Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 275-282
http://dx.doi.org/10.4236/jsemat.2013.34037 Published Online October 2013 (http://www.scirp.org/journal/jsemat)
FT-IR Spectroscopy Applied for Surface Clays
Characterization*
Paul Djomgoue#, Daniel Njopwouo
Laboratoire de Physico-Chimie des Matériaux Minéraux, Département de Chimie Inorganique, Université de Yaoundé I, Yaoundé I,
Cameroun.
Email: djomgouep@gmail.com
Received May 8th, 2013; revised June 11th, 2013; accepted July 10th, 2013
Copyright © 2013 Paul Djomgoue, Daniel Njopwouo. This is an open access article distributed under the Creative Commons Attri-
bution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
ABSTRACT
The present paper should be considered as a review of the application of Fourier Transform Infra-Red for surface clay
characterization. The application of surface clay materials for water treatment, oil adsorption, excipients or as active in
drugs has largely increased these recent years. The surface clay material presents hydroxyl groups, which can link very
easily water molecules. These hydroxyl groups can react with organic groups and by their vibration in the infra-red re-
gion, FT-IR can be easily used as a technical method for surface clay characterization. In this paper, we focus on the
determination of Lewis and Brönsted acid sites on the clay surface, a critical review of the sample preparation, the sur-
face characterization of bulk clay and the modified surface clay samples using FT-IR spectroscopy.
Keywords: Clay Materials; Surface Clays; Hydroxyls; Surface Acidity; IR Spectroscopy
1. Introduction
Clay minerals are a well-known class of natural inorga-
nic materials with well-known structural adsorption, rhe-
ological and thermal properties. These materials original-
ly have a hydrophilic character due to the presence of the
surface hydroxyl (-OH) groups, which can link very eas-
ily water molecules [1].
For many years, the clay materials have been used for
adsorption of heavy metals [2,3], dye molecules [4], her-
bicides [5,6], anions such as nitrates [7], like phosphates
and sulphates, or gas adsorption [8], like SO2.
In industry, these materials are also used as a catalys-
ator in organic syntheses or as excipient in pharmacy.
The application of clay materials is greatly governed by
their surface properties like adsorption capacities, surface
charges, large surface area, charge density, the type of
exchangeable cations, hydroxyl groups on the edges, si-
lanol groups of the crystalline defects or broken surfaces
and Lewis and Brönsted acidity [9]. Phyllosilicate surfac-
es contain two basic types, i.e., siloxane surface and hy-
droxyl surface. The 2:1 clay minerals (e.g., smectite group
minerals) only contain siloxane surfaces while the 1:1
clay minerals (e.g., kaolinite group minerals) contain
both the two kinds of surfaces (Figures 1 and 2).
The hydroxyl surfaces (e.g., Al-octahedral surface in
kaolinite) are excellent sites for grafting since the surface
hydroxyls can condensate with alkoxyl groups and/or the
Oxygen
Hydrogen
Aluminum
Silicon
*Review article.
#Corresponding author. Figure 1. Structure of 1:1 clay mineral [10].
Copyright © 2013 SciRes. JSEMAT
FT-IR Spectroscopy Applied for Surface Clays Characterization
276
Figure 2. Structure of 2:1 clay mineral [11].
hydroxyls in the hydrolyzed silane. Due to the high ratio
of the terminal surface area of the basal surface area,
hydroxyl groups located at breaking edges play an im-
portant role in the silane grafting reaction. However, due
to the variation of the structure and property of phyl-
losilicates, prominent differences of the grafting mecha-
nism and grafting sites exist among different clay min-
erals. For swelling clay minerals such as montmorillonite,
silane is readily intercalated into the interlayer space [12].
Chemical modifications on the surface of clay with acids,
bases, cationic surfactants and certain polyhydroxyl ca-
tions were also conducted to improve their sorption capa-
city. Surface modification of clay minerals has attracted
much attention because they obtained products exhibiting
properties suitable for many applications in material sci-
ence [12] and environmental engineering [13].
