Clay Minerals (1997) 32, 597-613
Mechanisms of oxidation of Ni(II)-Fe(II)
hydroxides in chloride-containing aqueous
media: role of the pyroaurite-type Ni-Fe
hydroxychlorides
PH. R E F A I T
AND J . - M .
R. GENIN
Laboratoire de Chimie Physique pour l 'Environnement, UMR 9992 CNRS-Universitb Henri Poincarb, Equipe sur la
Rbactivit~ des Espbces du Fer, and D~partement de Science des Mat~riaux, ESSTIN, 405, rue de Vandoeuvre,
F 54600 Villers-les-Nancy, France
(Received 1 January 1996; revised 27 January 1997)
A B S T R A C T : Ni-Fe pyroaurite-type hydroxychlorides were prepared by aerial oxidation of
Ni(II)-Fe(II) hydroxides precipitated in aqueous solution with various P = Fe/Ni ratios. When P~> 1/3,
Ni(l/)-Fe(II)-Fe(III) hydroxychlorides characterized by a specific Fe(IIl)/[Fe(II)+Ni(II)] ratio of 1/3,
corresponding to the idealized formula of NiII_xFe~XFem(OH)sCl.nH20 (with 0 ~< x ~< 3), were
obtained at the end of the first stage of oxidation, In a second reaction stage, these hydroxychlorides
oxidize with deprotonation of hydroxyl ions into O2- ions, i.e. the remaining Fen(OH)2 groups are
transformed into FemOOH groups. Along with the Ni(II)-Fe(IIO hydroxychloride which contains a part
of the FJnOOH groups a second phase is obtained. It is an amorphous Fe(II1) or Ni(II)-Fe(III)
oxyhydroxide when 1/3<P<~3/2, and a ferric oxyhydroxide identified as T-FeOOH (lepidocrocite)
when/9>3/2. On the other side of the domain, when P<l/3, the Fe(III)/[Fe(II)+Ni(II)] ratio cannot
reach the specific value of 1/3; this gives rise to a pyroaurite-type Ni(II)-Fe(III) hydroxychloride with a
lower chloride content, that is with an average composition of Nin+yFe~_y(OH)sCll_y.nH20 where y =
{[4/(l+P)] - 3}, down to minimum Fe(III) and C1 contents corresponding toy = 1/3 (P = 1/5). The in
situ mechanisms of oxidation of Ni(II)-Fe(II) hydroxides into Ni(H)-Fe(II)-Fe(III) hydroxychlorides are
discussed.
In hydroxides of divalent cations such as Mg(OH)2 will restore neutrality. Compounds composed of
(brucite) or I]-Ni(OH)2, cations are surrounded by hydroxide sheets which alternate regularly with
six O H - ions which form octahedra that share interlayers made of anions and water molecules will
edges and make sheets which lie perpendicular to result. Such clay-like minerals are collectively
the c axis of the hexagonal structure. The space referred to as the pyroaurite-sjbgrenite group of
group is P 3 m l and the stacking of O H - ions is hydroxides. In the pyroaurite sub-group, the layer
hexagonal close-packed, according to the sequence sequence is AcBiBaCjCbAk, where i, j, k designate
AcBF-]A where A, B represent the planes of OH- the interlayers, A, B, C the planes of O H - ions and
ions and c those of divalent cations. The set of a, b, c the planes of metal cations. This corresponds
octahedral sites between B and A are empty, I--1, to a rhombohedral R i m structure, as determined by
and constitutes an interlayer which alternates with Ingram & Taylor (1967) and Allmann (1968).
the set of occupied sites constituting the sheet. A
The intercalated anions are frequently carbonate,
sheet is electrically neutral as a whole but if some as in pyroaurite [MgnFem(OH)I6]2+[(CO3).4H20] 2trivalent cations such as Fe 3+ or A13+ substitute for itself. However, other anions can be found in the
some divalent cations, it gains positive charges. inteflayers, e.g. chloride as in iowaite (Kohls &
Negative charges, provided by intercalated anions, Rodda, 1967) which formula must be written as
9 1997 The Mineralogical Society
598
Ph. Refait and J.-M. R. G~nin
[Mg4HFem(OH)lo]+[CI-4H20]- (Allmarm & Donnay,
1969). Similarly, other metal cations can replace
Mg 2+ and Fe3+ as in hydrotalcite, a compound
isomorphous to pyroaurite where A13+ replaces
Fe3+, or in reevesite where Ni 2+ replaces Mg2+.
Finally, there also exist compounds, known as green
rusts (GR) (Bernal et al., 1959), including Fe2§ and
Fe3+ ions, which have recently been identified as
minerals in hydromorphic soils and given the
proposed name fougerite (Trolard et al., 1996,
1997).
The ratio of the amounts of trivalent and divalent
cations M(III)/M'(II) for natural samples is often
observed to be close to 1/3. In contrast, synthetic
samples, generally obtained by co-precipitation
from M(III) and M'(II) salts, may have quite
variable ratios. This is the case of hydrotalcite,
where the AI(III)/Mg(II) ratio is found in the range
from 0.6 to 0.25 (Miyata et al., 1971) and that of
chloride-containing green rust, GR(C1-), with
Fe(III)/Fe(II) ratio varying from 0.77 to 0.25
(Feitknecht & Keller, 1950). Green rust compounds
seem to prefer an Fe(III)/Fe(II) ratio of 1/2. Both
GR(CO]-) (Hansen, 1989; Drissi et al., 1995) and
GR(SO42-) (Cuttler et al., 1990; Hansen et al.,
1994; Grnin et al., 1996) were reported with such a
ratio. The chloride-containing green rust, GR(C1-),
seems to be an exception, showing a preference for
the 1/3 Fe(III)/Fe(II) ratio (Refait & Grnin, 1993a).
This report deals with Ni-Fe hydroxychlorides and
completes a preliminary first approach (Refait &
Grnin, 1993b). Varying the Fe and Ni proportions in
a range as wide as possible, it is aimed at
understanding the influence of the M(III)/M'(II)
ratio, confirmation of the specificity of the 1/3
value for the chloride system and understanding the
mechanisms of formation and oxidation of
pyroaurite-type hydroxides.
