Clay Minerals (1989) 24, 663-669
COMPOSITION, S T A B I L I Z A T I O N , A N D LIGHT
A B S O R P T I O N OF FE(II)FE(III) H Y D R O X Y C A R B O N A T E ( ' G R E E N RUST')
H. C. B. H A N S E N
Royal Veterinary & Agricultural Highschool, Chemistry Department, Thorvaldsensvej 40,
1871 Frederiksberg C, Denmark
(Received 12 December 1988; revised 13 February 1989)
ABSTRACT: The chemical composition of the pyroaurite-type compound Fe(II)Fe(III)
hydroxy-carbonate('green rust') synthesised from freshly precipitated ferrihydrite and Fe(II)
chloride solution at pH 7.0 ('induced hydrolysis') was determined. The compound was nearly
stoichiometric, with the formula FeInFe[l(OH)12CO3, the Fe(II):Fe(III) ratio being independent of the Fe(II):Fe(III) ratio in the initial reaction mixture. It shows all the XRD peaks
reported for this compound. Visible-nearIR showsa broad peak at 650 nm which is ascribable to
intervalence charge transfer and therefore the absorbance maximum decreases with increasing
degree of oxidation. Wetting the material with compoundscontaining hydroxylgroups (such as
glycerol or glucose) retards the oxidation of the otherwise very oxidation-sensitive compound.
Similar polar organic compoundsmay stabilize the Fe(II)Fe(III) hydroxy-carbonatein wet soils.
The importance of the green rust phase as an intermediate in the formation and transformation
reactions of oxides and oxyhydroxidesof iron in natural environments should not be ignored.
Green rust compounds are Fe(II)Fe(III) hydroxy-compounds containing a certain amount of
a non-hydroxyl anion which is reported to be sulphate, bromide, fluoride (Bernal et al., 1959),
chloride (Feitknecht & Keller, 1950), iodide (Vins et al., 1987). nitrate (Gancedo et al., 1983)
or carbonate (Stampfl, 1969). Certain organic anions probably also fit into the structure
(Feitknecht, 1953). The compounds, isostructural with the mineral pyroaurite (Allmann,
1970), are layered structures consisting of a positively charged brucite-like sheet and
negatively charged interlayers of the anion and a variable amount of water (Allmann, 1968).
The Fe(II): Fe(III) ratio of the synthetic green rust seems to be variable (Feitknecht & Keller,
1950). For synthetic pyroaurites produced by co-precipitation, the Mg :Fe(III) ratio has been
found to depend on the Mg:Fe(III) ratio in the synthesis mixture (Hashi et al., 1983).
The green rusts can be synthesised by oxidation of an Fe(II) solution (Feitknecht & Keller,
1950), by oxidation/anodic electrolysis of Fe (Butler & Benyon, 1967; Dasgupta & Mackay,
1959), or by 'induced hydrolysis' (Taylor & McKenzie, 1980).
Stampfl (1969) has detected a carbonate green rust in inner corrosion films of iron
waterpipes. The compound probably also exists in other iron-containing hydromorphic
environments, but the very high sensitivity towards oxidation makes isolation and detection
difficult. Taylor (1982) used high pressures of CO2 for storage and physical measurements. In
this paper the chemical composition of the Fe(II)Fe(III) hydroxy-carbonate synthesised by
'induced hydrolysis' has been studied and a simple technique for protecting the material
against oxidation is described. The visible-near I R absorption spectrum of the stabilized
green rust was obtained.
9 1989 The Mineralogical Society
664
H. C. B. Hansen
MATERIALS
AND METHODS
The Fe(II)Fe(IlI) hydroxy-carbonate was synthesised by a modification of the method
described by Taylor (1985). Stock solutions and synthesis mixtures were prepared in 100 ml
gas-proof cylindrical injection vials with rubber septums. A 0.2 u Fe(II) chloride stock
solution was prepared by reacting a deoxygenated hydrochloric acid solution with an excess
of Fe powder (Leussing & Kolthoff, 1953). Amorphous Fe(III) hydroxide was precipitated
1 h before by addition of 1 M sodium carbonate solution to a stirred 0.05 M Fe(III) nitrate
solution with nitrogen bubbling through it. Weighed amounts of Fe(II) chloride solution and
Fe(III) hydroxide suspension were injected in the synthesis vessel through 0.85 m m polythene
tubing connected to hypodermic needles using nitrogen. Nitrogen had already been bubbled
through a known quantity of water in the synthesis vessel for 1 h. Oxygen was removed from
the nitrogen by bubbling through three gas washbottles in series containing Cr(II) solutions
(Shriver, 1969), and from carbon dioxide by bubbling through 5 M sodium hydroxide. Finally
the gas was forced through pure water and into the solutions/suspensions through 0.85 m m
polythene tubing with a hypodermic needle. The flow rate was 30 ml/min. Radiometer
Titralab equipment was run in the 'pH-stat' mode for the automatic addition of 1 M sodium
carbonate into the reaction vessel.
After the reaction had reached completion (no further base consumption), the suspension
was stirred for 1 h with nitrogen bubbling and for a further I/2 h without nitrogen bubbling.