Several characterization techniques were employed in
order to identify the changes in clay: fourier transform-
infra-red (FT-IR), X-ray diffraction (XRD), raman spec-
troscopy, Si/Al NMR, transmission electron microscopy
(TEM), scanning electron microscopy (SEM), energy di-
spersive X-ray spectroscopy (EDX) and nitrogen adsorp-
tion-desorption isotherms [14]. FT-IR is a complementa-
ry method to X-ray diffraction (XRD) and other methods
used to investigate clays and clay minerals. FT-IR is ra-
pid, inexpensive and can be available in many laborato-
ries. For the FT-IR spectroscopy, it is still a discussion
about the technique procedure, the nature of bound be-
tween the clay surface and the organic molecule after ad-
sorption or grafting. In this paper, we intend to give some
recommendations for these questions.
The study presented in this paper is a review aimed at uti-
lization of FT-IR spectroscopic as the surface clay method
characterization of bulk materials or modified clay sam-
ples, determination of Lewis and Brönsted acid sites on clay
surface, and a critical review of the sample preparation.
2. Techniques
Today, infrared studies of the adsorption properties of
clay surfaces may be limited by sample preparation and
handling problems. IR absorption spectra of clay miner-
als are usually obtained from self-supporting films, films
sedimented on IR-transparent windows, alkali-halide disks
(e.g. KBr), or from oil mulls (the most common mulling
agent is Nujol-paraffin oil).
Transmission spectroscopy is the oldest and most com-
monly used method in the clay mineral studies. Naturally
occurring clay minerals are powders or solids that can be
ground into powders. The KBr pressed disc technique is
frequently used to prepare samples for IR analysis. Con-
venient means to prepare a clay sample for FTIR spectro-
scopic analysis is to mix the clay sample with potassium
bromide (KBr) and place the KBr/clay mixture into a dye
under pressure to form a pellet. In order to minimize the
amount of the adsorbed water, the discs should be heated
in a furnace overnight at 130˚C [15]. In our laboratory,
after heated at 130˚C, the sample should be immediately
submitted for FTIR measurement to avoid atmospheric
water adsorption. The pressing of KBr with clay to pre-
pare the KBr pellet may alter the spectrum through the
exchange of K into the structure. The danger occurs in
the stage of mixing the kaolinite with KBr before the
mixture is pressed into a disk [15]. If that mixing is gen-
tle it causes no effect, but if grinding is used in the mix-
ing, severe changes in the intensities of some of the absorp-
tion bands occur due to an interaction between the KBr
and the kaolinite. Bell et al. [16] show that the magnitude
of change depends on the absolute pressure, the pressing
time and whether a salt matrix is used. Pressing with KBr
causes larger differences than pressing neat. This simple
experiment shows that IR hydroxyl peaks change irrever-
sibly when kaolinite powder is pressed into pellets. This
may be due to optical or chemical effects. According to
these authors, due to the hydroxyl intensity changes, the
valuable structural information is lost when kaolinite IR
spectra are acquired using pressed KBr pellets [16].
The self-supporting lm (SSF) is another technique for
preparation of samples for FTIR measurement. In this
technique, a drop of that aqueous clay suspension is pi-
petted onto a thin polyethylene sheet and allowed to eva-
porate to dryness overnight at room temperature. But, the
problem with this technique is for smectite clay materials.
The important property of smectites is their swelling abi-
lity, when exposed to humidity, these minerals swell in a
series of steps as water enters the interlayer region to
form so-called one-, two- and three-layer hydrates. In
studies of smectite dehydration SSF technique is more
suitable because it allows examination of a sample with-
out a KBr matrix, known to be hygroscopic [17]. In other
points, absorption bands of water molecules adsorbed on
KBr and those present in the smectite overlap are not
possible to distinguish. Therefore it is good to heat the
Copyright © 2013 SciRes. JSEMAT
FT-IR Spectroscopy Applied for Surface Clays Characterization 277
sample before measuring, Madejova et al. [17] threat for
15 min at 150˚C a Montmorillonite for film Self-support-
ing film. Figure 3 presents the result of heated and un-
heated samples.We can see a completely eliminated band
near 3400 cm1 related to H2O adsorbed on the sample,
and a well-resolved band at 3629 cm1 assigned to OH
stretching vibrations of structural hydroxyls remained.
This technique was also useful for the 950 - 800 cm1 re-
gion, where well-resolved absorption bands are present.
