GEOLOGICA CARPATHICA, 53, 2, BRATISLAVA, APRIL 2002
ALUMINIUM RELEASE RATES FROM ACIDIFIED CLAY
STRUCTURES: COMPARATIVE KINETIC STUDY
, JIŘÍ HOSTOMSKÝ
and JANA SOUKUPOVÁ
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 250 68 Řež,
Czech Republic; email@example.com
Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science,
Charles University, Albertov 6, 128 43 Prague 2, Czech Republic
(Manuscript received October 4, 2001; accepted in revised form December 13, 2001)
Abstract: Dissolution rates of several clay materials (reference kaolinite samples, natural kaolinites of different origin,
illitic clays, halloysite) in 0.5 and 5 M sulphuric and 1 M hydrochloric acids were determined by measuring the release
rate of aluminium into solution. The X-ray powder diffraction and X-ray fluorescence methods have been employed for
mineralogical and chemical analyses of clay samples, respectively. The surface area of the samples was determined by a
adsorption technique (BET). The dissolved Al concentration was determined by spectrophotometry. The dissolution
rate of kaolinite in 0.5 M sulphuric acid at 25 °C
is approximately three times higher than in hydrochloric acid of
formally equivalent H
concentration. The dissolution in 5 M sulphuric acid is eight times faster if the solid phase is
periodically separated from the acid solution, washed by distilled water and dried. The aluminium release rate decreases
with the increasing amounts of clay micas in kaolinitic clays and is affected by the concentration of Al in the mineral
structure. Crystallinity, as a factor reflecting the quality of the crystal structure, influences significantly the aluminium
release rates during dissolution by acids.
Key words: kaolinite, illite, halloysite, aluminium release rates, sulphuric acid, hydrochloric acid.
Acid-sensitive surface and soil waters are characterized by in-
sufficient neutralizing capacity to compensate for increases in
acid input, for example due to acid rain or acid main drainage,
leading to dissolution of aluminium-containing clay minerals.
Since the residence time of the surface waters is not sufficient
for attaining chemical equilibrium with the solid phase, the ki-
netics of its dissolution may be crucial for the development of
weathering profiles and for aluminium mobility.
It has been suggested by many authors (e.g. Carroll-Webb &
Walther 1988; Carroll & Walther 1990; Nagy et al. 1991;
Wieland & Stumm 1992; Xie & Walther 1992; Ganor et al.
1995; Devidal et al. 1997; Huertas et al. 1998 and 1999) that
the dissolution rate of kaolinite, as well as other sparingly sol-
uble silicates, in acid solutions is governed by the presence of
surface complexes at Al hydroxyl sites. Therefore, the dissolu-
tion rate should be related to the accessibility of hydroxyl
groups on the basal octahedral and edge surfaces of the clay
If the reaction is far from equilibrium, the reaction rate R
), defined as the release rate of the selected ele-
ment (e.g. aluminium) per unit surface area of the solid phase,
has a direct relation to pH (or activity of protons), given, for
example, by Stumm (1990) as:
log R = log k — b pH (1)
where b is the reaction order and k is the reaction constant.
Various authors give significantly different values of the reac-
tion order b, for example, for kaolinite and pH < 4 as 0.5 (Gan-
or et al. 1995) or 0.38 and —0.02 (Wieland & Stumm 1992) or
0.09 (Carrol & Walther 1990). Therefore, other factors proba-
bly affect the dissolution kinetics. The chemical composition
of the solution and the physico-chemical properties of the sol-
id phase should be taken into consideration. For example, Rid-
ley et al. (1997) found that dissolution of gibbsite in acidic
low-temperature solutions is significantly enhanced by the
presence of sulphate ions, in comparison to chloride ions.
When all other parameters are fixed, gibbsite dissolves ten
times faster in 0.005 M H
solution than in 0.01 M HCl
solution. Moreover, natural samples of minerals of the same
chemical composition, such as kaolinites, exhibit different dis-
solution rates under the same experimental conditions (Hradil
& Hostomský 1999) and the dissolution process can be sub-
stantially faster for minerals of lower crystallinity (Soukupová
et al., in press). One may also ask what is the difference in re-
activity of a mineral surface if so-called ‘external’ hydroxyls,
which are typical for the structure of kaolinites, are absent in
2 : 1 clays, such as mica and smectite.
The aim of the present study is to compare dissolution rates
of different clay minerals and to study the influence of struc-
tural properties of the samples, their crystallinity and alumini-
um content, as well as the effect of the presence of complexing
agents (sulphates) in solution. Furthermore, the dissolution ki-
netics of kaolinite and halloysite with a variable content of
clay micas is investigated.
