S1 S2 H d D RA
Hyperfine parameters
H (kOe), d and D (mm s-1), RA(%)
FeII(1-y)FeIIIy(OH)2 (y/2)CO3 with 1/4< y < 1/3

4. GR(CO32-) R(-3)m a = 0.317588(2) nm c = 2.27123(3) nm
x = 0.33
Structure of GR(CO32-) FeII-III hydroxycarbonate at x = (1/3); (a) Three-dimensional view of the stacking of brucite-like layers. OH- ions lie at the apices of the octahedrons surrounding the Fe cations. CO32- ions in interlayers. (b) Projections along the c axis of the CO32- anions for three interlayers constituting a repeat.
Génin, J.-M. R.; Aissa, R.; Géhin, A.; Abdelmoula, M.; Benali, O.; Ernstsen, V.; Ona-Nguema, G.; Upadhyay, C.; Ruby, C. Fougerite and FeII-III hydroxicarbonate green rust; ordering, deprotonation and/or cation substitution; structure of hydrotalcite-like compounds and mythic ferrosic hydroxide Fe(OH)(2+x). Solid State Sci., 7 (2005)
With synchrotron
GR(CO32-) R(-3)m a = (2) nm c = (3) nm
R. Aissa, M. Francois, C. Ruby, F. Fauth, G. Medjahdi, M. Abdelmoula, J.-M. Génin, Formation and crystallographical structure of hydroxysulphate and hydroxycarbonate green rusts synthetised by coprecipitation  • J. Phys. Chem. Solids, 67 (2006)
(a)
(b)
FeII4 FeIII2 (OH)12 CO3
XRD and Mössbauer spectroscopy allowed us to determine the structure of all FeII-III hydroxysalts green rusts.

5. FeII-III oxyhydroxycarbonate
H2O2
Quadrupole splitting D (mm s-1)
x = 1 84 88 92 96
100
78 K
(e) 94 98
Transmittance %
Velocity (mm s-1) -4 4 -2 2 -3 -1 1 3
x ~ 0.78
(d) D3 D1 D2 99 95 97
(a)
x = 0.33
x ~ 0.50
D3 33 %
D %
Probability density (p)
(b)
D1 38 %
D %
D1 50 %
D2 17 %
D3 32 %
D4 31 %
(c)
D1 28 %
D2 9 %
x ~ 0.63
0.2
0.4
0.6
0.8
1.0
1.2
1.4
-0.2
-0.1
0.0
0.1
0.3
Eh(V)
{2 × [n(H2O2) / n(Fetotal)] + (1/3)}
D3 35 %
D4 43 %
D1 + D2 22 %
D4 67 %
with H2O2 a b c d e
The in situ oxidation of green rusts by deprotonation Use a strong oxidant such as H2O2, Dry the green rust and oxide in the air, Violent air oxidation, Oxide in a basic medium…
FeII6(1-x) FeIII6x O12 H2(7-3x) CO3
FeII-III oxyhydroxycarbonate
0 < x < 1
“Gütlich, Bill, Trautwein: M��össbauer Spectroscopy and Transition Metal 2009”

6. Diffraction Angle (2q°)
003
0.2 µm
(a)
GR(CO32-)
x = 0.33
(b)
H2O2
x = 0.50
(c)
0.5 µm
x = 1
(d)
Aerial 10 20 30 40
Intensity (arb. unit)
Diffraction Angle (2q°)
113
110
018
012
015
006
TEM and XRD patterns of the FeII-III oxyhydroxycarbonate due to the in situ deprotonation

7. Mass balance diagram of iron compounds
GR(CO32-)§
Ferrous GR
xFe(III) = {nFe(III) / (nFe(II) + nFe(III))}
A: FeII6 O12 H14 CO3
B: FeII4 FeIII2 (OH)12 CO3
C: FeII2 FeIII4 O12 H10 CO3
D: FeIII6 O12 H8 CO3
AD: FeII6(1-x) FeIII6x O12H2(7-3x) CO3
G: Fe(OH)2
H: Fe3 O4
E: Ferroxyhite d’ FeOOH
Fe(OH)2
GR(CO32-)
Stoichiometric
Fe(II)
Fe(III)
R = {nOH- / (nFe(II) + nFe(III))}
Fe3 O4
Ferroxyhite d’ FeOOH
0.2
0.4
0.6
0.8 1
0.5
1.5
2.5 3 2
GR(CO32-)*
Ferric GR B E D
0.33
0.67
1.67 G
Pro- & deprotonation of GR(CO32-) A
Mass balance diagram of iron compounds
FeII4FeIII2(OH)12CO3 + O2
FeIII6O12H8CO3 + 2 H2O
The oxidation or reduction of GR(CO32-) gives rise to GR(CO32-)* or GR(CO32-)§, i.e. FeII6(1-x) FeIII6x O12 H2(7-3x) CO3 where x  [0, 1]; the fougerite mineral is limited to the range [1/3, 2/3].
fougerite
Voltammograms obtained on an iron disc at 10 mVs−1 in 0.4 M NaHCO3 solution at 25 °C and pH = 9.6.