METHODS
The Ni-Fe hydroxychlorides were prepared by
oxidation of Ni(II)-Fe(II) hydroxides precipitated
from aqueous solutions of ferrous and nickelous
chlorides mixed with caustic soda. The chemicals
were provided by Prolabo |
i.e. NiC12.6H20
(Rectapur| 98% rain), FeC12.4HzO (Normapur|
98% min) and NaOH (Rectapur |
97% min).
Magnetic stirring in the open air ensured a
progressive homogeneous oxidation of the precipitate. A stirring rod, 6 cm long, rotating at a speed
of 550 rpm, was used. The volume of the glass
beaker was 400 ml, whereas that of the suspension
was 200 ml. A thermostat controlled the temperature which was kept at 25 _ 0.5~ Reactions were
monitored by recording the pH and the redox
potential E of the solution, measured using a
platinum wire and a saturated calomel electrode
as a reference. This allowed the reaction to be
stopped at any intermediate stage for characterization purposes. The NaOH and (NiCIE + FeClz)
concentrations were fixed at the values of
0.40mol 1-I and 0.23 mol 1-1, respectively,
whereas the initial ratio P = [FeC12]/[NiClz]
varied. These concentrations, used in a previous
work (Refait & G~nin, 1993b), were chosen in
order that a slight excess of dissolved NiC12 and
FeC12 was present in solution after precipitation of
the initial hydroxide. For instance, in the absence of
NiC12 (P = oo):
4 FeC12 + 7 NaOH ~ 7/2 Fe(OH)2 + 1/2 Fe2+
+ CI- + 7 NaC1 (1)
The excess (1/2 Fe z+ + C1-) allowed the
formation of the Fe(II)-Fe(III) hydroxychloride,
known as the chloride-containing green rust
GR(C1-), [Fe~IFem(OH)s]+[CI.nH20]- (Refait &
Grnin, 1993a). The reaction is then written as
follows:
7/2 Fe(OH)2 + 1/2 Fe2+ + C1- + 1/4 02
+ (I/2+n) n 2 0 -~ [Fe3IIFem(OH)a]+[Cl.nn20]- (2)
In this case, all metal cations were consumed in
the reaction and the part left in solution was
negligible. Therefore, the Fe/Ni ratio in the solid
phase can be expected to be close to the initial ratio
P = [FeC12]/[NiCI2] of the dissolved salts.
X-ray diffraction (XRD), M6ssbauer spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to analyse the
reaction products. Also, the inductively coupled
plasma-atomic emission spectrometry (ICP-AES)
method was used to determine the Fe/Ni ratio of
the products. All samples were aged for one month
in solution at 50~ before filtration and analysis.
However, some poorly crystallized samples, when
P ~< 1/6, needed to be aged for five more months
for XRD analysis.
XRD
Co-Ks radiation (E = 0.17902 nm) was used.
Filtration and drying of end products were carried
out in the open air over - 2 days. Products were
Mechanisms of oxidation of Ni(I1)-Fe(lI) hydroxides
599
coupled device (CCD) detector. Excitation of all the
samples was carried out with 487.98 nm radiation
from a Spectra Physics 2017 argon ion laser. The
Raman spectra of powder were obtained via a
confocal microscope (objective x 5 0 ; numerical
aperture 0.55; spatial resolution of ~3 p.m) in a
back-scattering geometry. The power o f the
radiation hitting the sample was less than one
milliwatt for an area of 4 p.m2. The spectral
resolution was 2 cm -1 with a precision on the
Raman wave number of ~0.3 cm -1. The acquisition
time of the Raman spectrum was 1 min.
finely powdered before reflection Debye-Scherrer
XRD at room temperature. The scanning speed of
the diffractometer was 0.5 deg min -1. Calibration
of the angular scale was carried out by mixing NaC1
with the samples.
M r s s b a u e r spectroscopy
The spectra were obtained by use of a constantacceleration MSssbauer spectrometer with a 50 mCi
source of 57Co in Rh. The spectrometer was
calibrated with a 25 ~tm foil of :t-Fe at room
temperature. MOssbauer spectra o f transient
compounds, which are sensitive to aerial oxidation,
were measured at 78 K. The transient compound
was obtained at point tl on the E vs. time curve as
displayed in Fig. 1, i.e. at the inflection point of the
first sharp increase between the first and second
plateau. It was filtered on a paper in an inert
atmosphere, set in the sample holder and introduced
in the cryostat for Mrssbauer measurements. During
this procedure, a very small level of oxidation
occurred in the short time which elapsed between
filtration and setting in the sample holder.
XPS
The samples were washed twice in water at 90~
in order to eliminate all traces of undissolved salts.
They were filtered, dried and analysed as a powder
using Mg-K~x or A1-K~ radiation at 1253.6 eV and
1486.6 eV, respectively. The C(ls) binding energy
of contamination carbon from hydrocarbides in the
atmosphere, 284.6 eV, was used as a calibration to
compensate the charging effect.
ICP-AES
Micro Raman spectroscopy
A Perkin-Elmer Plasma 2000 apparatus, equipped
with two monochromators, one operating from 160
to 800 nm, the other from 160 to 400 nm was used.
The detection limits were 3 ~tg 1-1 and 7 ~tg 1-1 for
The Raman spectra were recorded using a
T64000 triple monochromator with subtractive
dispersion of the first two stages and a charge
EsIaE
EsIaE(V)
(V)
P = 1/4
+0.2
P= 5/3
tl ~"
!
o
L2
/X
-0.2 ~
-0.4
0
i
I
I
I
30
60
90
120
Time (min)
Fic. 1. Curves of E vs. time recorded during synthesis, for P = 1/4 and P = 5/3. E is given with respect to the
standard hydrogen electrode (SHE).
Ph. Refait and J.-M. R. Grnin
600
Fe and Ni, respectively. The samples to be analysed
were dissolved in HNO3 solutions of pH = 1 at a
rate of 0.45 g 1-1. The calibration was made by
means of two standard solutions of dissolved NiCIE
and FeC12 of the same type as above.