Another method to obtain IR spectra of solids are Nu-
jol (mineral oil) mulls between KBr plates. Good results
are obtained by this method only if the average particle
size of the solid is somewhat less than the wavelength of
light the particles are to transmit. The sample should be
grounded in a mortar to reduce the average particle size
to 1 - 2 μm. About 5 to 10 mg of finely ground sample
are then placed onto the face of a KBr plate, a small drop
of mineral oil is added and the second window is placed
on top. With a gentle circular and back-and-forth rubbing
motion of the two windows, evenly distribute the mixture
between the plates [18].
The mixture should appear slightly translucent, with
no bubbles, when properly prepared. The FT-IR using
from oil mulls or Nujol technique modified of the spectra
of some organo-clays may occur in alkali halide disks
because they may react either with the clay (e.g. cation
exchange with the alkali halides, intercalation of the al-
kali halides, grinding or pressure effects) or with the or-
ganic matter (e.g. extracting the organic matter from the
clay by Nujol, replacing of adsorbed organic matter by
alkali halide) [19].
Attenuated Total Reflection FTIR (ATR-FTIR) is also
an alternative method for FT-IR sample preparation
where raw material is placed directly onto the diamond
crystal for data acquisition [20]. One advantage of ATR-
IR over transmission-IR, is the limited path length into
4000 3500 3000 2500 2000 1500 1000500
Wavenumbers (cm
-1
)
Absorbance
3627
3412
3629
1633
919
887
848
922
unheated
heated
Figure 3. Transmission IR spectra of SWy-2 (montmorillo-
nite) self-supporting film-sampling techniques; slight heat-
ing [17].
the sample. This avoids the problem of strong attenuation
of the IR signal in highly absorbing media, such as aqu-
eous solutions. During preparation, powdered core sam-
ples were settled (<2 μm grain size fraction), dried and
heated up at 80˚C to minimize the adsorbed water con-
tent and then kept in closed vessels covered by parafilm
before ATR-FTIR measurements [21].
3. Clay Surface Characterizations
3.1. Characterization of the Hydroxyls at the
Surface Clays
The triclinic layer structure of pure kaolinite reveals four
well resolved (-OH) bands in IR spectrum. Three of these
bands are assigned to the stretching vibrations of surface
hydroxyl groups (3652, 3671, and 3694 cm1) while the
fourth (3620 cm1) is attributed to the vibrations of inner
hydroxyl groups. The OH-bending region of kaolinite
shows vibrations of the inner surface OH groups at 913
cm1 and that of surface OH groups near 936 cm1; addi-
tional bands near 701 and 755 cm1 are associated with
the surface hydroxyls. Bands due to ν(AlFeOH) at 865 -
875 cm1 and stretching at 3607 cm1 are typical of Fe
bearing kaolinites [22]. The four OH-stretching bands in
the infrared spectrum of kaolinite are widely used in the
study of adsorption and interface reactions of this min-
eral. Raman spectra of kaolinites exhibit five OH-stre-
tching bands, the additional Raman feature at ~3685 cm1
being located between bands at about 3696 and 3670
cm1.
For the halloysite compared with kaolinite, two infra-
red bands are observed at 1648 and 1629 cm1 in the hal-
loysite spectrum. The assignment of these latter two
bands is to the water HOH bending mode. The fact that
two bands are observed suggests that there is water pre-
sent in the halloysite (10 Å) structure. This corresponds
with the hydroxyl stretching wavenumbers above. The
band at 1648 cm1 corresponds to strongly hydrogen bon-
ded water, whereas the band at 1629 cm1 is attributed to
less strongly hydrogen bonded water and corresponds to
the position of the water bending mode of liquid water
[23].
The structural OH bending mode in montmorillonite
absorbs between 700 and 950 cm1 and shows a series of
discrete peaks that indicates the cation composition in the
octahedral sheet. The OH bending mode of the A12OH
group absorbs near 920 cm1; The OH of Fe(III)AlOH
absorbs near 890 cm1; the OH MgAlOH absorbs near
840 cm1; and The OH Fe(II)Fe(III)OH absorbs near 800
cm1 [24]. The region above 3000 cm1 wavenumber of
FT-IR contains information about the silanols of mont-
morillonite. The characteristic vibration peaks of smec-
tite are at 3628 cm1 (OH stretching) [25].
