GEOLOGICA CARPATHICA, 53, 2, BRATISLAVA, APRIL 2002
93 — 98
SORPTION PROPERTIES OF REDUCED-CHARGE
and PETER KOMADEL
Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 842 36 Bratislava,
(Manuscript received October 4, 2001; accepted in revised form December 13, 2001)
Abstract: Four series of reduced-charge montmorillonites, with ranges in layer charge, were prepared from parent Ca-
montmorillonites of various chemical compositions. Fine fractions of bentonites from Otay (USA), Ivančice (Czech
Republic), Sarigus (Armenia) and Kriva Palanka (Macedonia) were used. The extent of ion exchange, 62—89 % of CEC
covered by Li
, was dependent on the mineral and on the Li
concentration in the liquid phase. Different levels of charge
reduction were achieved via Li-fixation by heating the samples for 24 h at 110—300 °C. Li
ions were fixed in the layers
of all four montmorillonites, but incomplete Li
exchange diminished the extent of charge reduction. Heating
the samples at temperatures up to 140 °C caused Li-fixation and reduction in relative cation-exchange capacity (CEC)
values by 32 to 48 % but only a decrease by 17 to 24 % in the sorption of water and by 4 to 14 % of ethylene glycol
monoethyl ether. The greatest changes in these properties were observed among the samples prepared at 130—200 °C,
while higher temperatures had little effect. The most extensive reduction in CEC, by 81 % after preparation at 300 °C,
was obtained for the Otay montmorillonite, the mineral with the highest octahedral and the lowest tetrahedral charge and
the greatest level of Li
exchange. Higher tetrahedral charge and a lower level of Li
tively affected the decrease in the cation exchange capacities, the specific surface areas and the water uptake capabilities
of the prepared reduced-charge montmorillonites.
Key words: water sorption, EGME specific surface area, cation-exchange capacity, Li-fixation, montmorillonite.
The abundance of montmorillonitic bentonite deposits has led
to extensive studies of its properties and industrial applica-
tions. Montmorillonites are common in many soils, sediments
and hydrothermal alteration products. The minerals of this
group have an expandable lattice, which has a variable c-axis
dimension depending on the number of layers of water mole-
cules hydrating the inorganic exchange cation between silicate
layers. Montmorillonites have interesting plastic, colloidal,
and other properties, which are frequently quite different from
one sample to another (Grim & Kulbicki 1961).
The layer charge and its distribution are among the most im-
portant characteristics of montmorillonites and indicate the ca-
pacity of a mineral to retain and to release cations and to ad-
sorb water and various polar organic molecules (Mermut
1994). The negative charge of the layers arises mainly from
the replacement of Al(III) by Mg(II) in the octahedral sheet.
Charge neutrality is achieved through the presence of hydrated
cations in the interlayer space. These cations can be relatively
easily replaced in the laboratory with other cations, such as
, in order to obtain a homoionic form of the mineral. Upon
the thermal treatment of Li-saturated montmorillonite, Li
ions move towards the negative charge centres in the layers
and become irreversibly fixed. Such cation fixation lowers the
cation exchange capacity of the clay (Hofmann & Klemen
1950; Bujdák et al. 1991). Preparation of reduced-charge
montmorillonites provides a possibility to investigate the
properties of these materials that relate to the layer charge
(Brindley & Ertem 1971; Clementz et al. 1974; Bujdák et al.
The amount of water present in smectite powders is very
variable. It depends strongly on several factors, such as rela-
tive humidity, kind and amount of exchangeable cations, size
and shape of the particles, and structural or crystal-chemical
constraints. For example, in the structurally related musco-
vites, two-thirds of the octahedral positions, and one-fourth of
the tetrahedral positions, are occupied by Al(III). The resultant
2e charge per unit cell is compensated by two K
do not hydrate and the physical properties of mica do not de-
pend on the water vapour pressure in the ambient atmosphere.
On the other hand, the Li-montmorillonites, where the layer
charge is generally less than 1e per unit cell, are particularly
susceptible to swelling by water and their physical properties
are accordingly affected (Bidadi et al. 1988).
