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Chapter 4: Dissolution of kaolinite, illite and montmorillonite
105
4 Dissolution kinetics of kaolinite, illite and montmorillonite
under acid-sulfate conditions: a comparative study1
ABSTRACT
Soils and sediments containing high levels of reduced inorganic sulfur pose a great risk to the
environment due to their potential to produce large acidity (H2SO4). Large quantities of
reduced inorganic sulfur have accumulated in inland wetland sediments from an input of
saline and sulfate rich water and long periods of submerged conditions in inland wetlands in
Australia. The exposure of these sulfidic materials to atmosphere results in highly acidic and
saline conditions in soils. Phyllosilicate dissolution is the major acidity neutralisation process
in inland wetland soils with a little or no carbonate mineral content. The acid neutralisation
capacity of phyllosilicates is dependent on the dissolution rates of these minerals. In previous
studies, phyllosilicate dissolution has been investigated in acidic conditions (HCl, HNO3 and
HClO4); however, the rates obtained from these studies cannot be directly applicable to acid
sulfate soils (ASS) with sulfate-based acidity. Additionally, high levels of salinity often
prevailing in inland ASS may have effect on the dissolution behaviour of phyllosilicates. This
study was aimed at determining the dissolution rates of kaolinite and montmorillonite in NaCl
solution (I = 0.01 M and 0.25 M) acidified with H2SO4 (pH range of 1 to 4.25). The solution
1 This chapter has been prepared for submission to Clay Minerals, under the title ‘A comparative study of the dissolution kinetics of kaolinite, illite and montmorillonite under acid-sulfate conditions’. Authors are Irshad Bibi, Balwant Singh and Ewen Silvester.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
106
compositions similar to the inland ASS solutions were used in these experiments. Flow-
through reactors were used to determine the mineral dissolution rates at 25°C. The dissolution
of kaolinite and montmorillonite was characterised by an initial rapid Al and Si release before
achieving steady state. The exception to this behaviour was montmorillonite dissolution at pH
2–4 in the lower ionic strength solutions, where a slow Al release continued throughout the
experimental duration. Kaolinite and montmorillonite dissolution rates decreased with
increasing pH at pH 1–3 and 1–4, respectively. Kaolinite dissolution rates obtained at pH 4
were similar or smaller than the pH 3 rates. Montmorillonite dissolution rates obtained from
Al release (RAl) were greater in the higher ionic strength solutions than in the lower ionic
strength solutions, which could be attributed to the adsorption of dissolved Al on cation
exchange sites in the lower ionic strength solutions. The greater RAl values at the higher ionic
strength than the lower ionic could have resulted from (i) the decreased accessibility of
interlayer exchange sites for (dissolved) Al adsorption due to particle aggregation, and (ii)
increased cation (Na+) competition for exchange sites. The greater Al release resulting from
phyllosilicates dissolution under high ionic strength conditions may be a contributing factor to
the ecological impacts of sulfide oxidation.
4.1 INTRODUCTION
Soils and sediments with elevated reduced inorganic sulfur (such as pyrite) are considered an
environmental hazard due to their acid generation potential (Dent and Pons, 1995; Fitzpatrick
and Shand, 2008). The input of highly saline and sulfate (SO42–) rich water, combined with
long inundation periods has provided ideal conditions for the accumulation of large amounts
of sulfide minerals in some inland wetlands in Australia (Lamontagne et al., 2006). The
oxidation of sulfide minerals on exposure to air and water produces sulfuric acid (H2SO4), and
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
107
in soils with very little or no carbonate buffering, can result in the dissolution of
phyllosilicates in soils and release of hydrolysable (Al3+, Fe3+) and base (K+, Na+, Mg2+, Ca2+)
cations (Fitzpatrick and Shand, 2008). In many acid sulfate soils, the dissolution of
phyllosilicate minerals is the only process that can neutralise the acidity generated by the
oxidation of sulfides on a long-term basis. The neutralisation capacity of phyllosilicates is
dependent on the rate of dissolution of these minerals. Phyllosilicate dissolution typically
follows a pattern of rapid and non-stoichiometric dissolution, followed by slower and
stoichiometric dissolution, with acid consumption (neutralising capacity) following the same
dynamics (Weber, 2003).
Kaolinite, montmorillonite and illite are common phyllosilicates in Australian soils (Norrish
and Pickering, 1983). The acidic dissolution of these phyllosilicates has been investigated in
several previous studies under diffrent conditions. Ganor et al. (1995) measured kaolinite
dissolution in flow-through reactors over the pH range 2 to 4.2 (in HClO4) and observed
stoichiometric dissolution with proton reaction order between 0.4 and 0.5. Huertas et al.