Materials and methods
Two samples of pure kaolinite which differ significantly in
crystallinity (KGa-1b and KGa-2, Georgia; USA) have been
used as reference materials (Fig. 1). Dissolution rates of both
118 HRADIL, HOSTOMSKÝ and SOUKUPOVÁ
materials were measured by Sutheimer et al. (1999) at pH 3 in
nitric acid solutions and by Hradil & Hostomský (in press) at
pH 0.65 in sulphuric acid solutions, respectively. KGa-1b is
classified as ‘well-ordered’ and KGa-2 as ‘poorly-ordered’
kaolinite by Van Olphen & Fripiat (1979) using the Hinckley
index, defined as the ratio of the sum of the heights of ( )
and ( ) diffraction peaks measured from their base and the
height of ( ) peak measured from the background of the
whole diffraction record. Sutheimer et al. (1999) described the
grains of KGa-2 kaolinite as more rounded than KGa-1b, with
curved edge steps (as analysed by atomic force microscopy).
According to Konta (1994), the structural disorder is a conse-
quence of the turbostratic structure, which is manifested by a
sheet translation in the b-axis direction.
Other natural samples from different sources were pre-treat-
ed by sedimentation in distilled water and the < 4
was separated. X-ray powder diffraction has been employed
for mineralogical analyses using a SIEMENS D-5005 instru-
ment under the following measurement conditions: CuK
ation, secondary monochromator, voltage 40 kV, current 30 mA,
degree range 2
3—90°, step 0.02° per 8 seconds. The raw data
were processed by the ZDS for Windows program (Ondruš
1997) employing the diffraction pattern database (JCPDS
2000). In addition, mineral compositions were calculated from
quantitative chemical analyses obtained by X-ray fluorescence
(Vacuum X-ray spectrometer PHILIPS PW 1404/10). Ideal
formulas of the mineral phases given by Deer et al. (1992) and
Velde (1992) were used; volatile compounds were excluded
from the calculation. The surface area of the samples was de-
termined by a N
adsorption technique (BET; Brunauer et al.
1938) using a Coulter SA3100 device. The physical parameters
and mineral and chemical compositions are listed in Table 1.
The following dissolution experiments were performed at
– Continuous dissolution of samples suspended in 0.5 M and
5 M H
and in 1 M HCl;
– Discontinuous dissolution of samples in 5 M H
In a continuous experiment, 100 cm
were filled with 70 cm
of the acid solution and 0.7 g of the
solid material was added. The bottles were placed into a con-
stant temperature shaking bath. At each predetermined time
interval, a bottle was taken off and the solid phase was separat-
ed by centrifugation.
In the discontinuous experiment, only one reaction vessel
was used. The solid to solution ratio was the same as in the
continuous experiments, that is 0.01 g cm
. At each sampling,
the whole volume of suspension was centrifuged. After decan-
tation of the solution, the solid residue was washed three times
with distilled water and dried in air at 90 °C. Then it was redis-
persed in the appropriate amount of the fresh acid solution to
restore the solid/liquid ratio (0.01 g cm
) and to start the next
The dissolved Al concentration was determined by spectro-
photometry (Spekol, Carl Zeiss Jena, Germany) using the Al
complex with Chromazurol S after masking Fe by ascorbic
acid (Malát 1973).
Results and discussion
Effect of sulphates and acidity
The dissolution rate of the well-ordered kaolinite KGa-1b,
defined as the release rate of Al per unit surface area of the
solid phase, in 0.5 M H
solution is three times higher than
in 1 M HCl (Table 2).
In 5 M H
, aluminium release rates differ significantly
for continuous and discontinuous experiments. If samples are
washed and dried periodically, their resulting dissolution is
much faster both for the pure kaolinite KGa-1b and for the il-
lite-rich clay IMt-1 (Table 2 and Fig. 2).
The dissolution rate R
), of the pure, well-or-
dered kaolinite KGa-1b in continuous experiments in the con-
centration interval 0.5 M < c
< 5 M may be approximated by
which was obtained using two experimental points given in
Taking into consideration that the reaction order (with respect
) in the acid concentration interval 0.05 M < c
< 0.5 M cal-
culated for different kaolinites is always close to 0.5 (Hradil &
Fig. 1. Powder diffraction patterns of well and poorly ordered ka-
olinites. (a) – reference clays KGa-1b vs. KGa-2, (b) – illite-rich
kaolins KIC-1 vs. KIC-8.
1 1 1
1 1 0
1 1 0
poorly ordered kaolinite
Kaollinite (well-ordered) with illite
Kaolinite (poorly-ordered) with illite
(KIC-8, Skalná ULK clay)
ALUMINIUM RELEASE RATES FROM ACIDIFIED CLAY STRUCTURES 119
(percentage of the
0.5 M H
1 M HCl
5 M H
5 M H
Locality and origin:
Skalná -ULK clay,
Hamr, Czech Rep.