With certain cations at interlayer exchange sites, smectites
readily adsorb water and polar molecules, resulting in a
marked expansion of their interlayers (Hendricks et al. 1940).
Because of the layer structure and charge of montmorillonite,
the sorption of vapours on the mineral can occur by different
mechanisms, depending on the molecular properties of the va-
pour. For polar molecules on montmorillonite the overall va-
pour sorption may reflect both surface adsorption and other ef-
fects (Monney et al. 1952). Chiou & Rutherford (1997) studied
the effects of layer charge and of Ca
ramethylammonium exchangeable cations on the sorption of
water and EGME (ethylene glycol monoethyl ether) vapours
on two montmorillonites. With the same exchangeable cation,
94 HROBÁRIKOVÁ and KOMADEL
the high-charge SAz-1 mineral showed a higher water capacity
than the low-charge SWy-1. The hydration of the tetrahedral
sheets of the minerals was found to be relatively weak. The
water uptake was enhanced by the increased charge in the
SAz-1 clay for all exchanged cations and at all p/p
to generally more gain in the energy of cation hydration than in
the energy of layer attraction when the clay is exposed to water
The purpose of this study was to determine how the sorption
characteristics of water, at various relative humidities, and of
EGME, were affected by layer charge in four series of re-
Materials and methods
Four series of reduced-charge montmorillonites were used.
The parent < 2
m fractions were separated from the bentonites
from Kriva Palanka (KP, Republic of Macedonia), Sarigus (Sa,
Republic of Armenia), Ivančice (Iv, Czech Republic) and Otay
(Ot, USA). Most of the exchangeable cations were replaced
using 5—7 times repeated washings with 1 M LiCl dur-
ing ion exchange in dialysis tubing and LiCl solutions. Iv was
ion-exchanged differently, with 0.1—1 M LiCl solutions. This
technique allowed elimination of spinning of big volumes of
dispersions at high rotations; however, full ion exchange was
not achieved. Their layer charge, given as multiples of charge
of one electron, e = 1.6018.10
C, per structural unit
, distribution and structural formulae are listed in Ta-
ble 1. One specimen of each parent material remained unheat-
ed, while the others were heated for 24 h at 110, 120, 130, 140,
150, 160, 180, 200, 250 and 300 °C to evoke different levels of
ion fixation. The details of separation and samples prepara-
tion are described (Hrobáriková et al. 2001).
The cation-exchange capacities (CECs) were determined by
repeated saturation of the samples with 1 M solution of ammo-
nium acetate at pH = 7. All extracts obtained from the same
sample were combined and analysed for Ca and Li by atomic
absorption and emission spectroscopy, respectively. Infrared
(IR) spectra in the 4000—400 cm
spectral range with a resolu-
tion of 4 cm
were obtained on KBr pressed disks (0.4 mg
sample and 200 mg KBr), using a Nicolet Magna 750 FTIR
spectrometer equipped with a DTGS detector.
The relative total specific surface area was determined by
ethylene glycol monoethyl ether (EGME) adsorption, follow-
ing the method of Novák & Číčel (1970). The samples were
dried under vacuum in a desiccator over P
for 48 h and
weighted afterwards. A few drops of EGME were added in
several minor portions to each sample (~250 mg) until a slight
excess of EGME was achieved. The point when the clay be-
came unable to accept more EGME was clearly visible. After-
wards the samples with EGME were stored in a vacuum desic-
cator over ignited CaCl
. They were weighed every 90 minutes
until constant mass was achieved and the total specific surface
area was then calculated according to Novák & Číčel (1970).
Sorption and desorption isotherms of water vapour were de-
termined under static conditions. Briefly, ~250 mg of the sam-
ples were dried at 60 °C overnight; then kept at 25 °C for 48
hours in a desiccator above P
, at which time the amount of
adsorbed water at 0 % RH was determined gravimetrically.