(1999) investigated the dissolution rate of kaolinite in batch reactors at 25°C and over the pH
range 1 to 13; HCl was used to adjust the pH in acidic range and all the experiments were
carried out in 1 M NaCl. Stoichiometric dissolution of kaolinite was reported at pH < 4 and a
surface coordination model was developed whereby dissolution was proposed to be controlled
by two separate surface complexes. Cama et al. (2002) investigated the combined effects of
pH and temperature on kaolinite dissolution in flow-through reactors over the pH range 0.5 to
4.5 (in HClO4) and at temperatures between 25 and 70°C. In experiments designed to
determine the effect of ionic strength, NaClO4 was used to adjust the ionic strength of the
solution. The authors proposed two independent and parallel reaction mechanisms for
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
108
kaolinite dissolution in the acidic pH region; the first mechanism controlled the reaction at pH
≥ 2.5, while the second mechanism dominated below pH 0.5. Between pH 0.5 and 2.5, both
reaction mechanisms influenced the dissolution rate. The effect of ionic strength on kaolinite
dissolution rate was found to be insignificant.
Zysset and Schindler (1996) conducted batch dissolution experiments using K-saturated
montmorillonite (SWy-1) in KCl solutions (0.03, 0.10 and 1.0 M) between pH 1 and 4 (using
HCl). The dissolution rate of montmorillonite was reported to increase with decreasing pH
and with increasing KCl concentration. Congruent dissolution of montmorillonite was
observed in 1.0 and 0.10 M KCl solutions, whereas a preferential Si over Al release was
observed in 0.03 M KCl solutions. Amram and Ganor (2005) studied the dissolution of
smectite over the pH range 1 to 4.5 (in HNO3), and temperature range 25 to 70°C, with
NaNO3 used to adjust the ionic strength. Smectite dissolution rates were found to increase
with decreasing pH, following a rate law with reaction order of 0.57. Similar to the kaolinite
dissolution study by Cama et al. (2002), these authors also reported an insignificant effect of
ionic strength on the smectite dissolution rate. In a recent study, Rozalen et al. (2008)
evaluated the effect of pH (1−13) on montmorillonite dissolution in both batch (using HCl)
and flow-through reactor (using HNO3) experiments; KNO3 was used as background
electrolyte in concentrations of 0.01 to 0.1 mol/L (batch) and 0.01 to 0.05 mol/L (flow-
through). In this work stoichiometric dissolution of montmorillonite was reported at pH < 4.5
with a proton reaction order of 0.40.
Although these studies have provided important information on effects of pH, ionic strength
and temperature on the dissolution of kaolinite and montmorillonite, the kinetic parameters
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
109
cannot be applied directly for the prediction of kinetics in natural systems where the acidity
generated from iron sulfide oxidation is in the form of H2SO4. In addition many inland acid
sulfate wetlands are also highly saline (Glover et al., 2011). In these systems, any effects of
ionic strength (particularly that imparted by NaCl) on dissolution rates, may be especially
important. The inconsistency in ionic strength effects observed in previous studies indicates
that this is an area that requires further investigation.
In the earlier study, it has been reported on the effects of pH (H2SO4) and ionic strength
(NaCl) on illite dissolution rates (Chapter 3) (Bibi et al., 2011). The current study builds on
the previous work and reports results from kaolinite and montmorillonite dissolution
experiments in NaCl solutions (0.01 and 0.25 M) over the pH range 1 to 4.25 (H2SO4).
Combined with the previous work on illite, this study allows to compare the dissolution rates
of three clay minerals naturally found in natural clay deposits, and assess their likely relative
roles in providing pH buffering.
4.2 MATERIALS AND METHODS
4.2.1 Clay pre-treatment and characterisation
Dissolution experiments were carried out using kaolinite (KGa-2) and montmorillonite (SWy-
2). The reference samples were obtained from the Source Clays Repository of the Clay
Minerals Society at the Purdue University, West Lafayette, USA. The clay fraction (< 2 µm)
of the mineral samples was separated by a sedimentation-resuspension procedure described
elsewhere (Bibi et al., 2011) (Chapter 3); the Na-saturated samples were freeze dried and
stored in polyethylene bottles.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
110
The mineralogy of the clay samples was determined by X-ray diffraction (XRD). The XRD
(GBC MMA: CoKα radiation, λ= 1.7890 Å, operating conditions of 35 kV and 28.5 mA)
patterns were obtained on randomly oriented specimens at 4 to 75° 2θ at a step size of 0.02°
2θ and a scan speed of 1.0° 2θ min−1. X-ray diffraction patterns of basally oriented clays were
obtained after treatments as described in Bibi et al. (2011) (Chapter 3). Minor amounts of
quartz were found in the montmorillonite sample, and the XRD patterns of kaolinite showed
the presence of small amounts of rutile (TiO2) as an impurity.
Sub-samples of the pre-treated clays were analysed for bulk chemical composition using X-
ray fluorescence (XRF, Philips PM2400) spectroscopy (Norrish and Hutton, 1977). The Fe(II)
content in the pre-treated clay samples was determined by 1,10-phenanthroline colorimetric
method (Amonette and Templeton, 1998). The uncertainty associated with the Fe(II) analysis
was ±0.03 mg L−1 (standard deviation, n = 3). The corresponding atomic ratios (Al/Si, Fe/Si,
Mg/Si and Na/Si) in the purified samples are presented in Table 4.1. The structural formulae
of the mineral samples was calculated from the XRF and chemical analyses (Cicel and
Komadel, 1994).