RESULTS
Preliminary analysis
The pH of the suspension, which was allowed to
vary during the experiments, decreased with time
down to a value depending on P. It reached 8.6 for
P = 1/12, 7.6 for P = 1/3 and 5.7 for P = 3/2. Since
some of the end product is likely to dissolve in
slightly acid or neutral solutions, a check must be
made as to whether the solid phase keeps an Fe/Ni
ratio close to P and the ICP-AES analyses
demonstrated this: experimental Fe/Ni ratios were
determined to be 0.217 __+ 0.006 for P = 1/5, 0.356
__+ 0.007 for P = 1/3 and 1.51 ___ 0.08 for P = 3/2.
Electrochemical study
The electrode potential E vs. time curves
recorded during the formation of the hydroxychlorides can be classified into two types (Fig. 1). For
P ~< 1/3, they present one reaction stage, which
ends with a sharp increase o f E, at the
corresponding time h measured at the inflection
point. For P>l/3, two stages can be distinguished
ending at tl and t2 respectively. Similarly, the plot
of time tl vs. the Fe(II) concentration produced a
curve comprising two parts (Fig. 2). When P ~< 1/3,
tl is proportional to the initial Fe(II) concentration. A
linear regression proved satisfactory (r = 0.965) and
yielding a time of 9 __+ 5 min. at the origin. On the
other part of the curve, when P>l/3, tx is constant
and independent of the initial Fe(II) concentration.
X R D analysis
Figure 3 displays the XRD patterns of the final
products of oxidation. The compound obtained for
the usual M(III)/M'(II) ratio P = 1/3 is the
Ni(II)-Fe(III) h y d r o x y c h l o r i d e with average
chemical formula of [NiII3FeIII(OH)s] +
[CI.nH20]-. Its main diffraction lines are visible,
even though they are broad. The distance between
two adjacent interlayers, i.e. c/3 in the hexagonal
lattice representation, is equal to 0.795 + 0.002 nm.
A value of 0.796 nm has already been given by
Schrllhorn & Otto (1987).
The XRD patterns can be classified into three
categories. (i) When P<l/5, the pattern displays
lines of both [3-Ni(OH)E and Ni(II)-Fe(III) hydroxychloride, and the relative abundance of ~-Ni(OH)2
increases as P decreases. (ii) When 1/5 ~< P ~< 3/2,
only lines of the hydroxychloride are visible. The
diffraction lines are broad, especially when P
Ot 2
200"
0
c-
E 15o.
v
E
0
o - -
t- 100'
o
50"
.•/3
0
0.00
0
0.~)5
4-
4-
q"
t1
ob
Fe concentration (mol/I)
FIG. 2. Reaction times tl and t 2 plotted vs. initial Fe(II) concentration in the synthesis solutions.
Mechanisms of oxidation of Ni(lI)-Fe(ll) hydroxides
P=Fe/Ni
NHOOl
NHlOl
NHll0 NH102
I
601
/ NH100 ~
I
~
3'5 ~
I
1/12
t
i
A"T-
~
2
1/6
3'5 ~
,~
i
,
~ 0 3)5 ~ _-I
I
35 ~
-/•
,
(~o2)
I
I [ "T"
I
(oo6)
~
IImo3)
t
I \ "
I',
~
0
~
I
i
I
I
3/2
' A'--I"
L 5/3
I
I
i
t
I
f"
35 ~
i
35 ~
I
30 ~
I
25 ~
i
20 ~
J
15 ~
w
10 o
5~
F~6. 3. XRD patterns of the end products for P = 1/12, 1/6, 1/5, 1/3,3/2, 5/3 and ~ . NH and y lines are those of
~-Ni(OH)2 and y-FeOOH, respectively. Other lines are those of hydroxychlorides indexed in the hexagonal
representation in the P = 1/3 spectrum. End products obtained for P = 1/12 and 1/6 were aged for six months,
lepidocrocite obtained for P = oo was not aged and other products were aged for one month. Co-K0~ radiation.
602
Ph. Refait and J.-M. R. G~nin
TABLE 1. Distance between two adjacent interlayers (c/3) in Ni(II)-Fe(IH) hydroxychlorides as a function
of P, calculated from doo6 and doo3 measured in each case from two different XRD patterns, and
thickness Dc of crystallites along the c hexagonal axis as determined by the Scherrer formula.
P
c/3 or De
P
c/3 or De
P
c/3 or D c
0.806
0.804
0.800
1/3
3/2
5/3
0.795
0.776
0.772
1
3/2
6.6
6.6
(a) c/3 in nm. The experimental error is <__ 0.003 nm
1/12
1/9
1/6
0.807
0.806
0.805
1/5
2/9
1/4
(b) Dc in nm. The error is <_+ 0.5 nm
1/6
I/5
2/9
6
7.3
7.8
1/4
1/3
1/2
9.3
10.8
10.3
(All the measured Dc values are taken from samples aged for one month)
departs from 1/3. This may correspond to a
distribution of crystal compositions, dealing with
average compositions and parameters. By using the
Scherrer formula, i.e. De = 0.9 X [e cos 0] -1 where
e is the full width at half maximum of a diffraction
line at Bragg angle 0, it is found (Table 1) that the
dimension of the particles along the c axis De has a
maximum length at P = 1/3, confirming that this
M(III)/M'(II) ratio plays indeed a specific role
which has an influence on the crystallite size
whatever the exact reason. (iii) For P > 3/2, lines
of y-FeOOH (lepidocrocite) are seen in addition to
those of the hydroxychloride until only y-FeOOH
exists at P = ~ .
From these XRD patterns of hydroxychlorides,
the distance c/3 between two adjacent interlayers
was computed (Table 1). The value c/3 varied from
P = 1/5 to P = 3/2, indicating that the average
composition of the hydroxychloride varies and its
decrease with increasing P is attributed, at least
partially, to the increasing proportion of Fe(III) at
the expense of the larger Ni(II) cation size. In
contrast, when P ~< 1/5, c/3 stays constant at
0.806 ___ 0.003 nm, indicating that the average
composition of the hydroxychloride probably does
not change any longer.