The samples were then stored under the next higher RH for
the following 48 hours. On the basis of the previous skills of
Bujdák et al. (1992) with similar amounts of samples and ex-
periments, this period of time was considered sufficient to ob-
tain the equilibrium. This procedure was repeated in dessica-
tors above the saturated solutions of CaCl
or above H
O, for RHs of 20, 43,
52, 66, 88 and 100 %, respectively. The results were collected
and the sorption curves plotted. Then, the samples were stored
under gradually decreasing RHs to obtain the corresponding
desorption curves. All the data were measured in triplicates;
the relative error was generally < 3.2 % of the measured value.
Results and discussion
Montmorillonite with Al-rich octahedral sheets was found
to be the dominant mineral in all four separated samples. No
admixtures were detected in KP, but minor amounts of pyro-
phyllite were found in Ot, amorphous SiO
in Iv and opal-CT
in Sa (Hrobáriková et al. 2001). Opal-CT, the commonest
form of hydrous silica, is composed of disordered stacking of
cristobalite-like and tridymite-like sequences (Jones & Segnit
Cation-exchange capacity (CEC) is a directly measurable
property dependent on the layer charge. Higher temperature
during preparation caused greater Li
fixation, greater layer
charge reduction and thus, a more extensive decrease in rela-
tive CEC in all four series (Fig. 1). Preparation below 140 °C
decreased significantly the CEC in all four series. The same
, was retained in rela-
tive CEC reduction for the samples prepared
140 °C. The
CEC values decreased systematically with increasing temper-
ature of preparation up to 250 °C, but further decreases were
negligible for the materials prepared at 300 °C (Fig. 1). The
most extensive CEC reduction occurred in the Ot series, with
the parent montmorillonite of the highest octahedral and the
Fig. 1. Relative cation exchange capacities for series of reduced-
charge montmorillonites prepared by heating for 24 hours at indi-
SORPTION PROPERTIES OF REDUCED-CHARGE MONTMORILLONITES 95
lowest tetrahedral charge (Table 1). After heating this sample
at 300 °C, the CEC decreased to 19 % of the value obtained for
the unheated Ot. The second greatest decrease was observed in
the KP series, yielding a CEC of 27 % of its original CEC after
heating at 300 °C. The KP montmorillonite has a lower octahe-
dral charge (—0.72 e/O
) than Ot as well as a slightly
higher tetrahedral charge (—0.15 e/O
). Relative CEC
values decreased considerably less (to 38 % of the original) in
the Sa series, and were presumably affected by the negative
contribution of the relatively high tetrahedral charge, partially
compensated by possibly unexchangeable K
least extensive decrease in relative CECs occurred in the Iv se-
ries (Fig. 1) with the lowest Li
(Hrobáriková et al. 2001). Relative CEC was reduced only to
45 % of CEC of unheated sample presumably due to lower ex-
cess of Li
in the used solutions.
The relative EGME specific surface areas (SSAs) obtained
for the four series of reduced-charge montmorillonites are
shown in Fig. 2. The values for the parent, unheated montmo-
rillonites were 745, 796, 706 and 799 m
for samples Ot,
KP, Sa and Iv, respectively. The presence of opal-CT and the
high tetrahedral charge of —0.43 e/O
together with the
relatively high K
content saturating 18 % of the negative
charge, suggest possible presence of illitic, non-swelling inter-
layers (Hrobáriková et al. 2001) in Sa and explain its lowest
SSA. The SSAs in all series decreased only slightly with in-
creasing temperature of preparation up to 130 °C. This de-
crease was by 10 % in KP, 8 % in Sa, 3 % in Ot and 1 % in Iv
(Fig. 2). Presumably the interlayer space was fully accessible
to EGME molecules for the montmorillonitic minerals in each
Higher temperatures of preparation resulted in further de-
creases in SSA and presumably led to the formation of low
charged pyrophyllite-like layers. The enhanced Li
these temperatures, accompanied by reduction of the negative
layer charge, is known to decrease the swelling ability of re-
duced-charge montmorillonite (Komadel et al. 1996). The
bands near 1120 and 420 cm
, assigned to absorption of pyro-
phyllite-like layers (Farmer 1974; Madejová et al. 1999) can be
distinguished in the IR spectra of Iv, KP, Ot and Sa samples
150 °C (Fig. 3). The original fine fraction of Ot
bentonite contained an admixture of pyrophyllite, thus the IR
spectra of all Ot samples in the series displayed absorption
bands at these wavenumbers. Increasing intensity of these
bands confirmed increasing content of pyrophyllite-like layers
in the Ot samples prepared at
150 °C (Hrobáriková et al.