The specific surface area (SSA) of the clay samples was determined using five point N2
adsorption isotherms and the Brunauer-Emmett-Teller (BET) method. Specific surface area
values for kaolinite and montmorillonite were 21 and 37 m2 g-1, respectively. The morphology
of the purified and partly-reacted clays was determined using a Philips CM12 transmission
electron microscope (TEM) operated at 120 kV at the Australian Centre for Microscopy and
Microanalysis of The University of Sydney. The clay specimens for TEM examination were
prepared by dispersing a small amount of clay in E-pure® (18.2 MΩ cm-1;
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
111
Barnstead/Thermolyne Corp., Dubuque, IA, USA) water by ultra-sonification. A drop of the
dispersed sample was put onto a carbon coated Cu grid using a Pasteur pipette and the sample
dried under light prior to TEM examination.
4.2.2 Flow-through reactor dissolution experiments
The dissolution experiments were conducted using flow-through reactors with three
compartments and an internal volume of 46 mL, the details are described elsewhere (Chapter
3) (Bibi et al., 2011). To maintain a constant temperature during the experiments the reactors
were kept immersed in a thermostatic water bath at 25 ± 1°C. The input solution was pumped
into the reactors at a flow rate of 0.02 to 0.05 mL/min with an 8-channel Gilson® peristaltic
pump. Flow rate and input solution composition were kept constant for the whole duration of
each experiment. Input solutions consisted either 0.01 or 0.25 M NaCl solution with pH
adjusted to 1.0−4.25 using AR-grade H2SO4 (2 M).
4.2.3 Solution phase analysis
The output solution was collected after every 24 hours and the dissolved concentrations of Si
and Al were determined. The pH of output solution was measured immediately after
collecting the sample using a combined pH electrode calibrated against standard buffer
solutions of pH 4.0 and 7.0. Output samples were analysed for Al and Si concentrations using
colorimetric analytical methods (Dougan and Wilson, 1974; Koroleff, 1976). The pH of the
output solutions for the experiments conducted at pH 1 was increased to > 1.5 by adding 0.5
M NaOH before analysing for Si and Al (Cama et al., 2002). The steady state samples were
also analysed for Fe, Mg and Na using an inductively coupled plasma atomic emission
spectrometer (ICP-AES, Varian® Vista AX CCD).
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
112
4.3 CALCULATIONS
4.3.1 Dissolution rate calculations
In acidic solutions, the dissolution reactions of kaolinite and montmorillonite can be
expressed as follows:
(Al1.91Fe0.04Ti0.05)(Si1.93Al0.07)O5(OH)4 + 6.28H+ → 1.98Al3+ + 0.04Fe3+ + 0.05Ti4+ +
1.93H4SiO4 + 1.28H2O (1)
K0.01Na0.56(Al3.13Mg0.45Fe3+0.32Fe2+
0.06Ti0.01)[Si7.98Al0.02]O20(OH)4 + 12.08H+ +7.92H2O →
0.01K+ + 0.56Na+ + 3.15Al3+ + 0.32Fe3+ +0.06Fe2++ 0.45Mg2+ + 0.01Ti 4++ 7.98H4SiO4
(2)
The dissolution rate of the mineral (mol m-2 s-1) under the steady state conditions was
calculated from the concentration of mineral component j using the following expression
(Cama et al., 2000):
( )j
j
j CSM
F
VR
1−=
(3)
where Vj is the stoichiometric coefficient of component j in the dissolution reaction for
kaolinite or montmorillonite, F is the flow rate of the input fluid (L/s), S is the specific surface
area (m2/g), M is the mineral mass (g) of the mineral and Cj is the concentration of component
j in the steady state solution. Steady state has been defined as where the difference in Si and
Al concentrations of consecutive samples was less than 6% (Rozalen et al., 2008); in this
study, this criterion was applied to six to seven data points to confirm the steady state
(Chapter 3) (Bibi et al., 2011). The dissolution rates of minerals were calculated based on the
average values across the steady state interval. The degree of the saturation of the solution
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
113
with respect to the relevant minerals was calculated in terms of Gibbs free energy of reaction,
Gr (kcal mol–1):
−=
eq
rK
IAPRTln∆G (4)
where R is the gas constant, T is the absolute temperature, IAP is the ion activity product and
Keq is the equilibrium constant (Nagy, 1995). Ion activities of the steady state solutions were
calculated using the geochemical code, PHREEQC (Parkhurst and Appelo, 1999). The errors
in the dissolution rates were estimated using the following equation (Miller and Miller, 1993):
222
+
+
=
SFCR
σ SFCR σσσ (5)
where σR is the uncertainty in the calculated rate, σC is the uncertainty in Al or Si
concentration of the output solution and σF and σS are the uncertainties associated with the
fluid flow rate and the mineral surface area, respectively.
