GEOLOGICA CARPATHICA, 50, 5, BRATISLAVA, OCTOBER 1999
373–378
ACIDITY OF PROTON SATURATED AND AUTOTRANSFORMED
SMECTITES CHARACTERIZED
WITH PROTON AFFINITY DISTRIBUTION
MARIÁN JANEK and PETER KOMADEL
Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 842 36 Bratislava, Slovak Republic
(Manuscript received September 3, 1998; accepted in revised form December 9, 1998)
Abstract:
Potentiometric titration of proton saturated fine fractions of three bentonites were used to characterize the
acid centre at the smectite-water interface in dispersions. The H-forms were prepared by H
+
→
OH
–
→
H
+
ion ex-
change with resins. The obtained titration data were used in a thermodynamic calculation of proton affinity distribu-
tion. Numerical solving of an integral adsorption equation revealed a continuous distribution of proton interaction
sites. Proton affinity distributions clearly detected up to 5 different proton interaction sites in all the smectite-water
systems, within accessible experimental range of pH’s between 2 and 12. The amount of the strongest acid sites, i.e.
those with the lowest pK values, decreased on aging, while the amounts of all weaker acid sites increased with the
progress of autotransformation. The strongest acid sites are connected with free protons present in the dispersion,
while the weaker acid sites are connected with the titration of released structural Al
3+
, Fe
3+
, Mg
2+
cations and/or their
hydrolyzed species, and deprotonation of SiOH groups. These results indicate the sources of acidity in acid activated
bentonites.
Key words:
acid sites, autotransformation, potentiometric titration, proton affinity distribution, smectite.
Introduction
Characterization of acidity of clay minerals is connected
with fundamental questions of the existence of proton inter-
action sites, which may influence their colloid properties,
rheology, reactivity, applicability, potential equilibrium reac-
tions in natural systems and aquatic environments, and
geochemical processes on solid oxide-water interfaces
(Stumm et al. 1980; Hayes & Leckie 1987; Bleam 1993;
Contescu et al. 1993a). This topic is also related to the char-
acteristics of acid activated bentonites due to their advanta-
geous properties and the wide scale of applications (Siddiqui
1968; Fahn & Fenderl 1983; Breen 1991; Breen & Watson
1998).
The main constituents of bentonite ores are clay minerals
from the smectite group. During the activation with strong
inorganic acid solutions, protons diffuse in the interlayer
space of smectite, exchange the original exchangeable cat-
ions, and acid attack on the layers starts (Siddiqui 1968;
Číčel 1991). Acid attack on smectite structure proceeds from
the marginal interface of the particles and structural cations
of octahedral sheets are leached out. However, a significant
contribution to leaching of structural cations via interlayer
space interface can occur (Číčel 1991; Komadel et al. 1996;
Janek et al. 1997). Depending on the extent of acid activa-
tion, affected by temperature, time, acid concentration, solid
to acid ratio, etc., the resulting solid product contains unaf-
fected layers and amorphous three-dimensional cross-linked
silica (Madejová et al. 1998; Tkáč et al. 1994), while the acid
solution contains ions according to the chemical composition
of smectite and the acid used.
Surface acidity of a solid can be assessed by investigation
of its interaction with suitable probe molecules (Tanabe
1970; Breen 1991; Brown & Rhodes 1997) or by potentio-
metric titrations of its suspension (Janek & Komadel 1993;
Wanner et al. 1994; Brady et al. 1996; Qing et al. 1997).
However, a sufficient number of reliable potentiometric titra-
tion data is important for their meaningful numerical treat-
ment, involving thermodynamic equilibrium calculations
(Contescu et al. 1993a).
Potentiometric titrations were employed by Brady et al.
(1996) to obtain pH dependent surface charge properties of
kaolinite, acid-base chemistry of illite (Qing et al. 1997) and
of montmorillonite (Wanner et al. 1994). A proton affinity
distribution calculation was used by Bandosz et al. (1994)
and Jagiełło et al. (1995) for characterization of pillared
clays and by Bandosz et al. (1996, 1997) to study surface
acidity of hydroxychromium taeniolites or bentonite. A sta-
ble numerical solution of the adsorption integral equation us-
ing splines (SAIEUS) was applied to obtain the proton affini-
ty distribution by the latter authors. The details of this
numerical approach were presented by Jagiełło (1994).