R a m a n spectroscopy
The end products obtained for P = 1/5, 1/3 and
3/2 were analysed. When viewed through the
optical microscope, the P = 1/5 and 1/3 samples
appear to be homogeneous while the P = 3/2
sample appears to be heterogeneous displaying
large greenish particles covered with brownish red
smaller regions of a secondary phase. Raman
analysis confirmed this observation. Only one type
of spectrum can be obtained when P = 1/5 or 1/3,
indicating that all particles are of the same nature
and correspond to the Ni-Fe hydroxychloride
observed by means of XRD. It is characterized by
two main peaks at 455 and 525 cm -1 (Fig. 4)
accompanied by two others of lower intensity at
~300 cm -1 and 425 cm -1 which are hard to see at
the scale of Fig. 4. According to Boucherit et al.
(1991) who obtained similar spectra for green rust
compounds, the peak at 525 cm -1 can be attributed
to Fe(III)-OH stretching mode and the peak at
455 cm -1 to the Ni(II)-OH stretching mode. In
contrast, two types of spectra were observed for the
P = 3/2 sample. The greenish particles produce the
previous characteristics and thus correspond to the
hydroxychloride observed by XRD. The brownish
red particles gave rise to a different spectrum with
three main bands at about 320, 460 and 540 cm -1,
which cannot be ascribed to either ct- or y-FeOOH
phases, nor to m a g n e t i t e (Johnston, 1990;
Townsend et al., 1994).
Finally, it must be noted that carbonate ions,
which would give rise to a band at ~1043 cm -1, are
not detected on the P = 1/3 and 3/2 samples.
However, such a band is visible in the case of
Mechanisms of oxidation of Ni(ll)-Fe(ll) hydroxides
600
to
sool
~i
-
400
300 i
200
o
401
>,
,
400
120
100
~
2o
C
120
100
80
60
40
20
0
1000
I
400
I
I
,
I
1200
1400
,
I
120
100
80
60
40
800
,
P = 1/3
~
200
rr
~600
I
to '~"
80
60
P = 1/5
m
200
9
603
,
m 600
I
,
800
I
1000
,
I
1200
,
I
1400
(Green particles)
o
cO
co
,~
"*~n
p = 312
ish red par,ticles)
I
200
,
I
400
,
I
600
I
I
800
I
I
I
1000
1200
1400
Wavenumber (cm 1)
Fro. 4. Raman spectra of the end products obtained at P = 1/5, 1/3 and 3/2.
P = 1/5, but is very weak. Thus, even though
pyroaurite-type compounds are known to show
great preference for carbonate (Miyata, t983;
Mendiboure & Schfllhom, 1986), these anions,
604
Ph. Refait and s
present in solution either due to NazCO3 impurities
in the caustic soda or introduced with the air used
for oxidation, do not induce the formation of an
hydroxycarbonate. Taking into account the limit of
detection, interlayers composed of 95% chloride
and 5% carbonate, as observed by Miyata (1983)
for hydrotalcite-like hydroxychlorides, can be
admitted. Moreover, from the maximum content
of Na2CO3 impurities given by the supplier, the
carbonate concentration introduced in solution can
be estimated at ~0.002 tool 1-1, but the amount of
chloride incorporated into the solid phase is
-0.05 tool 1-~, so that the maximum carbonate
available would effectively represent a little less
than 5% of the interlayers anionic composition. In
the following discussion, this possible amount of
interlayer carbonate will be neglected.
M b s s b a u e r analysis
Figure 5 shows the M6ssbaner spectra of the final
products of oxidation measured at room temperature.
Each spectrum is fitted with various quadrupole
doublets which have hyperfine parameters characterizing a ferric state, i.e. isomer shifts (IS) in the range
from 0.1 to 0.7 rnm s -1 (Table 2). There is no Fe(II)
R. Gknin
in any final compound. Four different ferric
quadrupole doublets Do, D1, D2, and D s were used
for a correct computer fitting of all the spectra with
Lorentzian-shaped lines. The spectra obtained for
P ~< 1/3 are composed of three of these doublets.
The main one, Do, represents at least 72% of the total
area. The other doublets of small intensity, D1 and
D2, have larger and smaller quadrupole splitting
(QS), respectively, than that of Do. The doublet D1 is
responsible for the shoulder visible in the curve at
~0.75 mm s -1 while D2 is responsible for the
asymmetry of the spectrum. When P > 1/3, Do is no
longer predominant. The relative abundance of
doublet D1 increased and an extra doublet D3
appeared with increasing P while doublet D2 was
no longer visible. This is illustrated by the curves of
the relative abundance vs. P in Fig. 6. The doublets
D1 and D3 have large QS values, -0.8 and
1.1 mm s -1, respectively. These values are similar
to those observed by Murad & Taylor (1984) in
oxidized Fe(II)-Fe(III) hydroxycarbonate (QS = 0.63
and 1.02 mm s-l).
Intermediate compounds, i.e. the products
obtained at the end of the first reaction stage
when P > 1/3 at point tl in Fig. 1, were analysed by
M6ssbauer spectroscopy at 78 K (Fig. 7). The
TmL~ 2. Mfssbauer hyperfine parameters at room temperature of the final products of oxidation.
IS
P = 1/9
QS
RA
Do
Da
D2
0.34
0.39
0.26
(FWHM = 0.22)
0.42
79
0.86
9
0.26
12
DO
D1
D2
0.34
0.34
0.25
(FWHM = 0.24)
0.42
72
0.79
12
0.27
16
Do
Dl
D2
0.34
0.36
0.26
Do
D1
D2
0.35
0.37
0.25
P = 1/6
P = 1/5
P = 1/4
IS
QS
RA
Do
Dl
D2
0.34
0.38
0.24
P = 1/3
0.45
0.90
0.27
(FWHM = 0.25)
76
16
8
Do
Dl
0.35
0.37
P = 1/2
0.48
0.87
(FWHM = 0.27)
68
32
(FWHM = 0.25)
0.43
73
0.90
12
0.26
15
Do
DI
D3
0.36
0.37
0.37
P = 1
0.49
0.81
1.11
(FWHM = 0.27)
47
38
15
(FWHM = 0.24)
0.43
73
0.80
16
0.26
11
Do
Dl
D3
0.35
0.37
0.36
P = 3/2
0.48
0.76
1.06
(FWHM = 0.29)
38
40
22
IS = isomer shift with respect to metallic a-iron at room temperature in mm s- 1 ; QS = quadrupole splitting in
mm s- I ; RA = relative abundance in o~. Full widths at half maximum (FWHM) in mm s- l- are constrained to be
equal for each Lorentzian-shaped line.