4.4 RESULTS
4.4.1 Initial release rates of elements
Solution pH values, Al and Si concentrations and the Al/Si atomic ratio in the output solutions
with time for the kaolinite dissolution experiments over the pH range 1 to 4 are presented in
Figs. 4.1 and 4.2 (Fig. 4.1 shows I = 0.25 (higher ionic strength) and Fig. 4.2 shows I = 0.01
(lower ionic strength)). Aluminium and Si release rates from kaolinite dissolution changed
over the period of the experiment, from initially high rates that decreased over the first few
hundred hours prior to achieving steady state. The experiments at the higher ionic strength
showed a preferential initial release of Al over Si at all pH values (Fig. 4.1). Preferential
release of Al was also observed at pH 1 and 2 for the lower ionic strength experiments (Fig.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
114
4.2a–d). At pH 3 (and low ionic strength) similar Al and Si release rates were observed
throughout the experiment (Fig.4. 2e,f) while at pH 4.25 preferential Si release was observed
during initial 300 h of the experiment (Fig. 4.2g,h). Beyond this time the Si concentrations
decreased and the Al concentrations increased to stoichiometric Al/Si values (Fig. 4.2g,h).
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
115
Fig. 4.1 Solution pH and the concentrations of Al and Si (a,c,e,g) and Al/Si ratio (b,d,f,h) in the
output solution as a function of time for kaolinite dissolution at pH 1–4 and at I = 0.25 M.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
116
Fig. 4.2 Solution pH and the concentrations of Al and Si (a,c,e,g) and Al/Si ratio (b,d,f,h) in the
output solution as a function of time for kaolinite dissolution experiments at pH 1–4.25 and at I =
0.01 M.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
117
Solution pH values, Al and Si concentrations and the Al/Si atomic ratio in the output solutions
with time for the montmorillonite dissolution experiments over the pH range 1 to 4 are
presented in Figs. 4.3 and 4.4 (Fig. 4.3 shows I = 0.25 (higher ionic strength) and Fig. 4.4
shows I = 0.01 (lower ionic strength)). Similar to that observed for kaolinite, montmorillonite
dissolution was characterised by an initial rapid release of Al and Si prior to achieving steady
state after a few hundred hours. The exceptions to this behaviour were the experiments at pH
4 at the higher ionic strength (Fig. 4.3g) and at pH 2–4 at the lower ionic strength (Fig.
4.4e,g). An initial preferential release of Al over Si was observed for the experiments at pH
1–3 at the higher ionic strength, and at pH 1 at the lower ionic strength (Figs. 4.3 and 4.4).
The opposite trend (preferential initial Si release) was observed at pH 4 at the higher ionic
strength and at pH 2–4 at the lower ionic strength.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
118
Fig. 4.3 Solution pH and the concentrations of Al and Si (a,c,e,g) and Al/Si ratio (b,d,f,h) in the
output solution as a function of time for montmorillonite dissolution experiments at pH 1–4 and
at I = 0.25 M.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
119
Fig. 4.4 Solution pH and the concentrations of Al and Si (a,c,e,g) and Al/Si ratio (b,d,f,h) in the
output solution as a function of time for montmorillonite dissolution experiments at pH 1–4.25
and at I = 0.01 M.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
120
4.4.2 Relative release rates of elements at the steady state
The stoichiometry of mineral dissolution was assessed by comparing the elemental ratios
(Al/Si, K/Si, Fe/Si and Mg/Si) of the output solutions at steady state (Table 4.2) to the
elemental ratios in the original mineral samples (Table 4.1).
Table 4.1 The atomic ratios of cationic elements with Si for Georgia Kaolinite (KGa-2),
Wyoming montmorillonite (SWy-2) and Silver Hill Illite (IMt-2) Na-saturated samples
calculated from their chemical compositions determined by X-ray fluorescence (XRF)
analyses.
Mineral Al/Si Fe/Si Mg/Si Na/Si K/Si
Kaolinite 1.03 0.02 – – –
Montmorillonite 0.39 0.05 0.06 0.07 –
Illite¶ 0.55 0.10 0.07 – 0.20 ¶This data has been presented in the earlier study (Bibi et al., 2011) (Chapter 3).
For kaolinite dissolution, the steady state Al/Si ratios ranged between 1.00 and 1.66 at the
higher ionic strength and between 0.94 and 1.19 at the lower ionic strength, compared to 1.03
in the clay fraction of the original sample. For montmorillonite dissolution, the Al/Si ratio
ranged between 0.46 and 0.57 at the higher ionic strength (compared to an Al/Si ratio of 0.39
in the clay fraction of the original mineral sample). At the lower ionic strength, the Al/Si ratio
ranged between 0.2 and 0.4 with the higher value obtained at pH 1. The Fe/Si ratio under
steady state conditions, and at the higher ionic strength, showed preferential Fe release at pH
1; stoichiometric Fe release at pH 2 and 3; and preferential Si release at pH 4. For the lower
ionic strength experiments the Fe/Si ratio at steady state was close to stoichiometric ratio at
all pH conditions except at pH 3 where preferential Fe release was observed. Magnesium was
preferentially released from montmorillonite compared to Si at both ionic strengths and at all
pH values.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
121
Table 4.2 Atomic ratios (Al/Si, Fe/Si and Mg/Si) in the steady state solutions for the two
ionic strength solutions at pH 1–4.25 at 25°C.