An important aspect of these potentiometric measurements
is their simplicity and achievement of apparent and/or intrin-
sic equilibrium constant values. Jagiełło et al. (1995) showed
by potentiometric titration of citric acid and by analysis of at-
tained proton affinity distribution that this approach revealed
the pK values which were in an excellent agreement with for-
merly reported pK values of citric acid deprotonation. In ad-
dition, the obtained values of molar ratios of detected car-
boxylic groups were in a very good agreement with the
known structural composition of this acid and can be regard-
ed as a suitable test of reliability of this approach.
In the presented study, potentiometric titrations of freshly
proton-saturated forms of smectites of various chemical
compositions were used to obtain their proton affinity distri-
374 JANEK and KOMADEL
bution. However, proton saturated smectites are known as
unstable materials, spontaneously changing to (H, Al, Fe,
Mg)-forms (Barshad & Foscolos 1970; Bednáriková et al.
1993; Janek & Komadel 1993) upon aging, that is with the
progress of autotransformation. This process is closely relat-
ed to proton attack on smectite structure in acids, which oc-
curs during the preparation of acid-activated bentonites.
Therefore, our interest was also focused on proton affinity
distributions of equilibrium product autotransformation reac-
tion. The main objective of this paper was to characterize the
acid sites of three fresh proton saturated and autotransformed
smectites of various chemical composition using proton af-
finity distribution analysis.
Materials and methods
Fine fractions of three bentonites were used in this study:
JP montmorillonite (Jelšový Potok, Slovakia), ST iron rich
beidelite (Stebno, Czech Republic), and SWa-1 ferruginous
smectite (Grant County, Washington). The clays were Ca
2+
saturated, fractionated to < 2
µ
m, washed free of excess ions,
dried at 60 °C and ground to pass a 0.2 mm sieve. XRD and
IR spectroscopy revealed dioctahedral smectite as the domi-
nant mineral in all of these fine fractions of bentonites, how-
ever, minor admixtures of quartz and kaolinite were identi-
fied in ST sample. About 21 % of total iron present in ST
was bound in goethite and 79 % in smectite (Číčel et al.
1992). The structural formulas of all samples used were pub-
lished elsewhere (Janek et al. 1997).
Before preparation of H-forms, the samples were Na
+
satu-
rated using 1 M NaCl solution, washed free of excess ions
with water and ethanol, air dried at 60 °C, and ground to pass
a 0.2 mm sieve. Subsequent treatments, employed to remove
readily soluble oxo/hydroxo phases, such as iron and/or alumi-
num oxides/hydroxides, were performed with sodium oxalate
solution. This procedure ensured maximal saturation of smec-
tite with protons (Aldrich & Buchanan 1958; Barshad 1969).
The dispersions of Na-smectites were passed through a train of
three columns under applied suction. The first and the last col-
umn was filled with the resin Amberlit IR-120 in the H
+
form,
the middle one with the resin Wofatit SBW in the OH
-
form
(Barshad 1969).
The autotransformation process was monitored by potentio-
metric titration at 25 °C using a Mettler DL-21 automatic titra-
tor. About 170 data points were collected for each sample in a
step-mode titration and pH values were recorded after 30 sec-
ond reaction and electrode response stabilization times. A
combined-glass microelectrode Schott-Geräte (N 6000 A)
with a platin diaphragm was used as a pH reading sensor. Cali-
bration was performed prior to measurements with Merck
buffer solutions of pH
1
= 4.00 and pH
2
= 9.18 according to
National Bureau of Standards (NBS). The slope of the elec-
trode response was 58.9 mV/pH, which means a 99.6 % re-
sponse of theoretical ideal value was achieved and changed by
less than 0.1 % per month during the period of about two
months needed to complete the titration experiments.