Copyright © 2013 SciRes. JSEMAT
FT-IR Spectroscopy Applied for Surface Clays Characterization
278
3.2. Characterization of Modified Clay Surface
Samples
Murakami et al. [26] modified a kaolinite with butane-
diols. The IR spectrum of Kao-1,2BD (kaolinite modi-
fied with 1,2-Butanediols) showed the bands at 2970,
2941 cm1 (rasCH2) and 2885 cm1 (rsCH2), indicating
the presence of the organic component (Figure 4(d)). The
spectrum of Kao-1,3BD (kaolinite modified with 1,3-
Butanediols) also showed a pattern similar to that of
Kao-1.2BD (kaolinite modified with 1,2-Butadiols); the
bands at 2971, 2931 cm1 (rsCH2) and 2856 cm1 (rsCH2)
were observed (Figure 4(d)). The spectrum of methoxy-
modified kaolinite (Figure 4(b)) shows the disappear-
ance of the bands at 3668 and 3653 cm1 and the ap-
pearance of the bands at 3645 and 3631 cm1. The shifts
are ascribable to the variation in the interlayer environ-
ments of the hydroxyl groups. The broad band centered
at 3550 cm1 can be due to OH of the interlayer water.
The bands at 3645 and 3631 cm1 found for methoxy-
modified kaolinite disappeared in the spectrum of Kao-
1,2BD (kaolinite modified with 1,2-Butanediols), and a
new band at 3599 cm1 was observed (Figure 4(c)). The
band is ascribable to OH of hydroxyl groups on the in-
terlayer surface hydrogen bonded with grafted 1,2-Bu-
tanediols. A broad band at 3550 cm1 due to rOH of in-
terlayer water was also observed. In the spectrum of
Kao-1,3BD, the bands at 3645 and 3631 cm1 observed
in the spectrum of methoxy-modified client disappeared
and a new broadband at 3650 cm1 and a sharp band at
3599 cm1 were observed (Figure 4(d)) [26].
Priyanthi et al. [27] show that infrared spectroscopy is
a powerful method for monitoring and analyzing clay-
fluid interactions. They studied the characteristic of IR
bands, such as SiO stretching, OH stretching of struc-
tural OH group, OH stretching and HOH bending
3800
Wavenumbers / cm
-1
Transmittance
3599
3600 3400 3200 3000 2800
2971
2931
2856
2970
2941
2885
2954
2910
2840
3599 3620
3631
3645
3695 3695
3668
3653
3620
(a)
(b)
(c)
(d)
Figure 4. Infrared spectra of (a) kaolinite, (b) dry meth-
oxy-modified kaolinite, (c) Kao-1,2BD, and (d) Kao-13,BD
[26].
of interlayer water, in clay, and characteristic IR vibra-
tion bands such as C=O and CCO stretching bands in
solvents, they use these results to understand the clay-
fluid interactions at the molecular level. Important is to
dry the clay sample in order to remove water before
FTIR measurements, however, slight changes in structure
during manipulation cannot be ruled out and the results
could be wrongly interpreted. We suggest many other
techniques in order to compare the results like powder
X-ray diffraction.
3.3. Determination of Brönsted and Lewis Acid
Sites
There are several methods to determine the surface acid-
ity of clay minerals such as Hammett indicator technique,
n-butylamine back titration, microcalorimetry and FT-IR
spectroscopy of adsorbed basic probe molecules [28].
Brönsted and Lewis acid sites can be distinguished from
the IR spectrum of pyridine adsorbed on the clay surface.
Brönsted and Lewis acid site concentrations are calcu-
lated using the IR bands centered at 1545 cm1 (charac-
teristic of the pyridium ions pyH+) and 1455 cm1 (char-
acteristic of the pyL species), respectively, using the fol-
lowing formula:
Lewis ll
CAS*1000
(1)
Br nstedbb
CA S*1000
ø
(2)
where Al is the area of the IR band centered at 1450 cm1,
Ab is the area of the IR band centered at 1545 cm1, S is
the area of the pellet (cm2), ω is the weight of the pellet
(mg), and ε are the extinction coefficients (in cm μmol1)
εl = 1.28 cm μmol1, εb = 1.13 cm μmol1 [28].
The infrared absorption bands in the 1400 –1700 cm1
region for pyridine adsorbed on silica-alumina and alu-
minosilicates have been used to study their acidity since
the 1960s [2]. But it is still a problem. It is known that spec-
tra of MMT before pyridine adsorption contain in this re-
gion only one band near 1630 cm1 due to the bending
OH vibrations of water molecules; therefore it important
to dry the sample before the pyridine adsorption [28].