Mechanisms of oxidation of Ni(II)-Fe(lI) hydroxides
605
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~
V (mm/s)
Fro. 5. Selection o f M6ssbauer spectra at room temperature o f the end products: P = 1/6, 1/3 and 3/2. 9 9 9 .:
experimental curve;
: elementary quadrupole doublets; . . . . . .
: global computed curve.
Ph. Refait and J.-M. R. G~nin
606
80"
7O ~
601
i
I--*-~
50"
<~ 40"
rr 30"
201
10"
0
A..~-r
0.0
0.2
.~ ,
04
~
015
,
015
--
1:0
9
,
12
9
114
"4"
P
F1G. 6. Relative abundances of the various ferric quadrupole doublets characteristic of Ni(II)-Fe(III)
hydroxychlorides plotted vs. P = [Fe]/[Ni].
intermediate product obtained for P = oo is the socalled chloride-containing green rust, GR(C1-),
whose XRD pattern is similar to those presented
in Fig. 3 (ASTM card n ~ 13-88 from Bernal et al.,
1959). The difference between the M6ssbaner
spectra o f Fig. 7 measured at 78 K and those of
Fig. 5 run at 300 K lies in the existence of ferrous
quadrnpole doublets R1 and Rz, characterized by
large IS and QS values. The hyperfine parameters,
given in Table 3, are consistent with previous
studies (G6nin et al., 1986; R6zel et al., 1988). It
can be seen that the ferric doublet is Do as observed
for the final product, with its characteristic QS
value o f ~ 0 . 4 0 - 0 . 4 5 m m s - ' . Note that at 78 K, IS
are always larger than those at room temperature
(second-order Doppler shift).
X P S study
This was performed without any Ar-ion sputtering on the final product obtained for P = 1/3, 1/2
and 3/2. The three spectra are similar and, as an
e x a m p l e , t h a t o f the P = 1/3 c o m p o u n d
Nin3Fem(OH)sCl.nH20 is presented in Fig. 8a.
The sole noticeable difference between samples is
in the O(ls) region, as displayed in Fig. 8b. The
oxygen sub-spectrum is composed of only one peak
for P = 1/3 while a second peak appears for P = 1/2
at a lower binding energy (BE), representing 5% of
the total area and increases with P. At P = 3/2, its
relative abundance is 32%. Spectra were fitted with
Gaussian-shaped or Gaussian-Lorentzian-shaped
lines. Background determination was carried out
TABLE 3. M6ssbauer hyperfine parameters at 78 K of some intermediate products of oxidation at time hIS
P = 0% i.e. GR(C1-)
QS
RA
(FWHM = 0.27)
P = 5/3
IS
QS
RA
(FWHM = 0.28)
Fe n
R1
R2
1.26
1.27
2.80
2.55
36.5
37
R1
R2
1.27
1.27
2.83
2.53
37
19.5
Do
0.48
0.38
26.5
Do
0.48
0.46
43.5
Fem
IS = isomer shift with respect to metallic c~-iron at room temperature in mm s-l; QS = quadrupole splitting in
rnm s-l; RA = relative abundance in %. Full widths at half maximum, given in mm s -1, are constrained to be
equal for each Lorentzian line.
Mechanisms of oxidation of Ni(ll)-Fe(II) hydroxides
607
A
100
o~ 98
96
94
!
~
"
GR(CI
)-
I
!
,, ;,..
v
92
I
~
i
9 I
9o
88_4
n
I
l
-2
~.J
,T
'r
6~
4,!
I
n
0
I
D
n
1
j
I
I
2
I
4
B
I00
Oe
"•
=o
98
96
P = 5/3
92
,
-4
I
-2
,
V i~,
0
,
I
2
,
I
4
V(mm/s)
FIG. 7. M6ssbauer spectra at 78 K o f the intermediate products obtained at time tl. (A) P = 0% i.e. GR(C1-);
(B) P = 5/3. eee: experimental curve;
: elementary quadrupole doublets; . . . . . .
: global computed curve.
608
Ph. Refait and J.-M. R. G~nin
4ooooq
Ni 2p
3oooo~
~
2p:~
~
s~176176176
P 113
40000
/~
=
=
30000
20000
20000
1000
10000
0
880
870
860
540
850
,
535
Binding energy (eV)
k.
oooor-
~a
(0
5~0
s~5
Binding energy (eV)
5~o
3000
2000
r
735
5;~0
3,oloo5~176176
~=
Fe 2p
15OOO
E
,
525
Binding energy (eV)
O
~
(o
530
730
725
E
720
715
710
705
Binding energy (eV)
lOOO
'=~
E
540
5~5
50000
5000
40000
....
ooo
2
//,\
3OOO0
2OOOO
2000
lOO
210
", .
205
200
.
.
195
.
.
1OOOO
.
190
Binding energy (eV)
0
540
535
530
525
520
Binding energy (eV)
FIG. 8. X-ray photoelectron spectra (XPS) of Ni(II)-Fe(III) hydroxychlorides. (a) Spectrum of Ni~IFem(oH)sCl.nH20 obtained for P = 1/3: Ni 2p, Fe 2p and C1 2p subspectra. (b) Evolution of O ls subspectrum with varying P
from 1/3 to 3/2. - - :
background and global computed curves; . . . . .