50 ml of freshly prepared H-smectite dispersion were im-
mediately titrated with 0.1 M KOH (Titrisol Merck standard
solution) and the rest of the dispersion was left to age in a
tightly closed polyethylene flask at 90 °C. After four days
and again after an additional 48 hours of aging 50 ml por-
tions of dispersion were used for the next titrations. If the last
two potentiometric curves obtained were identical, that is the
part of the titration curves indicating consumption of free
protons was not present any more, the autotransformation
process was considered to be completed. To facilitate the cal-
culation of the molar amount of protons interacting with the
solid from potentiometric titration curves, parallel determi-
nation of dispersion density and mass fraction of the solid
phase were performed gravimetrically.
Proton affinity distributions were calculated from potentio-
metric titration curves of freshly proton saturated and au-
totransformed smectite dispersions using the SAIEUS pro-
gramme (Jagiełło 1994). Theoretical considerations of proton
affinity distribution calculations on alumina and alumina
modified surfaces, published by Contescu et al. (1993a,
1993b), can be briefly summarized as follows:
(i) It is assumed that the proton affinity of various oxygen/
hydroxyl groups at the solid–solution interface is determined
by the same factors as the number and type of the subadja-
cent coordinating structural atoms, which is a fundamental
base for the differences of acid/base properties of surface ox-
ygen atoms (Bleam 1993).
(ii) Proton transfer reaction may be described with the
equilibrium reaction characterized by experimentally distin-
guished apparent equilibrium constant; one-pK model (Van
Riemsdijk et al. 1987; De Wit et al. 1990).
(iii) A single population of proton transfer sites in a proto-
nated state at a given pH is described by a Langmuir type ad-
sorption isotherm:
[1]
where q is the fraction of sites with characteristic apparent
affinity constant pK, which are protonated at the given pH.
(iv) Investigation of apparent rather than intrinsic affinity
constants ensures that coulombic interactions as long range
interactions are neglected, but chemical binding forces as
short range interactions are reflected by this model. The ionic
strength of used solution was found to have a minor effect on
deconvoluted distributions (Contescu et al. 1993a).
(v) The experimentally measured proton binding isotherm
derived from potentiometric titration curve involves the theo-
retical proton balance equation:
[2]
where m is the mass of the solid in dispersion, V
0
is the initial
volume of the solution, V is the volume of added titrant, C
t
is
the concentration of the titrant and [H]
f
and [OH]
f
are the con-
centrations of actual pH determining species calculated from
the values of dispersion pH after its correction for activity co-
efficients using Davie’s equation (Contescu et al. 1993b).
(vi) Q represents the total amount of protonated sites with-
in an experimental window varied within limits of experi-
mentally accessible pH range and depends on pK of distin-
guished individual sites via the integral equation:
1
)
p
pH
(
]
10
1
[
−
+
+
=
K
q
)}
]
OH
[
]
H
).([
(
.
{
m
1
f
f
0
t
−
+
−
=
V
V
C
V
Q
ACIDITY OF PROTON SATURATED AND AUTOTRANSFORMED SMECTITES 375
Fig. 1.
Recalculated proton desorption isotherms of JP sample (a)
and revealed proton affinity distribution (b).
[3]
which is a Fredholm integral equation of the first kind (Ned-
erlof et al. 1990). Here f(pK) is distribution function of acid
sites and is the differential quantity, which represents the
number of sites with pK values in the interval (pK–dpK,
pK+dpK). Q
0
is an arbitrary reference level constant refer-
ring to the amount of bonded sites outside the integration
limits
α
,
β
(Bandosz et al. 1994; Jagiełło et al. 1995). Solu-
tion of the Fredholm integral equation is a formidable mathe-
matical task and is complicated by problems with the ill-con-
ditioned character of the matrix obtained from the system of
linear equations during its numerical solution. Nederlof et al.
(1990) discussed related problems with respect to determina-
tion of the adsorption affinity distributions and local iso-
therm approximations. Jagiełło (1994) proposed a numerical
method for stable solution of the adsorption integral equation
utilizing a non-negativity constrain of B-spline functions for
f
(pK) representation and regularization principle as stabiliza-
tion procedures of the solution.
(vii) Release of structural cations in course of autotransfor-
mation should be reflected in the proton affinity distribution
as proton interaction sites due to hydrolysis and/or their am-
photeric character. These simultaneous equilibrium reactions
are expected to be superimposed in the resulting proton affin-
ity distribution. The released cations are supposed to be ad-
sorbed on the surface of the particles as hydrated or partially
hydrolyzed outer-sphere complexes (Sposito 1992).
