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-54600 Villers-lès-Nancy, France. E-Mail:
[email protected] Green rusts and fougerite in the biogeochemical cycle of iron, A. Herbillon and J.-M. R. Génin Editors, C. R. Geoscience, 338 (2006) 393-498. “Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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 2003-05). 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 [0.330.67]. 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) 291-327. Total Organic Carbon (wt-%) 0 0
Depth (m)
•
0, 2
0, 4
CaCO3 (wt-%) 0
0
1 0
2 0
Exchangeable Nitrate(mg kg-1) -1 Fe (mg kg ) 0 N1 2 0 1 2
0
0
0
0
Exchangeable Mn (mg kg-1)
Fe(II) of total Fe 2 00
1
1
1
1
1
2
2
2
2
2
(%) 4 0
6 0
0
2
4
Organic matter Humus
6
0
2
Ferric oxyhydroxides 3
3
3
3
3
3
4
4
4
4
4
4
Vibeke Ernstsen, Geological Survey of Denmark and Greenland
fougerite 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.
Hydromorphic gley soil profile Valley of the Vraine river, 10 km north of Vittel (France)
D2
97
Transmittance %
Transmittance %
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. x 0.25
x = 0.25
87 82 -4
D1 78 K
-3
-2
Transmittance %
101 99 97 x = 0.4 95 93
91
78 K
89 87 -12
D3
-8
(a) -1 0 1 2 Velocity (mm s-1)
3
4
98
94
S2
S1 D3
D’1
(c) -4 0 4 8 -1 Velocity (mm s )
12
-3
101 99
97
-1 0 1 2 Velocity (mm s-1)
3
4
S2
H
d 1.30 D 2.9 RA 52
D3 D’1
91 -12
(d)
x
-4 0 4 8 -1 Velocity (mm s )
FeII-FeIII ions coprecipitation giving for x > 1/3 a mixture of phases: GR(CO32-) and goethite
0.4 D3 S1 490 0.50 0.43 0.47 0 25 17
S2 482 0.54 0 6
0.5 D3 S1 S2 12 H 473 453 d 1.29 0.49 0.39 0.53 D 2.87 0.48 0 0 RA 39 19 26 16 Hyperfine parameters H (kOe), d and D (mm s-1), RA(%) D1+D2
-8
26
0.33 D1 D2 D3 d 1.28 1.28 0.47 D 2.97 2.55 0.43 RA 48 18 34
D1+D2
x = 0.5
78 K
12
x
S1
95 93
(b) -2
RA 62
D3
x
78 K
-4
D2
D3 D1
96 95
D1
d 1.28 1.28 0.47 D 2.97 2.55 0.43
D2
x = 0.33
97
Transmittance %
92
100 99
FeII(1-y)FeIIIy(OH)2 (y/2)CO3 with 1/4< y < 1/3
(a)
XRD and Mössbauer spectroscopy allowed us to determine the structure of all FeII-III hydroxysalts green rusts. (b)
x = 0.33
FeII4 FeIII2 (OH)12 CO3
GR(CO32-) R(-3)m
With synchrotron a = 0.317588(2) nm
c = 2.27123(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) 1016-1019.
Structure of GR(CO32-) FeII-III hydroxycarbonate at x = (1/3); (a) Three-dimensional view of the stacking of brucitelike 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) 545572.
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…
0.1
d
b
0.0 -0.1
a
-0.2
0.2
0.4
0.6
0.8
1.0
1.2
{2 × [n(H2O2) / n(Fetotal)] + (1/3)}
96 95
78 K
94 -4
-3
-2
-1 0 1 2 Velocity (mm s-1)
x = 0.33
Transmittance %
92
x ~ 0.63
88 84 -4
(c)
78 K -3
-2
-1 0 1 2 Velocity (mm s-1)
D4
x ~ 0.63
31 %
D3
3
D1 28 %
(c)
32 %
D2 9%
78 K -1
0
1
2
100
3
Quadrupole splitting D (mm s-1)
(a) 0
4
D1
D2 17 %
1
2
Quadrupole splitting D (mm s-1)
x ~ 0.78
92 88
-4
(d)
78 K -3
-2
-1 0 1 2 3 Velocity (mm s-1)
x ~ 0.78 D3
D4
4
(d)
43 %
D1 + D2
35 %
22 %
78 K -1
0
1
2
3
Quadrupole splitting D (mm s-1)
88
x ~ 0.50
(b)
78 K -3
-2
-1 0 1 2 3 Velocity (mm s-1)
D3
38 %
D4 16.5 %
78 K
4
D1
(b)
33 %
x ~ 0.50
3
96
84 4
D3
78 K
3
92
84 -4
50 %
33 %
1.4
Probability density (p)
Transmittance %
Probability density (p)
96
D3
(a)
-1
100
Transmittance %
c
x = 0.33 D1
Probability density (p)
0.2
97
D2
12.5 %
-1
0
1
2
3
Quadrupole splitting D (mm s-1)
Transmittance %
e
100 98
x=1
96
(e)
78 K
94
Probability density (p)
Transmittance %
0.3
Probability density (p)
Eh(V)
FeII-III oxyhydroxycarbonate FeII6(1-x) FeIII6x O12 H2(7-3x) CO3 100 100 0