: individual peaks; ME: chargetransfer multielectron excitation.
using the Shirley method, except for the Fe 2p
regions where the background was estimated by a
polynomial function of degree 5. The oxygen peak
present in all spectra is found at a binding energy of
531.2 eV whereas a value of 530.0 eV is found for
the additional peak of the P = 1/2 and P = 3/2
spectra. Similar values were found in ferric
oxyhydroxides and are attributed to O H - ions and
0 2 - ions, respectively (Maclntyre & Zetaruk, 1977;
Brion, 1980). Note that no oxygen peak due to
intercalated water molecules was observed since no
peak was detected in the 533-535 eV range. It is
most likely that water loss occurs when the vacuum
(3 • 10 -9 ton') is established in the spectrometer.
The value of 712.5 eV found for BE of Fe 2p3/2 at
P = 1/3 and 1/2 is slightly larger than those usually
found in ferric oxyhydroxides which lie from 711.3
to 711.9 eV (Maclntyre & Zetaruk, 1977; Konno &
Nagayama, 1979; Brion, 1980). In contrast, BE of
711.4 eV found at P = 3/2 fits perfectly to values in
FeOOH. It is likely that the 0 2 - species observed
in the O ls region are due to FemOOH groups. The
average value of 855.9 eV found for Ni 2p3/2 is
typical of BE found for Ni(OH)2 , which lies from
855.5 eV to 856.6 eV (Kim & Winograd, 1974;
Shalvoy et al., 1979; Lorenz et al., 1979; Salvati et
al., 1981). Finally, the value of~197.6 eV found for
Ct 2p3/2 is somewhat similar to that found in
various chlorides, e.g. 197.8 eV in KC1 and
198.3 eV in NiC12 (Kishi & Ikeda, 1974; Wren et
al., 1979). From the areas o f N i 2p, O ls and C1 2p
peaks, the composition of the surface of the
compounds was estimated. The area corresponding
to the Fe 2p region is not considered since the
position of its background is unreliable and
approximated with a polynomial function. For the
NiI[aFem(OH)sCI.nH20 compound obtained at
P ='1/3, Ni/C1, O/Ni and O/C1 ratios of 2.95, 2.59
Mechanisms of oxidation of Ni(ll)-Fe(lI) hydroxides
and 7.63 are observed. These values are close to the
values of 3, 2.67 (8/3) and 8 of the ideal formula
written above. The O/Ni and O/C1 ratios are found
at 3.05 and 10.9 for P = 1/2, and 4.2 and 14.1 for
P = 3/2. The O/C1 ratios, larger than the expected
O/C1 ~ 8, indicate that the chloride content of the
surface decreases with P. The O/Ni ratios allows
estimation of the Fe/Ni ratios at the surface, since
in hydroxides, oxyhydroxides or hydroxychlorides,
the O/[Fe + Ni] ratio is always equal to 2. It was
found to be 0.3 for P = 1/3, 0.5 for P = 1/2 and 1.2
for P = 3/2, so that the surface is similar or slightly
richer in Ni than the bulk.
DISCUSSION
The M(III)/M'(II) ratio o f 1/3
As in many other cases of pyroaurite-type
hydroxides, Ni-Fe hydroxychlorides present a
preference for a specific M(III)/M'(II) ratio close
to 1/3. This is essentially revealed by the existence
of an oxidation process consisting of two stages for
Ni(II)-Fe(II) hydroxides with P ratios larger than
1/3. The M6ssbauer spectra at 78 K of the
intermediate compounds obtained at the end of the
first stage lead to the same M(III)/M'(II) ratio
(Fig. 7, Table 3): the ferric doublet Do of GR(C1-),
i.e. the compound obtained for P = 0% represents
26.5% of the total area of the spectrum, which gives
an Fe(III)/Fe(II) ratio of 0.36. On the other hand, Do
represents 43.5% of the spectrum of the compound
obtained for P = 5/3, and by taking into account the
presence of Ni, this gives an Fe(III)/[Fe(II)+Ni(II)]
ratio of 0.37. Knowing that a partial oxidation of the
compounds cannot be avoided during sample
preparation, the ferric amount is slightly overestimated and the exact M(III)/M'(II) ratio is more
likely to be close to 1/3. In a previous study (Refait
& Grnin, 1993a), on the basis of other experimental
arguments, this value for 1/3 was already proposed
for GR(C1-); it appears that it must be extended to
Ni(II) containing hydroxychlorides.
Hence, the difference between GR(C1-) and the
two other well known green rusts, GR(CO32-) and
GR(SO]-), is confirmed. In similar experimental
conditions, those two GRs show a preference for a
Fe(III)/Fe(II) ratio of 1/2. It must be noted that
chloride is a monovalent anion, whereas sulphate or
carbonate are divalent and it is known that divalent
anions are preferred to monovalent anions in the
interlayers (Miyata, 1983); the simplest reason
609
would be that electric neutrality between intedayers
and hydroxide sheets must be provided by two
monovalent anions whereas just one divalent anion
is sufficient. Thus, in this latter case, the same
charge involves a smaller interlayer area than in the
former case. Therefore, the accumulation o f
chloride ions in the interlayers to reach a 1/2 ratio
as it is in other GRs is more difficult. According to
Miyata (1983), the affinity for anions decreases as
the diameter of anions increases, e.g. the affinity is
in the sequence O H - > F - > CI- > B r - > I - . This
sequence may be connected with the difficulty in
accumulating large anions in the interlayers.
M(III)/M'(II) < 1/3
At the lower limit P = O, no Fe is present and
~-Ni(OI-I)2 is the only phase obtained. Between the
two limits, P = 0 and P = 1/3, two regions are
observed. For the lowest P values (P<I/5), two
distinct solid phases form, as testified by XRD, and
are identified as I3-Ni(OH)2 and Ni(II)-Fe(III)
hydroxychloride. This indicates that Ni(II) is in
too large an excess to allow the formation of a pure
hydroxychloride which has a composition corresponding to P. It can be seen in Table 1 that this
hydroxychloride probably keeps the same composition since its lattice parameter c/3 remains constant.