8. GR* is also obtained by bacterial reduction of ferric oxyhydroxide
20 µm
(e)
(A. Zegeye)
(G. Ona-Nguema)
(a)
(b)
5 µm
(d)
Production of Fe(II) and consumption of methanoate during culture of Shewanella putrefaciens in presence of lepidocrocite gFeOOH. The initial amount of FeIII (as lepidocrocite ) and of methanoate were respectively 80 mM and 43 Mm.
(b) X-ray pattern of the solid phase of incubation experiments with S. putrefaciens: mixture of green rust (GR1) and siderite (S) obtained after 15 days of incubation.
(c) Mössbauer spectrum after 6 days of bioreduction.
(c) TEM observations and
(d) optical micrograph of GR crystals obtained by reduction of lepidocrocite by S. putrefaciens; One sees the bacteria that respirate GR*. 3 6 9 12 10 20 30 40 50 60
Intensity (a.u.)
2 q
GR1 (012)
GR1 (015)
GR1 (018)
GR1 (003)
GR1 (006)
S (104)
S (018)
Time (days) 70 18 24 36
Fe(II)
Methanoate
Abiotic control
Methanoate (mM)
Fe(II) (mM) 80
GR* is also obtained by bacterial reduction of ferric oxyhydroxide
x ~ 0.50
(c)
Six days
Velocity (mm s-1)
Transmittance (%) -4 -2 2 4 92 94 96 98
100 D2 Dg
D’3 D1
78 K
bioreduction
G. Ona-Nguema, M. Abdelmoula, F. Jorand, O. Benali, A. Géhin, J.-C. Block and J.-M. R. Génin, Iron (II,III) hydroxycarbonate green rust formation and stabilization from lepidocrocite bioreduction, Environ. Sci. and Technol. 36 (2002) 16-20

9. Fougerite : FeII6(1-x)FeIII6xO12H2(7-3x)CO3
(d) The fougerite mineral is able to reduce pollutants within the water table such as nitrates. Dissimilatory iron reducing bacteria regenerate the fougerite active mineral4.
(a)
x ~ 0.50 -4 -2 2 4
Fougères D3 D1
98.0
98.5
99.0
99.5
100.0
Transmittance %
78 K
Velocity (mm s-1)
(b)
x = 0.50 -3 -1 1 3
synthetic
293 K
(c) D2
fougerite
NO3-
N2,
NH4+
+ CO32-
CH2O
(d)
1, ,
FeIII
FeII +
CO32-
Fougerite : FeII6(1-x)FeIII6xO12H2(7-3x)CO3
Comparison between field experiments and laboratory assays
The similarity between the original spectrum obtained in (a) and that of the deprotonated oxyhydroxycarbonate2 (b) is striking. More recently, field experiments were done in Fougères using back-scattering miniaturized Mössbauer spectrometer MIMOS3 (c) to follow the value of ratio x with time and depth in situ within the gley soil.
1. J.-M. R Génin., G. Bourrié, F. Trolard, M. Abdelmoula, A. Jaffrezic, Ph. Refait, V. Maître, B. Humbert and A. Herbillon, Thermodynamic equilibria in aqueous suspensions of synthetic and natural Fe(II) - Fe(III) green rusts; occurences of the mineral in hydromorphic soils, Environ. Sci. Technol. 32 (1998)
2. J.-M. R. Génin, R. Aïssa, A. Géhin, M. Abdelmoula, O. Benali, V. Ernstsen, G. Ona-Nguema,C. Upadhyay and C. Ruby, Fougerite and FeII–III hydroxycarbonate green rust; ordering, deprotonation and/or cation substitution; structure of hydrotalcite-like compounds and mythic ferrosic hydroxide Fe(OH)(2+x), Solid State Sci., 7 (2005)
3. D. Rodionov, G. Klingelhöfer, B. Bernhardt, C. Schröder, M. Blumers, S. Kane, F. Trolard, G. Bourrié, and . J.-M. R. Génin, Automated Mössbauer spectroscopy in the field and monitoring of fougerite, Hyperfine Interactions, 167 (2006)
4. C. Ruby, C. Upadhyay, A. Géhin, G. Ona-Nguema and J.-M. R. Génin, In situ redox flexibility of FeII-III oxyhydroxycarbonate green rust and fougerite, Environ. Sci. Technol., 40 (2006)

10. 3 m
78 K
Transmittance (%)
velocity (mm s-1)
DFeII(clay)
DFeII(foug)
DFeIII(clay)
DFeIII(foug) -4 -2 2 4 97 98 99
100
(c)
2 m
-15
-10 -5 5 10 15
DFeII
S1 + S2
DFeIII
(a)
2.50 m S
(b)
fougerite
clays
Oxidised zone
Reduced zone
FeII
FeIII
Fe+2aq
goethite
paramagnetic
Ferric oxyhydroxides
(d)
δ Δ H G RA
(mm s−1) (mm s−1) (kOe) (mm s−1) (%)
Depth 2 m
DFeIII (para +clay)
(para +clay)
DFeII (clay)
S1 (goethite) −
S2 (goethite) −
S1 +S2 (goethite)
Depth 2.50 m
DFeIII
(para+clay+foug)
DFeII (clay+ foug)
S (goethite) −
Depth 3 m
DFeII (foug)
DFeIII (foug)
DFeII (clay)
DFeIII (clay)
Hyperfine parameters of Mössbauer spectra measured at 78 K of samples extracted in Denmark at different depths of 2 m, 2.50 m, 3 m out of hydric soils from (a) an oxidised zone to (c) a reduced zone. A mixture of fougerite, ferric oxyhydroxides and clay minerals is observed. H: hyperfine field (kOe); δ: isomer shift (mm s−1) with respect to α Fe at room temperature; Δ or ε: quadrupole splitting or shift (mm s−1); Γ : half-width at half maximum (mm s−1); RA: relative abundance (%).
Fougerite is the active mineral that is mixed with clay minerals and responsible for the natural reduction of nitrates in gley soils within the water table.
J.-M. R. Génin, R. Aïssa, A. Géhin, M. Abdelmoula, O. Benali, V. Ernstsen, G. Ona-Nguema,C. Upadhyay and C. Ruby, Fougerite and FeII–III hydroxycarbonate green rust; ordering, deprotonation and/or cation substitution; structure of hydrotalcite-like compounds and mythic ferrosic hydroxide Fe(OH)(2+x), Solid State Sci., 7 (2005)