A sample of the suspension was then withdrawn with a syringe through the septum for
determination of the carbon dioxide content. Iron and carbon dioxide were also determined
in the supernatant (after settling of the solids) from samples also withdrawn with a
hypodermic syringe but through a 0-45/~m Millipore filter. Carbon dioxide was determined
by absorption in base and back titration of the excess with hydrochloric acid. Iron was
determined by the o-phenanthroline method using flow injection analysis (Mortatti et al.,
1982). The uncertainty in the determination of iron is negligible compared to that of the
carbon dioxide determination.
The product was separated on an ice-cooled Bfichner fritted-glass filter (por. 4), washed
with 25 ml of ice-cooled water and finally, while still on the filter, wetted with glycerol,
glucose-saturated water or fatty acids. X-ray diffraction (XRD) was performed using a
Philips diffractometer (Co-K~ radiation) with a smear of the product on a glass slide. A
visible-near IR absorption spectrum was recorded on a Spectronic 1201 spectrophotometer
using a transparent smear.
RESULTS AND DISCUSSION
Due to the transient nature of the green rust, only an indirect evaluation of its chemical
composition was possible. It was assumed that all the added Fe(III) was contained in the
green rust (the material dissolved readily in the 2 M hydrochloric acid forming a clear
solution), that the compound contained carbonate only (no hydrogen carbonate), that no
Fe(II) was oxidized to Fe(III), that the carbon dioxide-carbonate system was in equilibrium
at the time of sampling and, finally, that the amounts of Fe(II) species and carbon dioxide
adsorbed on to the hydroxy-carbonate were negligibly small. It was not possible to determine
in this way whether the compound contained oxo groups as claimed by Misawa et al. (1974).
No chloride or nitrate was found in the products. The contents of Fe(II), Fe(III), hydroxide
and carbonate were determined in the following manner:
Composition, stabilization and light absorption of 'green rust"
665
Fe(III) = Fe(III) added as Fe(OH)3
Fe(II) = Fe(II) added - Fe(II) unreacted at termination of reaction
CO32-
= Cr(suspension) - Cr(supernatant)
where Cr = HCO~ + CO3~- + CO*
and CO* = CO2 + H2CO3
OH-
= {Cr(total Na2CO3 addition) - Cr(suspension)
+ CO*(supernatant)} 2 + HCO~(supernatant)
Concerning the supernatant, the following relationships were used:
CO~- = Cr([H+]2/K*K2 + [H+]/K2 + 1)-1;
HCO~ = Cr([H+]/K * + K2/[H +1 + 1)- t ;
CO* = Cr(K* K2/[H+]z + K*/[H +] + 1)-1
where K* = (HCO~-)(H+)/{(H2CO3) + (CO2)}
and K2 = (CO2-)(H+)/(HCO3-)
The results of 5 syntheses performed at pH 7.0 are given in Table 1. The charge of the
hydroxy-carbonate is assumed to be negligible compared with the calculated charge
imbalance. Because the uncertainty in the determination of the hydroxyl content is at least
twice the uncertainty in the determination of the carbonate content, the charge imbalance is
apportioned 2/3 to hydroxide and 1/3 to carbonate. The following compositions were
calculated:
~
:~
Synthesis no. 1 : F_IHFerl
~2 4.04,OH
I,
)12-31,x-~3)0-9
9
n :OH )13.0\'~-r
~ ::~
Synthesis no. 2: F -r2m F e 4.27~
n
H )11.8(CO3)o.8.
Synthesis no. 3: Fe 2IIl F e 3.72(0
:OH ~
,~,-,
Synthesis no. 4: ~-nlFe
l c 2 n3.8ot
)~.Tkk-~3)t.o.
Synthesis no. 5: Fe~tIFe~t.12(OH)12.0(CO3)i.I.
TABLE 1. Chemical composition of green rusts from different
experiments.
Synthesis number
Fe(II) :Fe(III) in
synthesis mixture
Content in mmol of:
Fe(III)
Fe(II)
OHCO32charges
1
2
3
3-9
5.4
5.5
1.02
2-07
6.66
0.53
-0,52
1.02
2,19
6-72
0.43
-0,13
1.15
2.13
6.78
0-48
-0.05
4
5
5-9
5.0
1-15
2.18
6.61
0.54
0,11
1-15
2.36
6.67
0.61
0-27
666
H. C. B. Hansen
1.46
/
1.$5 1 , 5 8
1.64
1.74
1.118
1,97
2.09
2.34
2,47
2,~7 2 . 7 2
J
FIG. 1. XRD traces of Fe(II)Fe(III) hydroxy-carbonate treated in different ways. A,B: Glycerol
treated, freshly prepared and after storage for 35 days at 4~ respectively. C: Glucose treated,
freshly prepared. D,E: Untreated, freshly prepared and after storage for 24 h at 4~
respectively, d-spacings in/~.
TABLE 2. Comparison of XRD data for Fe(II)Fe(III)
hydroxy-carbonate collected in this investigation, with
those of McGiU et al. (1976).