Table 1: Tetrahedral, octahedral and total charges and structural formulae of parent Li-montmorillonites.
Fig. 3. Infrared spectra of samples Iv, Sa, KP and Ot prepared by
heating for 24 hours at 150 °C.
Fig. 2. Relative EGME specific surface areas for series of reduced-
charge montmorillonites prepared by heating for 24 hours at indi-
2001). EGME molecules could not penetrate into the non-
swelling interlayer spaces. This was reflected in decreased
EGME SSAs obtained for the samples prepared at
The effect of heating at 150—200 °C on the samples was
clearly different (Fig. 2). The relative EGME SSA decrease
96 HROBÁRIKOVÁ and KOMADEL
was more pronounced for the KP and Ot series than for the Sa
or Iv series, as was observed for the relative CEC values (Fig.
1). The relative SSAs of the samples prepared at 200 °C de-
creased to 38, 43, 61 and 68 % of the values obtained for the
parent KP, Ot, Sa and Iv samples, respectively. The exact rea-
son for this behaviour remains unclear, however, possible dif-
ferences in the homogeneity of the layer charge distribution in
the parent samples could be one of the explanations. As was
observed for CEC, differences between the samples in the
same series prepared at 250 and 300 °C were minor, proving
that heating of Li-montmorillonites beyond 250 °C results in
little further change to their properties (Madejová et al. 1999).
For the samples prepared at 300 °C the values obtained were
27, 34, 45 and 44 % for KP, Ot, Sa and Iv (Fig. 2). The final
extent of reduction in relative SSAs reflected the negative in-
fluence of the highest tetrahedral charge in Sa and the lowest
substitution in Iv.
Comparison of relative CECs and relative EGME SSAs for
four series of reduced-charge montmorillonites is shown in
Fig. 4. It demonstrates clearly that CEC was affected to a
greater extent than SSAs at low and intermediate preparation
temperatures; indicating that decreases in layer charge had a
greater effect on the exchange capacity than the total surface
area available for the sorption of EGME molecules. This effect
is most significant for the Ot series, i.e. for the mineral of the
highest octahedral charge, in which the relative charge de-
crease was the greatest before the substantial decrease in rela-
tive EGME surface area could be observed (Fig. 4).
Representative water sorption-desorption curves (SDCs) of
the Sa series are illustrated in Fig. 5. The shapes of the SDCs
for the other three reduced-charge montmorillonite series (not
shown) were similar. The inclination of isotherms was influ-
enced by the layer charge reduction. The irregular shapes of
both the adsorption and desorption branches of the isotherms
suggested complex water adsorption mechanisms. More water
remained on all the samples during desorption, i.e. when the
drying parts of the sorption/desorption curves were measured.
Such hysteresis was reported to occur due to more extensive
water adsorption mainly on the external surfaces of the mont-
morillonite particles throughout the drying rather than wetting
experiments (Ormerod & Newman 1983; Bujdák et al. 1992).
The amount of water sorption was dependent on RH, as is of-
ten the case for swelling clay minerals (Cases et al. 1992). As
was expected from the high hydration energy of the Li
hydration of the interlamellar space of reduced-charge samples
started at very low relative pressures. The samples prepared at
the lowest temperatures of 110 and 120 °C had a higher affini-
ty for water (Fig. 5, data for 120 °C are not shown, they over-
lap with those for 110 °C) than the untreated samples. Higher
water uptake by these samples, in comparison to the parent Li-
montmorillonites, could be caused by lower hydration of their
interlayer cations brought about via more extensive dehydra-
tion upon preparation at increased temperatures of 110 or 120
vs. 60 °C.