Results and discussion
Typical proton desorption isotherms of freshly prepared
and autotransformed H-smectites of characteristic shapes for
these materials are presented in Figs. 1a–3a. The isotherms
of freshly prepared H-forms show titration of both strong and
weak acid sites. The addition of 0.1 M KOH solution result-
ed in an access of sites which were neutralized by the added
basic solution without changing the pH of the titrated disper-
sion. This is indicated by the steep decrement of Q function.
It is notable that the used clays differ in their ability of proton
retention which depends on the different structural ‘resistivi-
ty’ of each sample against the attack of protons upon prepa-
ration (Janek & Komadel 1993; Janek et al. 1997). The sam-
ple preparation procedure ensured maximal saturation with
protons for the very beginning of the experiment with their
molar amount equal to the cation exchange capacity of smec-
tite. Once the protons exchange the original exchangeable
cations, they are consumed in the reaction of autotransforma-
tion. Therefore pH values of freshly proton saturated and au-
totransformed samples were found to be 2.48, 2.61, 2.62; and
3.40, 3.66, 3.11; for JP, ST and SWa-1, respectively. The rate
of autotransformation is different for each smectite used and
the structural cations are released in various ratios (Barshad
& Foscolos 1970; Janek & Komadel 1993).
Proton desorption isotherms of autotransformed samples
differ from those of the fresh samples. They revealed depen-
dencies with missing parts of the strongest acid sites at the
lowest pH’s. An obvious decline of Q was found for all sam-
ples near pH = 6, which indicates the presence of weak acid
sites in this pH region. Distinguishing of proton interaction
sites by the evaluation of the slope of tangent to Q function
only provides the same qualitative information as investiga-
tion of the original volumetric titration curves. However, in the
case of reliable solution of the adsorption integral equation [3],
the proton interaction sites can be detected on the pK scale.
The obtained proton affinity distributions in the studied
smectite-water systems are shown in Figs. 1b–3b. Several re-
solved peaks representing different proton interaction sites of
various pK values protonated at the intervals pK–dpK,
pK+dpK were found within the accessible experimental pH
window. In agreement with characterization of deprotonation
equilibrium reactions, the strongest acid sites are of the low-
est pK values and the weakest acid sites are those of the
highest pK values. With increasing pH of the dispersion, the
single sites are gradually deprotonated. Jagiełło et al. (1995)
has proposed that the theoretical pK distribution of a pure
chemical compound should be represented by a sum of delta
Dirac functions. Nevertheless, we believe that representation
of such a dependence using continuous spline functions are
strongly realistic due to the existence of Boltzmann energy
distribution of any investigated proton interaction site.
2
4
6
8
10
12
0.0
0.2
0.4
0.6
0.8
b
f(p
K
) /
mmo
l.
g
-1
p
K
-2.0
-1.6
-1.2
-0.8
-0.4
0.0
2
4
6
8
10
12
a
pH
JP
(Q +
Q
0
) /
mmo
l.
g
-1
Freshly prepared H-form
Autotransformed sample
0
p
)
p
(
)
p
,
pH
(
)
pH
(
Q
K
d
K
f
K
q
Q
+
=
∫
β
α
376 JANEK and KOMADEL
The results of observed pK values are summarized in Table
1. Strong acid proton interaction sites with pK’s between 2
and 3 and weak acid sites with pK’s above 5 were found in
all freshly proton saturated smectites. The existence of both
sites is in agreement with the previously reported qualitative
characteristics of proton saturated smectites (Janek & Ko-
madel 1993; Janek et al. 1997; Komadel et al. 1997). Several
weak proton interaction sites are detected in the affinity dis-
tribution. Upon autotransformation, the amounts of weak
acid sites of pK’s above 5 increased while the strongest acid
sites disappeared or decreased in amount for all samples
studied (Figs.1b–3b). This is connected with consumption of
protons in the reaction of autotransformation. The strong
acid sites are also consumed during the storage of acid-acti-
vated bentonites.