The Brönsted sites (B) show bands near 1490, 1540,
and 1635 cm1. The 1540 cm1 band is typical of this site;
the corresponding species is the pyridinium ion (PyH+).
It is assumed that tricoordinated aluminum atoms with an
electron-free orbit constitute Lewis acid centers (L). Pyri-
dine coordinated to the Lewis sites absorbs near 1455,
1490, and 1610 - 1625 cm1; the 1455 cm1 band is typi-
cal of these sites. The third type of site, corresponding to
hydrogen-bonded pyridine (H) on the clay solid, has vi-
brations near 1440 and 1590 cm1 (Figure 5) [28]. These
bands are probably due to a strong interaction between
the cation and the pyridine molecule, by attraction of the
cation into the electrostatic field [17].
Copyright © 2013 SciRes. JSEMAT
FT-IR Spectroscopy Applied for Surface Clays Characterization 279
1650
Wavenumber, cm
-1
1635
1600 1550 1500 1450
P
H
L
B
1590
1575
1540
1490
1455
1440
1610-25
Figure 5. Bands in the IR spectra of acid solids with ad-
sorbed pyridine in the 1420 - 1650 cm1 region. Pyridine
adsorbed on B, Brönsted sites; L, Lewis sites; H, hydrogen-
bonded, and P, physisorbed pyridine [28].
The band at 3620 cm1 corresponds to the “inner hy-
droxyls” located on the plane, common to octahedral and
tetahedral sheets, as described previously. Bands record-
ed at 3668 and 3652 cm1 are suggested could be attribut-
ed to the vibration of the “outer hydroxyls” located at the
surface and along broken edges of kaolinite monocrystals
[17].
3.4. Clay Surface Complexation
Adsorption of metal ions from aqueous solution on ox-
ides, clay minerals and clays has been a subject of inter-
est in chemistry as well as in other research areas. It is
considered that the adsorption of heavy metal ions and
complexes on clay minerals occurs as a result of ion ex-
change, surface complexation, hydrophobic interaction,
and electrostatic interaction [9]. The adsorption modes of
ions on mineral surfaces are mainly divided into outer-
sphere and inner-sphere surface complexes. In general,
the chemical interactions in inner-sphere complexes are
stronger than those of outer-sphere complexes (Figure 6)
[9]. These differences in binding strengths inuence the
mobility of ionic species in the environment. Hence, a
distinction between outer-sphere and inner-sphere com-
plexes is signicant and FT-IR is helpful for the evalua-
tion of the outer-sphere and inner-sphere surface com-
plexes. The evaluation of outer-sphere and inner-sphere
surface complexes needs techniques such as attenuated
total internal reection spectroscopy (ATR–FTIR). Zhang
et al. [29] studied the Cd(II)-sulfate interactions on the
goethite-water interface, ATR-FTIR studies indicated that
sulfate adsorption on goethite occurred via both outer-
and inner-sphere complexation; the authors assign peaks
at 1170, 1132, and 1050 cm1 to inner-sphere species.
More recently Brechbühl et al. [30] studied the compet-
tive sorption of carbonate and arsenic to hematite: com-
bined ATR-FTIR and batch experiments, the ATR-FTIR
spectra indicated the predominant formation of bidentate
binuclear inner-sphere surface complexes for both sorbed
arsenate and sorbed carbonate.
Moreover, the FT-IR could be used to show the bound
between the clay and the metal ion after adsorption, for
example, Eren et al. [13] modified a bentonite clay using
MnCl2 for copper adsorption, the FT-IR of the sample
after copper adsorption presented in Figure 7 shows that
the stretching OH band was shifted up to 3668 cm1 and,
moreover, new bands appeared near 3591 and 3460 cm1
in the spectrum of MMBCu(II) sample. The IR spec-
trum of the MMBCu(II) sample showed a strong band
of water near 3400 cm1, due to the overlapping asym-
metric ν3 and symmetric ν1(HOH) stretching vibra-
tions. We are also modified the magnetite clay with
Black Eriochrome T, NET and the FTIR spectroscopy
help us to show the modification of our sample using
KBr technique [4].