Minerals Ionic strength = 0.25 (M) Ionic strength = 0.01(M)
Output
pH
Al/Si Fe/Si Mg/Si Output
pH
Al/Si Fe/Si Mg/Si
Kaolinite
1.01 1.01 – – 1.00 0.99 – –
2.03 1.00 – – 2.09 0.94 – –
3.01 1.66 – – 2.98 1.09 – –
4.00 1.46 – – 4.28 1.19 – –
Montmorillonite
1.01 0.51 0.12 0.09 1.00 0.40 0.08 0.09
2.02 0.52 0.07 0.13 2.09 0.20 0.07 0.25
3.00 0.57 0.06 0.12 2.98 0.14 0.13 0.10
3.99 0.46 0.03 0.18 4.31 0.11 0.05 0.20
Illite¶
1.03 0.50 0.20 0.36 1.00 0.50 0.13 0.07
2.01 0.66 0.32 0.37 2.09 0.54 0.37 0.13
2.95 0.72 0.11 0.22 2.95 0.52 0.29 0.28
3.98 0.50 0.13 0.30 4.31 0.27 0.47 0.65 ¶This data has been presented in our earlier study (Bibi et al., 2011) (Chapter 3).
4.4.3 Dissolution rates of minerals
Mineral dissolution rates were obtained from Al and Si release rates at the steady state, using
equation 3 (see Table 4.3). A significant decrease in the dissolution rate of kaolinite (for both
RSi and RAl) was observed with increasing pH over the pH range 1 to 3, at both ionic strengths
(Table 4.3). At pH 4 the kaolinite dissolution rate was similar to that at pH 3 at the lower
ionic strength, and slightly higher than that at pH 3 at the higher ionic strength.
Montmorillonite dissolution rate data obtained at the two ionic strengths indicate that the RSi
values are similar at both ionic strengths across the pH range studied, whereas RAl is
significantly greater at pH 2–4 at the higher ionic strength (Table 4.3).
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
122
Table 4.3 Experimental conditions, steady state concentrations of cations (µmol/L.g) in the output solution and mineral dissolution rates
(mol m–2
s–1
) at 25°C.
Mineral Output pH Duration (h) Initial mass (g) Flow ratec Si Al Fe Mg RSi
a RAl
a RSi %
b RAl %
b
I = 0.25 M
Kaol.
1.01 1880 0.401 0.048 25.29 25.46 - - -12.30 -12.31 11.1 11.3
2.03 2170 0.389 0.038 6.76 6.74 - - -12.98 -12.99 11.0 11.0
3.01 1870 0.413 0.022 2.03 3.24 - - -13.74 -13.54 10.8 10.8
4.00 1700 0.413 0.027 2.19 3.23 - - -13.63 -13.47 10.8 10.9
Mont.
1.01 1880 0.122 0.043 74.67 37.79 8.61 7.05 -12.74 -12.64 13.2 11.0
2.02 1950 0.123 0.036 44.47 23.33 5.82 2.93 -13.05 -12.92 11.1 11.1
3.00 1700 0.127 0.022 46.79 26.50 2.67 4.23 -13.23 -13.08 11.3 10.8
3.99 1500 0.127 0.028 24.04 10.98 0.69 3.43 -13.42 -13.35 10.9 10.8
I = 0.01 M
Kaol.
1.00 2325 0.336 0.020 62.71 61.88 - - -12.28 -12.30 16.0 15.6
2.09 1700 0.376 0.029 10.22 9.63 - - -12.92 -12.95 11.0 11.2
2.98 1500 0.412 0.013 2.72 2.96 - - -13.85 -13.82 10.9 11.2
4.28 1660 0.408 0.022 1.30 1.57 - - -13.89 -13.82 10.9 10.8
Mont.
1.00 2000 0.114 0.021 205.09 82.37 17.11 17.54 -12.62 -12.61 17.4 14.0
2.09 1820 0.127 0.029 77.48 15.83 5.04 18.50 -12.89 -13.17 12.3 10.9
2.98 1570 0.127 0.029 48.41 6.63 1.99 5.83 -13.11 -13.57 10.9 10.8
4.31 1660 0.138 0.020 30.51 3.19 1.52 6.23 -13.47 -14.05 11.6 10.8
Kaol. = Kaolinite; Mont. = Montmorillonite; (a) Log RSi and log RAl (mol m-2 s-1) are the dissolution rates calculated from the release rates of Si and Al, respectively;(b) RSi
and RAl are the errors in the calculated RSi and RAl, respectively; (c) Fluid flow rate (mL/min).
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
123
4.4.4 Saturation state of the steady state solutions
The saturation state of the steady state solutions was calculated for both kaolinite and
montmorillonite systems for the clay minerals and associated mineral phases containing Al,
Si, Fe and Mg; the kaolinite saturation data are shown in Table 4.4 and montmorillonite
saturation data shown in Table 4.5. The value of Gr for both kaolinite and montmorillonite
increased with increasing pH as did the Gr values for Fe and Mg oxide minerals. Amorphous
SiO2 and quartz showed the opposite trend, with decreasing Gr values with increasing
solution pH (Tables 4.4 and 4.5). All steady state solutions were undersaturated with respect
to kaolinite and montmorillonite as well as the Al, Si, Fe and Mg bearing mineral phases
considered in the modelling.