Assignment of single interaction sites from calculated pro-
ton affinity distributions is important for better understand-
ing of acidity in acid treated clays. As mentioned above, the
strongest acid sites with pK of about 2.7 are connected with
protons in the diffuse doublelayers on the surface and in the
interlayers, potentially interacting with the edges of the parti-
cles. Relatively strong acid sites in autotransformed ferrugi-
nous smectite SWa-1 with pK = 2.9 are noteworthy. Such
strong acid sites are absent from the other two autotrans-
formed clays (Figs. 1b–3b, Table 1). This is due to differenc-
es in the chemical composition of the smectites used. SWa-1
contains much more iron than JP and ST samples. The strong
acid sites present after autotransformation result from titra-
tion of released [Fe(H
2
O)
6
]
3+
cations which have the highest
tendency for hydrolysis in comparison to [Al(H
2
O)
6
]
3+
and
[Mg(H
2
O)
6
]
2+
(Baes & Mesmer 1976). Absence of these
sites on JP and ST samples, which also contain some struc-
tural iron, is either because concentrations of iron are below
their possible detection by potentiometric titrations, or their
aqueous chemistry is influenced by the common iron substi-
tution by aluminum in their oxo/hydroxides originated by
precipitation using a solution of alkali metal hydroxides
(Schwertmann 1984), thus the presence of other cationic spe-
cies hindered the detection of small amounts in JP and ST.
Weak acid sites detected in the range of pK’s between 5
and 9 are due to the amphoteric character of the released
Fig. 2.
Recalculated proton desorption isotherms of ST sample (a)
and revealed proton affinity distribution (b).
Fig. 3.
Recalculated proton desorption isotherms of SWa-1 sample
(a) and revealed proton affinity distribution (b).
Table 1:
pK values of five proton interaction sites obtained from
proton affinity distributions for freshly proton saturated (pK
Fps
)
and autotransformed (pK
Au
) samples.
Sample
pK
Fps
/ pK
Au
protonation 2
←
pH
→
12 deprotonatio n
JP
2.7 / –
6.1 / 5.3
8.1 / 8.1
9.2 / 9.6
11.3 / 11.2
ST
2.7 / –
5.1 / 5.6
8.5 / 8.5
9.7 / 9.8
11.3 / 11.3
SW a-1
2.7 / 2.9
5.6 / 5.4
8.1 / 7.0
9.4 / 9.6
11.3 / 11.3
2
4
6
8
10
12
0.0
0.2
0.4
0.6
0.8
b
f(p
K
) /
m
m
o
l.g
-1
pK
-1.6
-1.2
-0.8
-0.4
0.0
2
4
6
8
10
12
a
pH
ST
(Q +
Q
0
) /
m
m
o
l.g
-1
Freshly prepared H-form
Autotransformed sample
2
4
6
8
10
12
0.0
0.3
0.6
0.9
1.2
b
f(p
K
) /
m
m
o
l.g
-1
p
K
-2.0
-1.6
-1.2
-0.8
-0.4
0.0
2
4
6
8
10
12
a
pH
SWa-1
(Q +
Q
0
) /
m
m
o
l.g
-1
Freshly prepared H-form
Autotransformed sample
ACIDITY OF PROTON SATURATED AND AUTOTRANSFORMED SMECTITES 377
structural cations and/or their hydrolysis. In the course of a
potentiometric titration, firstly are neutralized [Al(H
2
O)
6
]
3+
cations with pK of about 5.5, forming a hydroxide (Low
1955). By following additions of the titrant, Al(OH)
4
anions
are probably formed at a pK of about 8.3. The next proton in-
teraction site with pK = 9.5 is supposed to be due to titration
of [Mg(H
2
O)
6
]
2+
cations, however, some iron cations can
possibly get titrated in this region. On the basis of these con-
siderations, released structural cations can be involved in the
following reactions taking place during titration, which could
be considered for simulations of titration curves:
[4]
[5]
[6]
[7]
[8]
M is one of the released structural cations with charge z
(Al
3+
, Fe
3+
, Mg
2+
). Possible reactions including different ion
species are not shown.