In contrast, when 1/5<P < 1/3, the hydroxychloride
compound in the single phase end product, as
testified by micro-Raman spectroscopy (Fig. 4). Its
average composition varies with P and induces a
variation of the lattice parameter c. The general
chemical formula may be deduced since, for
electroneutrality, the number of C1- ions must be
equal to the number of ferric ions, that is:
NiII3+yFenlI_y(OH) sC11_y.nH20
where y--- {[4/(l+P)] - 3}.
As the departure y from the specific composition
at P = 1/3 increases, the hydroxychloride becomes
less and less stable. This may arise from the
corresponding decrease of the electrostatic interactions between interlayers [Cll_y.nH20] (l-y)- and
hydroxide sheets [Ni~I+yFetm_y(OH)s]O-y)+. P ~ 1/5
can be interpreted as the lower limit for the stability
of Ni(II)-Fe(llI) hydroxycblorides and corresponds
to an idealized limiting formula of:
9II
lII
Nxlo/3Fe2/3(OH)sC12/3.nH20
The Mrssbauer spectrum of such Ni(II)-Fe(III)
hydroxychlorides consists mainly of a ferric doublet
610
Ph. Refait and J.-M. R. G~nin
Do with well defined hyperfine parameters,
corresponding to a unique, well determined crystallographic site (Fig. 5, Table 2). It gives rise to a QS
of 0.43 __+ 0.02 mm s -1, smaller than those found
in paramagnetic spectra of ferric oxyhydroxides
which are in the range of 0.50-0.70 mm s -~ for
crystallized compounds (Rossiter & Hogdson,-1965;
Johnson, 1969; Madsen et al., 1985; Olowe et al.,
1990) or larger (up to 1 mm s -1) than those in the
case o f p o o r l y c r y s t a l l i z e d or amorphous
compounds such as ferric hydroxide Fe(OH)3 or
ferrihydrite (Murad & Schwertmann, 1980;
Au-Yeung et al., 1984). The relative abundance of
Do is practically constant and equal to 75%.
Additional doublets D1 and D2 may be due to the
contribution of Fe atoms close to the surface or to
variations in the crystal composition. In other
compounds such as goethite, ferrihydrite (Murad
& Schwertmaun, 1980) or hematite (Kraan, 1973),
it was observed that small particles of very poor
crystallinity gave spectra that could be fitted with
two doublets rather than the usual one. These
doublets have different quadrupole splittings,
-0.5 mm s -1 for one of them as in well crystallized
compounds, and -.4).85 mm s -1 for the other one.
They were considered to result from well-ordered
inner regions and poorly-ordered surface regions of
the particles, respectively. Hence, doublet Dl, with
QS between 0.79 mm s -1 and 0.90 m m s -1 may be
attributed to the surface Fe(III) ions of the
hydmxychloride particles.
M ( I I I ) / M ' ( I I ) > 1/3
In this case, the oxidation leads in a first stage
ending at time tl (Figs. 1,2) to a hydroxychloride
characterized by the Fe(III)/[Fe(II)+Ni(II)] ratio of
1/3. The end products result from a subsequent
oxidation stage ending at time t2 (Figs. 1,2) when
all Fe is in the ferric state. As in the previous case,
two domains were observed between the limits
P = 1/3 and P = oo. When P >/ 5/3, XRD patterns
allow identification of a second phase, obtained
along with the Ni(II)-Fe(III) hydroxychloride. It is
T-FeOOH which is obtained in its pure form at the
limit P = oo. In this domain, Fe(III) is in too large
an excess to allow the formation of a Ni(II)-Fe(III)
hydroxychloride with a composition corresponding
to P. In contrast, according to XRD, such a
hydroxychloride is obtained in the other domain
when 1/3 < P < 3/2. The patterns of the end
products reveal the presence of only one crystalline
phase, the Ni(II)-Fe(III) hydroxychloride. Its
average composition depends on P and since the
c/3 parameter decreases, the final hydroxychloride
becomes richer in Fe(III) as P increases. It must be
noted that there is no theoretical limit to the upper
content of Fe(III): a ferric hydroxychloride, more
likely Femoa(OH)sCI.nH20, can result from violent
oxidation of GR(C1-) by means of hydrogen
peroxide (Bemal et al., 1959; Refait & Gtnin,
1993b).
However, optical microscopy coupled with
Raman spectroscopy analyses carried out on the
P = 3/2 product revealed the presence of an
additional phase. It is probably amorphous since it
is not observed in the XRD pattern. It is more likely
to be free of chloride, which would explain the
large O/C1 ratios observed by XPS at P = 3/2 and
1/2. But it could also be an amorphous FeOOH
phase or a Ni-Fe oxyhydroxide of formula
(1-x)Ni(OH)2.xFeOOH. The first hypothesis is
consistent with the XPS analysis at P = 3/2,
where the binding energy for Fe is that of
FeOOH. However, the Raman spectrum supports
the second hypothesis; the two first peaks, at 320
and 460 cm -1 correspond to 13-Ni(OH)2 (Johnston
& Graves, 1990). The third one, at 540 cm -1, is
usually observed in the Fe(OH)2 spectrum but is
absent when pure Fe(OH)2, totally free of Fe(III), is
analysed (Lutz et al., 1994). This last peak is then
attributed to the presence of Fe(III) which is known
to occur commonly in ferrous hydroxide, up to 20%
without noticeable changes in the XRD patterns
(Bernal et al., 1959), giving its typical blue-green
colour to the compound. The additional phase could
then be considered as an Fe(III)-containing nickel
hydroxide, an intermediate between Ni(OH)2 and
FeOOH.
The MSssbauer spectra of such end products are
characterized by the increased relative abundance of
ferric doublets, D1 and D3, with large quadrupole
splittings. They were the subject of a previous study
(Refait & Gtnin, 1993b) and, even if the way the
doublets are determined here is slightly different,
the main characteristics remain: the total relative
abundance of these additional doublets, i.e. 32% at
P = 1/2, 53% at P = 1 and 62% at P = 3/2,
corresponds approximately to the excess Fe(III)
formed during the second oxidation stage, i.e. 25%
at P = 1/2, 50% at P = 1 and 58% at P = 3/2. In
fact, the experimental abundance is always found to
be slightly larger, due to the surface contribution
which gives rise to a similar quadrupole doublet.