H
y
drotalcite-like com
p
ounds
Outer-sphere complex
Inner-sphere complexes
S
Al P
MgMg MgMg Mg MgAl Al Al
Figure 6. Outer-sphere and inner-sphere surface complexes
at the hydrotalcite-like compounds [31].
4000 3000 2000 100040
0
Wavenumbers (cm
-1
)
Transm
stance
3400
3310
3100
3668
3591
3617
3259
3361
1038 470553
1855 1625
1625
1038
470
527
936
629
585
1651
1080
1038
3415
3635
(a)
(b)
(c)
470
553
Figure 7. IR spectra of the RB (a) bulk samples, MMB (b)
modified with MnCl2, and MMB–Cu(II) (c) [13].
Copyright © 2013 SciRes. JSEMAT
FT-IR Spectroscopy Applied for Surface Clays Characterization
280
Nicolini et al. [32] studied the dehydrated halloysite
intercalated mechanochemically with urea: Thermal be-
havior and structural aspects; the absorption bands at
3696 and 3622 cm1 in the FTIR spectrum were assigned
to the stretching vibration due to external and internal
OH groups of halloysite, respectively. The intensity of
the band at 3696 cm1 is lower than the band at 3622
cm1, because it is mainly the external hydroxyl groups
that are responsible for the interactions with urea through
NH groups and new bands were observed at 3503 and
3390 cm1 indicated the adsorption of urea.
Table 1 gives the wave number values of the hy-
droxyls group of the most used clay minerals like kaolin-
ite, halloysite and montmorillonite. It appears in this ta-
ble that these values are between 3000 - 3800 cm1 and
these values are really close, most are around 3600 cm1.
It’s not sufficient to use only FTIR to characterize clay
minerals, other methods like powder X-ray, thermal gra-
vimetric etc. should be combined to have a complete cha-
racterization.
4. Conclusion
The present review was aimed to the technical procedural
methods used by FT-IR for clay surface characterization
because, FT-IR remain is an economical, rapid and com-
mon technique. A spectrum can be obtained in a few mi-
nutes and the instruments are sufficiently inexpensive as
to be available in many laboratories.
An IR spectrum can serve as a fingerprint for mineral
identification, but it can also give unique information
Table 1. Wavenumber peaks of the hydroxyl, OH group of
most used clays.
Clay minerals Wavenumber (OH) cm1 Refs
Kaolinite 3696, 3671, 3650 [33]
Halloysite 3696 (OH-internal), 3622 (OH-external) [32]
Montnorillonite 3624 (Al-OH), 3422 (Water) [34]
Verniculite 3675 (Mg-vermiculite),
3594 (Na-vermiculite) [35]
Bentonite 3625 [15]
Illite 3600 [36]
Chlorite 3670, 3580 (Interlayer) 3440 (water) [37]
Smectite 3622 [38]
saponite 3740 (Si-OH), 3670 (Mg(OH)2), 3570 [39]
Sepiolite 3719 (Si-OH) 3689 (Mg3OH) [40]
Nacrite 3701 and 3647 (out-of-plane),
3647 (in-plane) [23]
Dickite 3708, 3654, 36228 (Inner surface OH) [41]
Lizardite 3686 (Mg-OH) [42]
about the mineral structure, including the family of min-
erals to which the specimen belongs, the degree of regu-
larity within the structure, the nature of isomorphic sub-
stituents, the distinction of molecular water from consti-
tutional hydroxyl, and the presence of both crystalline
and non-crystalline impurities [17].
Among different techniques samples preparing for IR
measurement are self-supporting films, films sediment on
IR-transparent windows, alkali-halide disks like: KBr,
NaCl etc., or from oil mulls. Many parameters influence
the results in each case as the pressing, grinding effect
and atmospheric water adsorption, therefore the choose
of preparing samples should be greatly depending on the
type of clay and the results we need to show. As a rec-
ommendation for using FT-IR for the surface clay charac-
terization, the Attenuated Total Reflection, the ART-FTIR
method, could be a best technique for the sample prepa-
ration. It is important to dry the sample up to 100˚C to
eliminate the adsorbed water on clay sample and this
gives a good result and interpretation in the range 3000 -
4000 cm1 where appear the surface hydroxyl bands of
clays.
5. Acknowledgements
The authors thank the anonymous reviewers for their in-
sightful suggestions to improving the initial manuscript.
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