Mineralogical composition (wt. %):
Content of Al
BET surface area
Grain size (
Van Olphen &
Van Olphen &
Effect of composition of structural layers
In order to compare different clay minerals from the point
of view of the aluminium release rate, the Al concentration in
the solid sample should be taken into account. The difference
becomes important in minerals relatively poor in aluminum.
In the Fig. 3, differences in rates related to the value of the dis-
solution rate of the reference kaolinite (KGa-1b) in 0.5 M
solution are shown. Well-crystallized sedimentary
clays (KGa-1b, KIC-1, IMt-1 – solid circles), which differ in
illite concentration (Table 1) and, therefore, also in Al concen-
tration, exhibit significant differences in their aluminium re-
lease rates; the dissolution rates decrease with increasing con-
tent of illite (Hradil & Hostomský, in press). We suggest that
these differences are caused partially by the different content
of aluminium (Fig. 3) and probably by the absence of ‘exter-
nal’ hydroxyls in 2 :1 structures of clay micas.
Effect of structural order
Crystallinity’ is a term describing broadly the structural per-
fection of a crystalline phase. One may assume in general that
a ‘poorly-ordered’ phase is more easily dissolved than a ‘well-
ordered’ material. In the case of kaolinite group minerals,
‘crystallinity’ decreases in the succession kaolinite—hal-
Table 2: Aluminium release rates of reference well-ordered kaolin-
ite KGa-1b in different dissolution regimes and 25 °C.
Table 1: Composition and properties of studied clay samples.
Fig. 2. Aluminium release in 5 M H
solution in two experi-
mental arrangements for (a) pure kaolinite (KGa-1b); (b) illite-rich
R = 1.9*10
continuous dissolution experiment
discontinuous dissolution experiment
R = 5.58
continuous dissolution experiment
discontinuous dissolution experiment
Hostomský 1999), one can conclude that the dissolution rate at
the highest acid concentrations (0.5 M < c
< 5 M) increases
with increasing acidity even more steeply than in the concen-
tration interval 0.05 M < c
< 0.5 M.
120 HRADIL, HOSTOMSKÝ and SOUKUPOVÁ
loysite—allophane (amorphous). All these phases are nearly of
the same chemical composition; halloysite has a tubular struc-
ture of rolled sheets instead of typical hexagonal flat sheets of
kaolinite and its particles are in general smaller. Even among
kaolinite samples, well- and poorly-ordered materials may be
identified, but their structural difference is not as significant as
is the difference between kaolinite and halloysite samples. In
Fig. 1, diffraction patterns of the well- and poorly-ordered ka-
olinites (reference materials KGa-1b vs. KGa-2 and illite-rich
kaolins KIC-1 vs. KIC-8) are compared. As can be seen in Fig.
3, the aluminium release rates of poorly-ordered structures
(open circles and squares) are always higher than those of
well-ordered materials (solid circles and squares). Extremely
reactive kaolinite KAO-6 (with the dissolution rate similar to
that of halloysite), has been formed by alteration processes in
sandstones of Cenomanian age related to later volcanic activi-
ty in the Tertiary, in the area of uranium underground leaching
(Stráž pod Ralskem, Czech Republic); this is in contrast to
other samples of sedimentary origin.
It may be concluded that crystallinity, which in some way
refers to the clay’s origin, plays an important role as a factor
affecting the mineral’s reactivity and, specifically, its dissolu-
– The dissolution rate of kaolinite in 0.5 M sulphuric acid at
is aproximately three times higher than in hydrochloric
acid of formally equivalent H
concentration. The dissolution
in 5 M sulphuric acid is eight times faster if the solid phase is
periodically separated from acid solution, washed by distilled
water and dried.
– Aluminium release rate decreases with the increasing con-
centrations of 2 :1 clays (illites) in kaolinitic clays.
– The dissolution rate defined as the release rate of alumini-
um per surface area of the solid phase is affected by the con-
centration of Al in the mineral structure. It should be taken
into account if clay minerals with the different Al content are
– Crystallinity, as a factor reflecting the quality of the crystal
structure, influences significantly the aluminium release rates
in dissolution by acids. Halloysite and poorly-ordered clays
formed by authigenic alteration processes are extremely reac-
tive in comparison with well-ordered sedimentary clays.
Acknowledgments: This work was supported by the Grant
Agency of the Czech Republic (Grant No. 203/98/P203). The
authors are very grateful to their colleagues in the Institute of
Inorganic Chemistry, Academy of Sciences of the Czech Re-
public (Petr Bezdička and Antonín Petřina – X-ray diffrac-
tion, Tomáš Grygar – spectroscopic and spectrophotometric
methods, Václav Štengl – BET surface measurements) and in
the Gematest Ltd., Prague (Alexander Manda – X-ray fluo-
rescence) for performing the measurements and for their help
with the interpretations of the results.
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