Mechanisms of oxidation of Ni(ll)-Fe(ll) hydroxides
This indicates that the excess Fe(III) is found in a
rather different local environment than that of the
Fe(III) resulting from the first oxidation stage. This
confirms that the two oxidation processes involve
different mechanisms. During the first one, whether
the specific M(III)/M'(II) ratio of 1/3 is reached or
not, the oxidation of Fe(II) into Fe(III) is connected
to the increase of the chloride content of the solid
phase. Hence, the hydroxide sheets only contain
Fen(oH)~ groups, which give rise to doublet Do,
and no O2- species are formed. In contrast, the
second oxidation process involves the formation of
0 2 - species, as testified by XPS, and Fell(OH)2
groups are transformed into FelnOOH groups,
which give rise to D1 and D3 doublets. There is
no contradiction between XRD and Raman observations: FenlOOH groups are found in Ni-Fe
hydroxychlorides as well as in an additional
phase, more likely an Fe(III)-containing nickel
hydroxide.
In conclusion, the second oxidation stage is not
connected to an increase of the chloride content as
the first one is. This suggests that the specific
Fe(III)/[Fe(II)+Ni(II)] ratio of 1/3 corresponds to a
maximum value for the chloride content. This must
be understood as a saturation when the maximum
possible amount of C1- ions is present in the
interlayers. It could correspond to a balance
between the electrostatic attraction of interlayers
[CI-nH20]and hydroxide
sheets
[(Ni,Fe)txlFenI(OH)z~+2]+, which is increased by
accumulating C1- ions, and the electrostatic
repulsion among C1- ions within one interlayer,
which opposes such an accumulation.
The mechanism of oxidation of Ni(ll)-Fe(lI)
hydroxides
The previous discussion allows a model to be
proposed which describes the mechanisms of
oxidation of nickelous-ferrous hydroxides in the
presence of C1- ions. This model may be extended
to other type of anions like CO~-, SO,2-, etc., and
constitutes a basis of understanding the formation
of some clay-like minerals of the pyroauritesjrgrenite group to which fougerite belongs
(Trolard et al., 1996). Starting from the initial
N i ( I I ) - F e ( I I ) h y d r o x i d e s m a d e o f the
AcB[3AcBDAcB[3A stacking sequence which
characterizes the hexagonal structure where A and
B represent the OH- ions, c the metallic cations
Ni(II) or Fe(II), and [] the empty interlayer
611
octahedral sites, the oxidation process consists of
introducing C1- ions and water molecules in the
empty interlayers. By the same token, the
progressive intercalation changes the stacking
s e q u e n c e and, for i n s t a n c e , the i n i t i a l
AcB []AcB[[]AcB[ZA
becomes
AcBiBaC[]BaC[3B where i represents one
[CI.nH20]- intercalated layer. The process can be
repeated to yield AcBiBaCjCbAFqC and finally
AcBiBaCjCbAkA, which has the structure observed
in pyroaurite. This progressive in situ rearrangement of the initial Ni(II)-Fe(II) hydroxide structure
towards Ni(II)-Fe(II)-Fe(III) hydroxychloride is
solely due to the intercalation of C1- ions. At
saturation with C1- ions, a specific composition is
reached and imposes a corresponding value for the
Fe(III)/[Fe(II)+Ni(II)] ratio. This process corresponds to the first stage of oxidation and two
limiting cases of substitution exist: when P is equal
to 1/3, a N i ( I I ) - F e ( I I I ) h y d r o x y c h l o r i d e
[Ni~IFem(OH)s]+[CI.nH20]- and when P = oo,
one obtains an Fe(II)-Fe(III) hydroxychloride
[Fe~IFenI(OH)s]+[Cl.nH20]- which is GR(C1-), is
obtained. For P < 1/3, even though C1- ions are
available, there exists a lack of Fez+ ions to become
Fe 3+ ions, and the possible number of Fe3+ ions at
the end of the first stage will limit the matching
number of C1- ions for electrostatic neutrality
which can be incorporated in the hydroxychloride.
Consequently the intercalation of C1- ions does not
reach saturation.
CONCLUSIONS
The Ni-Fe hydroxychlorides of the pyroaurite group
with various P = Fe/Ni ratios can be prepared by
aerial oxidation of Ni(II)-Fe(II) hydroxide precipitates in aqueous solution. They show a preference
for a specific Fe(III)/[Fe(II)+Ni(II)] ratio of 1/3.
When P is <1/3, this ratio cannot be reached and
only one reaction stage is observed in which Ni(II)Fe(III) hydroxychlorides with Fe(III)/Ni(II) ratios
<1/3, down to 1/5, are formed. In these compounds
the chloride content stays equal to the Fe content
and diminishes with P. The limiting compound,
II
III
with ideal composition Ni10/3Fe2/3(OH)sClz/3.nH20,
can be considered to correspond to the minimum
Fe(III) and C1 content compatible with the
pyroaurite-type structure. In contrast, when the
ratio P is >1/3, the oxidation reaction comprises
two stages. The first one corresponds to the
formation of an Fe(II)-Ni(II)-Fe(III) hydro-
612
Ph. Refait and J.-M. R. Gknin
xychloride characterized, whatever the value of P,
by the specific Fe(III)/[Fe(II)+Ni(II)] ratio of 1/3.
At the limit P = o% it results in the formation of the
F e ( I I ) - F e ( I I I ) h y d r o x y c h l o r i d e green rust,
Fe~IFem(oH)aCI.nH20. The second stage corresponds to the oxidation of the remaining Fe(II)
but does not involve an increase of the chloride
content in the final solid phase. The specific
Fe(III)/[Fe(II)+Ni(II)] ratio of 1/3 thus corresponds
to a saturation, the maximum chloride content that
can be accumulated in the interlayers under the
experimental conditions considered here.
ACKNOWLEDGMENTS
The authors would like to thank B. Humbert for
carrying out the Raman spectroscopy measurement and
M. Alnot and J. Lambert for their expert assistance in
the XPS experiment. The very constructive advice of
the reviewers is fully acknowledged.
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