The speciation of Al in the steady state solutions (calculated using PHREEQC) from the
kaolinite and montmorillonite dissolution experiments suggested that the dominant Al species
at pH 1–2 was Al(SO4)+, whereas at pH 3–4, Al3+ was the dominant species (Appendix 2).
The percentage of Al(OH)2+ in the steady state solutions remained below 1 at pH 1–3,
however, this species increased to 8 % and 13 % at pH 4.00 (I = 0.25 M) and 4.25 (I = 0.01
M), respectively (Appendix 2) for both minerals (montmorillonite and kaolinite).
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
124
Table 4.4 Saturation state of the steady state solutions for kaolinite dissolution
experiments with respect to selected minerals.
Output pH Kaolinite SiO2 (am) Quartz Gibbsite
I = 0.25
1.01 -30.95 -3.78 -2.00 -16.07
2.03 -25.40 -4.58 -2.82 -12.47
3.01 -19.85 -5.26 -3.51 -9.02
4.00 -11.53 -5.22 -3.45 -4.91
I = 0.01
1.00 -28.68 -3.40 -1.62 -15.30
2.09 -23.76 -4.42 -2.65 -11.83
2.98 -18.33 -5.14 -3.38 -8.37
4.28 -9.63 -5.59 -3.82 -3.95
Table 4.5 Saturation state of the steady state solutions for montmorillonite dissolution
experiments with respect to selected minerals.
Output
pH
Mont SiO2 (am) Qtz Gibb Kaol Bruc Akag Goe Hem
I = 0.25
1.01 -46.94 -3.85 -2.07 -16.53 -32.03 -29.09 -13.74 -17.83 -28.82
2.02 -39.02 -4.16 -2.39 -12.41 -24.42 -26.69 -10.01 -13.68 -20.51
3.00 -29.99 -4.11 -2.33 -8.47 -16.44 -23.91 -7.01 -10.30 -14.87
3.99 -23.35 -4.50 -2.73 -4.88 -10.04 -21.28 -4.09 -6.97 -7.09
I = 0.01
1.00 -43.28 -3.33 -1.57 -15.78 -29.50 -28.44 -13.67 -17.23 -27.62
2.09 -36.73 -3.86 -2.09 -12.17 -23.34 -25.29 -10.26 -13.39 -19.96
2.98 -30.02 -4.13 -2.37 -8.59 -16.75 -23.36 -7.26 -10.02 -13.23
4.31 -19.85 -4.37 -2.59 -3.81 -7.62 -19.82 -2.70 -4.95 -3.08
*Mont = montmorillonite; SiO2(am) = amorphous silica; Qtz = quartz; Gibb = gibbsite; Kaol = kaolinite; Bruc = brucite; Akag = akaganéite; Goe = goethite; Hem = hematite.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
125
4.5 DISCUSSION
4.5.1 Initial release rates
As shown in Figs. 4.1 and 4.2, kaolinite dissolution at pH ≤ 4 showed a preferential release of
Al over Si during initial few hundred hours of the experiments. Similar preferential Al release
has been reported for dissolution experiments conducted on pure and natural kaolinites in
H2SO4 solutions during the initial fast release stage (Hradil and Hostomsky, 2002). Carroll
and Walther (1990) also observed a preferential release of Al over Si at the initial stage of
dissolution experiments conducted on Georgia kaolinite in the acidic pH region. These
authors suggested that the preferential Al release during the initial rapid release stage was due
to the fact that Al and Si were removed from the mineral solution interface by two different
types of complexes that were not completely independent to each other and that the
complexes involved in the Al release were easily detached from the structure.
Aluminium was also released at a higher rate compared to Si during the initial dissolution of
montmorillonite at pH 1–3 at the higher ionic strength and at pH 1 at the lower ionic strength
(Figs. 4.3 and 4.4). The initial incongruent release of cations has been observed previously in
several batch and flow-through reactor dissolution studies conducted on other 2:1
phyllosilicates (Chapter 3) (Bibi et al., 2011; Kohler et al., 2003; Oelkers et al., 2008). It has
been reported that fast exchange reactions from either adsorption sites or from the mineral
structure itself may result in an early rapid release of Al from clay minerals (Charlet et al.,
1993). Oelkers (2001) suggested that the relative ease of cleavage of Al–O bonds compared to
Si–O bonds leads to a preferential Al release at the initial stage of clay dissolution
experiments.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
126
It has been observed that the interlayer cations are the first to be released from clay minerals
due to rapid ion-exchange reactions between solution and the mineral interlayer, with the rates
of these reactions being diffusion controlled. Some examples of this effect include the rapid
release of Mg and K from the interlayer of vermiculite (Kalinowski and Schweda, 2007), fast
leaching of Ca, Na and Mg from smectite (Metz et al., 2005) resulting in a depletion of the
interlayer cations and a preferential K release compared to framework cations from the
interlayer of muscovite, phlogopite and biotite micas (Kalinowski and Schweda, 1996). A
rapid and preferential release of interlayer K over tetrahedral Si was also observed from illite
in saline-acidic dissolution experiments at two different ionic strengths (I = 0.25 M and 0.01
M) (Chapter 3) (Bibi et al., 2011). In the current study, Na-saturated montmorillonite was
used and due to the high input concentration of Na in the background solution in the
experiments, it was impossible to detect small changes in Na concentrations in solution
resulting from Na release from the interlayer sites.