Reactions [4] to [7] can accompany the interactions of
present hydrolytic and/or hydroxo species with the surface of
mineral particles (Thompson & Tahir 1991; Thompson &
Mitchell 1993) as simultaneous equilibrium reactions. Titra-
tion of homosaturated and/or in defined cation ratios pre-
pared (Al/Fe/Mg)-forms can possibly help to identify inter-
action sites on the proton affinity curves in future
experiments. Furthermore, all these effects are clearly re-
sponsible for the detected shifts of the peak positions with
pK’s from 5 to 9 (Figs. 1b–3b, Table 1).
The weakest acid sites appearing within experimentally
accessible pH window were identified at pK = 11.3. These
sites are supposed to be due to deprotonation of structural
SiOH groups present on the edges of smectite particles.
These functional groups occur in some acid untreated clays
as well as in amorphous silica, the reaction product of acid
treatment of clays. The amount of these groups increased
upon autotransformation (Figs. 1b–3b). A similar trend was
found by Tkáč et al. (1994) and Komadel et al. (1996) who
reported formation of SiOH groups with the extent of acid
treatment of smectites detected by
29
Si MAS NMR.
Moreover, Rand et al. (1980) have concluded from their
rheological investigations of montmorillonite dispersions, that
isoelectic points of the edge-surface of montmorillonite parti-
cles should exist at pH’s below 4 and above 10. Here, the low-
est (2.7) and the highest (11.3) pK values reported in this work
are in agreement with edge-surface proton interaction reaction,
as was schematically presented by Lagaly (1989). Hence, pro-
ton affinity distribution offers detailed characterization of pro-
ton interaction reactions at clay mineral/water interface which
are fundamental for rheological studies of clay mineral parti-
cles in dispersions (Brandenburg & Lagaly 1988; Güven &
Pollastro 1992; Permien & Lagaly 1994).
Conclusions
Proton saturated smectites were prepared from the < 2
µ
m
fractions of three bentonites. Potentiometric titration of pro-
ton saturated forms was used to characterize the acid centers
at the smectite-water interface. The H-forms were prepared
by H
+
→
OH
-
→
H
+
ion exchange with resins. Autotransforma-
tion of smectite dispersions was performed at 90 °C. Titra-
tion data were further used in the thermodynamic calculation
of proton affinity distribution. Numerical solution of an inte-
gral adsorption equation offered continuous distribution of
proton interaction sites.
Proton affinity distributions clearly detected up to 5 differ-
ent proton interaction sites in all proton saturated smectite/
water systems, within the accessible experimental range of
pH’s between 2 and 12. The amount of the strongest acid
sites, that is those with the lowest pK values, decreased on
aging, while the amounts of all weaker acid sites increased
with the progress of autotransformation. The strongest acid
sites are connected with free protons present in the disper-
sion, while the weaker acid sites are connected with the titra-
tion of released structural Al
3+
, Fe
3+
, Mg
2+
cations and/or
their hydrolyzed species, and with deprotonation of function-
al SiOH groups. These results indicate the nature of acid
sites in H-bentonites and can be helpful for interpretation of
the rheological properties of acid/base conditioned smectites
dispersions.
Acknowledgements:
The authors thank Dr. J. Jagiełło and
Dr. K. Putyera for their kind assistance in proton affinity dis-
tribution calculation. The support of Slovak Grant Agency
(Grant 2/4042/98), and NATO Linkage Grant ENVIR. LG
960569 for partial support to purchase volumetric titration
apparatus is gratefully acknowledged.
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O
6H
M(OH)
zOH
]
O)
[M(H
2
z
z
6
2
+
⇔
+
−
+
+
−
−
+
+
⇔
+
O
nH
]
O)
(H
[M(OH)
O
nH
]
O)
[M(H
3
n)
(z
n
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2
n
2
z
6
2
+
−
−
+
+
⇔
+
O
nH
]
O)
(H
(OH)
[M
O
nH
]
O)
m[M(H
3
n)
(zm
n
6m
2
n
m
2
z
6
2
−
−
⇔
+
4
3
M(OH)
OH
M(OH)
−
+
+
⇔
OH
O
H
O
2H
3
2
-
378 JANEK and KOMADEL
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