Glycerol-treated
hydroxy-carbonate
McGill et aL (1976)
d(A)
I/Io
d(A)
I/Io
003
006
101
012
104
015
107
018
7.504
3.755
2.718
2-666
2"462
2.343
2.086
1"964
100
25
3
20
l0
15
2
15
7.53
3"759
2.72
2"668
2.47
2"344
2"09
1.967
100
32
1
15
3
12
1
9
10,10
01,11
110
113
116
1.740
1-641
1"584
1.549
1-462
5
3
5
5
5
hkl
1"88
1
1.740
1"644
1"583
1.550
1.462
2
1
2
2
2
Composition, stabilization and light absorption of 'green rust'
667
I.O
0.9
(],El
13.7
0.6
0.5
0.4
0.3
0.2
0.1
i
0.0
~
i
i
51~1 550
i
~
i
i
i
6~0
71~
750
1
i
B00,8180
i
900
9~1
FIG. 2. Visible-near IR absorption spect~m of a transparent smear of freshly prepared
Fe(II)Fe(III) hydroxy-carbonate.
The synthesised Fe(II)Fe(III) hydroxy-carbonate seems to be rather uniform in
composition, with an Fe(II):Fe(III) ratio in the range 1.86-2-14. The stoichiometry is close to
that of Fe[UFe~(OH)l 2CO3 determined by Stampfl (1969) on natural material, but diverging
from that of pyroaurite, M(III)2M(II)6(OH)16CO3. On the basis of the XRD data of McGill
et al. (1976), Brindley & Bish (1976) have predicted an Fe(II):Fe(III) ratio of 2:1 for the
hydroxy-carbonate which has been confirmed by M6ssbauer measurements by Murad &
Taylor (1984) on freshly prepared compounds. The same ratio has also been found by
Tamaura et al. (1984) and Kanzaki & Katsura (1986) for the green rust II.
Treatment of the green rust crystals with glycerol inhibits the oxidative breakdown of the
pyroaurite structure since after storage at 4~ for I month, only a thin yellow brown skin had
developed on the blue-green material. XRD data confirmed this structure-stabilizing effect of
glycerol. No changes in d-spacings or relative intensities were observed after exposing the
hydroxy-carbonate to the atmosphere for 35 days at 4~ (Fig. 1A and B). On the contrary,
Fig. 1D and 1E show that the untreated material rapidly oxidizes. Glucose-saturated water
has an effect similar to glycerol (Fig. 1C), whereas fatty acids such as 9-octadecenoic acid(c/s)
or 9,12-octadecadienoic acid(cis,cis) have no significant effect.
In Table 2, the d-spacings and relative intensities of the glycerol-treated hydroxycarbonate are compared with the XRD data reported by McGiU et aL (1976) indexed in the
hexagonal system. The relatively high 00! intensities of the glycerol-treated material are
probably caused by a preferred orientation of the crystals produced during preparation of the
smear. The corresponding d-spacings indicate that neither glycerol nor glucose penetrate the
interlayer region of the pyroaurite structure.
The oxidation-retarding effect of the compounds containing hydroxyl groups is not only
668
H. C. B. Hansen
caused by a slow velocity of diffusion of oxygen towards, or carbon dioxide out of, the
hydroxy-carbonate in the viscous liquid films surrounding the crystals. The glycerol/glucose
may chemisorb on to the hydroxy-carbonate, their hydroxy groups having a high affinity for
the hydroxy-carbonate surface. Some of these hydroxy groups are easily oxidized to aldehyde
or carboxylic acid groups, further lowering the probability of oxygen reaching and oxidizing
the structural Fe(II).
Fig. 2 shows the visible-near I R absorption spectrum of the hydroxy-carbonate
(unpolarized light). A broad peak at 650 nm was ascribed to intervalence {Fe(II)-Fe(III)}
charge transfer (Bums, 1980). It has also been reported by Misawa et al. (1973) for green rust
II. With increasing oxidation the peak gradually disappears, since after oxidation for 1 day
the peak height above background had dropped to 45% of the original value, and after 5 days
the peak had vanished.
CONCLUSIONS
Fe(II)Fe(III) hydroxy-carbonate prepared by induced hydrolysis at pH 7.0 seems to be a
stoichiometric compound with the approximate composition FeI2"Fen(OH)12CO3 in which
the Fe(II):Fe(III) ratio is not dependent on the overall Fe(II):Fe(III) ratio in the synthesis
mixture.
The hydroxy-carbonate has a broad adsorption peak at 650 nm, the absorbance of which
decreases upon oxidation, and therefore can be used to follow the oxidation kinetics.
Wetting the blue-green material with compounds containing hydroxyl groups (such as
glycerol or glucose) protects it against rapid oxidation and this effect should be useful during
sampling in the field and also during characterization. The interaction between the green
rust and the glycerol/glucose may be worth studying, because a similar effect of organic
compounds on green rust materials in soil seems possible. The implication of this
stabilization effect may be crucial to understanding the genesis of oxides and oxyhydroxides
of iron in hydromorphic soils, because green rust phases are known from laboratory
experiments to be important intermediates in the formation and transformation reactions of
iron oxides.
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