4.5.2 Steady state dissolution
The stoichiometry of the steady state dissolution reaction was estimated from the Al/Si ratio
in the case of kaolinite dissolution and from the Al/Si, Fe/Si and Mg/Si ratios for
montmorillonite dissolution. Steady state Al/Si ratios for kaolinite dissolution at the lower
ionic strength, showed values close to that of the original kaolinite. Huertas et al. (1999)
conducted batch dissolution experiments on Georgia kaolinite (KGa-1) in 1 M NaCl
solutions, using HCl to adjust the solution pH in the acidic pH region. These authors also
reported a stoichiometric Al/Si ratio for experiments conducted in the acidic pH range (pH 1–
4). The Al/Si ratios at pH 1 and 2 at the higher ionic strength in the current study were also
close to the stoichiometric value, however, at pH 3 and 4 the Al/Si ratio was greater than that
of the original kaolinite (Fig. 4.1). In contrast to the results for kaolinite dissolution at pH 3–4
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
127
(I = 0.25 M), several previous studies have reported a preferential Si over Al release from
kaolinite dissolution at the steady state and attributed this effect to the adsorption of dissolved
Al onto the reactive kaolinite surfaces (Carroll-Webb and Walther, 1988; Carroll and Walther,
1990; Wieland and Stumm, 1992; Yang and Steefel, 2008). Although the absorbance values
for both Al and Si in the output solutions of the experiments at pH 3 and 4 were very close to
the detection limits of the colorimetric methods used in the analyses, which might have
resulted in the uncertainities in the observed values, especially at the higher ionic strength.
However, it is worth noting that the steady state pH (≥ 4.0) in both ionic strength systems was
very close to the point of zero charge (PZC) value of kaolinite and thus the mineral
dissolution rate was very slow. At pH values > pHPZC; the dissolution of mineral is promoted
by the adsorption of OH– ions, whereas at pH < pHPZC dissolution is driven by the adsorption
of H+ ions (Carroll-Webb and Walther, 1988).
The steady state Al/Si ratio from montmorillonite dissolution at the higher ionic strength
indicates a preferential Al over Si release (Table 4.2). The Al/Si ratio from experiments
conducted at the lower ionic strength showed an inhibited Al release except at pH 1 where
stoichiometric release of Al was observed. These findings are consistent with the results
obtained by Zysset and Schindler (1996), who reported output Al/Si ratios significantly (in
0.03 M KCl solutions) lower than that of the original mineral sample. Golubev et al. (2006)
also observed an Al/Si ratio lower than that of the original mineral at pH < 4, under conditions
under-saturated with respect to gibbsite. In both cases these authors have suggested that the
lower Al/Si ratio was due to the adsorption of dissolved Al on cation exchanger sites; it is
most likely that a similar mechanism has operated in the lower ionic strength solutions over
the pH range 2 to 4 in our experiments. The apparent adsorption of Al on exchange sites in
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
128
the lower ionic strength systems suggests that the interlayer sites are more easily accessible in
the lower ionic strength system than the higher ionic strength system where mineral is well
flocculated and fewer sites are accessible. At higher ionic strength increased competition from
Na+ ions may also have reduced adsorption of dissolved Al on the exchange sites.
Non-stoichiometric dissolution of montmorillonite was also indicated by the Mg/Si ratio,
which ranged from 0.09–0.25 (Table 4.2) in the output solutions compared to the Mg/Si ratio
of 0.06 in the original mineral (Table 4.1). A preferential Mg release has been observed in
previous studies of montmorillonite dissolution, and this was attributed to the rapid release of
Mg through ion exchange reactions (Rozalen et al., 2008). A similar mechanism involving ion
exchange reaction between octahedral Mg2+ and Na+ or H+ ions from the solution may have
resulted in a preferential Mg release in this study. These results also suggest a slightly faster
dissolution of octahedral cations (e.g. Al, Mg, Fe) than the tetrahedral cations (e.g. Si, Al) of
the mineral. However, an important difference between Al and Mg released from the
octahedral sheet is that Al is (apparently) re-adsorbed onto exchange sites while Mg is
released to solution, and Mg content may be used as a proxy for the dissolution of octahedral
cations.
4.5.3 pH dependence
Fig. 4.5 shows a plot of log RSi versus pH for kaolinite and montmorillonite from the current
study and log RSi values for illite dissolution from our previous work (Chapter 3) (Bibi et al.,
2011). The reaction orders obtained from the linear regression of these plots confirm a
stronger pH dependence of the kaolinite dissolution rates (n = 0.72 and 0.78 at I = 0.25 M and
0.01 M, respectively) compared to illite (n = 0.32 and 0.36 at I = 0.25 M and 0.01 M,
respectively) and montmorillonite (0.22 and 0.26 at I = 0.25 M and 0.01 M, respectively).