The samples prepared at 130—300 °C were less hydrated at
all investigated RHs. Water content in these samples gradually
decreased with decreasing layer charge and with decreasing
surface area. The effect of charge reduction on hydration was
better observed at high relative humidity: at a high RH, more
water was adsorbed, but differences between samples were
greater; as the RH decreased, both the total amount of water
adsorbed and differences between the materials decreased. At
high RH, above ~80 %, the water vapour hydrated interlayer
cations and condensed more effectively in the micropores and
interlayer voids by wicking, resulting in saturation of water
sorption capacities of the samples (Güven 1992). The water
molecules filled the interlayers as well as the spaces between
the mineral particles.
Relative water sorption data obtained at 100 % RH for four
series of reduced-charge montmorillonites are shown in Fig. 6.
All samples prepared at 110 and 120 °C adsorbed a similar
amount of water to their parent counterparts. Preparation at
higher temperatures, resulting in Li
fixation and layer charge
reduction as well as more extensive dehydration of the remain-
cations in the interlayers, led to materials of gradually
decreased water uptake ability. The data for the samples pre-
pared at 250 and 300 °C overlapped, again confirming the sim-
Relative humidity [%]
Fig. 5. Effect of preparation temperature on water sorption and de-
sorption of the Sa series at various relative humidities.
Fig. 4. Comparison of relative cation exchange capacities and rel-
ative EGME specific surface areas for four series of reduced-
SORPTION PROPERTIES OF REDUCED-CHARGE MONTMORILLONITES 97
ilarity in hydration properties of these pairs of samples. The
changes in water sorption within the series (Fig. 6) and among
the series were similar to those observed for the relative
EGME specific surface areas (Fig. 2), showing the negative
effect of the increasing number of pyrophyllite-like layers on
sorption properties of all four series investigated. This infor-
mation along with IR and RTG results shows the negative ef-
fect of the increasing number of non-expandable layers on the
water uptake. Comparison of the relative cation exchange ca-
pacities and the relative levels of water sorption at 100 % RH
is made in Fig. 7 for all four series of reduced-charge montmo-
rillonites. As was observed for EGME uptake, the relative wa-
ter uptake decreased at a lower rate that the decrease of rela-
tive CECs for sample preparation below 200 °C. Heating of
the samples for 24 hours at temperatures < 200 °C evoked
more pronounced decreases in the relative CECs than in the
relative amount of water sorbed. Similarly to EGME sorption
(Fig. 4), the effect is most pronounced for the Ot series, where
the most extensive reduction of the relative CEC was observed
before the relative water sorption effectively decreased from
its initial value (Fig. 7).
Four series of reduced-charge montmorillonites with gradu-
ally decreasing cation exchange capacity were prepared from
different parent Li-montmorillonites by heating at 110—300 °C.
Preparation at 110—130 °C produced materials with signifi-
cantly decreased CEC, but with rather minor modification in
both the total specific surface area as well as water and EGME
sorption capabilities. All investigated properties of the prod-
ucts prepared at higher temperatures were more significantly
modified, presumably due to the development of collapsed and
non-swelling pyrophyllite-like interlayers. Negligible differ-
ences in properties of materials prepared at 250 and at 300 °C
proves that heating at 250 °C for 24 hours is sufficient to
achieve a maximal extent of Li
fixation and layer charge re-
Fig. 7. Comparison of relative cation exchange capacities and rel-
ative water sorption at 100 % RH for four series of reduced-charge
Fig. 6. Relative water sorption data at 100 % relative humidity for
series of reduced-charge montmorillonites prepared by heating for
24 hours at indicated temperatures.
Acknowledgment: The authors appreciate helpful comments
of Drs. Bujdák, Gates and Serwicka on an earlier version of
this paper and financial support of the Slovak Grant Agency
VEGA (Grant No. 2/7202).
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