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
129
Fig. 4.5 (a) The plot of log RSi versus pH for kaolinite, illite and montmorillonite at the higher ionic
strength (0.25). The regression equations for kaolinite, illite and montmorillonite are: log RSi = -0.72pH -
11.55; log RSi = -0.32pH-12.47; and log RSi = -0.22pH-12.55, respectively. (b) The plot of log RSi versus pH
for kaolinite, illite and montmorillonite at the lower ionic strength (0.01); the regression equations for
kaolinite, illite and montmorillonite are: log RSi = -0.78 pH - 11.43; log RSi = -0.36 pH - 12.38; and log RSi =
-0.26 pH - 12.35, respectively. Kaolinite dissolution data at pH 4 at both ionic strengths was not included
in the regression analysis.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
130
It is important to note that the pH 4 data for kaolinite dissolution experiments were not used
for the linear regression estimates; as the rates at pH 4 were either similar or greater than the
dissolution rate at pH 3. Huertas et al. (1999) reported the strong pH dependence of kaolinite
dissolution rates below pH 4 whereas, the rates were much less pH dependent at near neutral
conditions. According to the model developed by Huertas et al. 1999, kaolinite dissolution
under acidic conditions is controlled by two distinct Al complexes. The rate limiting step is
thus associated with the adsorption of a proton on an Al centre, detachment of Al, and the
subsequent detachment of Si under both neutral and acidic conditions. This shows the
contribution of basal plane of kaolinite to the dissolution reaction (Huertas et al. 1999).
4.5.4 Mineral dissolution rates
Fig. 4.6 shows a comparison of kaolinite dissolution rates (log RSi (Fig. 4.6a) and log RAl (Fig.
4.6b) obtained in this study with previously reported values in the literature. Shown on these
plots are values reported by Huertas et al. (1999) for kaolinite (KGa-1) dissolution rates
measured in batch reactors in 1 M NaCl, pH adjusted using HCl and CH3COOH. Also shown
are values from Cama et al. (2002), who conducted dissolution experiments on Georgia
kaolinite (KGa-2) in flow-through reactors with HClO4 solutions. The kaolinite dissolution
rates obtained in this study are slightly higher than the previously reported values at pH 1, 2
and 4, whereas a dissolution rate similar to the literature values was obtained at pH 3.
Importantly, the dissolution data from all studies show very weak pH dependence between pH
3 and 4, the reasons for this behaviour have been discussed earlier. In the natural
environment, the concentration of dissolved Al is controlled by the solubility of gibbsite and
kaolinite, which are common alteration products of rocks and soils.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
131
Fig. 4.6 A comparison of kaolinite (KGa-2) dissolution rates logRSi and logRAl between pH 1 and 4,
obtained in this study (I = 0.25) with published data (Cama et al., 2002; Huertas at al., 1999). The data
from Cama et al. (2002) is for flow-through reactor experiments using Georgia kaolinite (KGa-2) with pH
adjustment using HClO4, whereas the data from Huertas et al. (1999) is from batch dissolution
experiments using Georgia kaolinite (KGa-1) in a background electrolyte of 1M NaCl, with pH
adjustment using HCl.
Higher sulfate concentrations resulting from pyrite oxidation in acid sulfate systems are
reported to increase the release of Al during gibbsite and kaolinite dissolution through
increased complexation of Al in aluminium sulfate species (Nordstrom, 1982). In a gibbsite
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
132
dissolution study, it was reported that the total Al released in H2SO4-NaCl solution was 10
times greater than total Al in HCl-NaCl solution at the same pH (2) and at 0.1 M ionic
strength (Ridley et al., 1997). The distribution of Al species derived from potentiometric data
indicated that the aluminium sulfate (Al(SO4))+, species dominated the H2SO4-NaCl solutions
(Ridley et al., 1997). Gibbsite dissolution rates have also been reported to be 15–30 times
greater in H2SO4 solutions compared to HClO4 solutions of equal molar anion concentration
(Packter and Dhillon, 1969). The higher kaolinite dissolution rates obtained in this study at
pH 1, 2 and 4 compared to the rates reported in earlier studies (Cama et al., 2002; Huertas et
al., 1999) could be due to the presence of SO42– ions the studied system. As discussed
previously, the dominant Al species in the steady state solutions were Al3+ and Al(SO4)+, with
a greater proportion present as Al(SO4)+ at pH 1 and 2 (Appendix 2).
Fig. 4.7 shows a comparison of RAl values for montmorillonite dissolution at the higher (Fig.
4.7a) and lower ionic strengths (Fig. 4.7b). While similar RAl values are obtained at pH 1,
with increasing pH the difference between the RAl values obtained at the two ionic strengths
increases, with higher values obtained at the higher ionic strength. RSi values for
montmorillonite dissolution are unaffected by ionic strength (Table 4.3), and given that the
Al/Si ratio at steady state differs more strongly from the stoichiometric value at the lower
ionic strength it is more likely that the differences in RAl values are due to Al re-adsorption at
the lower ionic strength rather than enhanced Al dissolution at the higher ionic strength. The
increased RAl at the higher ionic strength is probably due to a combination of: (i) the
decreased accessibility of interlayer exchange sites for (dissolved) Al adsorption due to
particle aggregation, and (ii) increased cation (Na+) competition for exchange sites.
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
133
Complexation of Al3+ with Cl– ions in the high ionic strength solution may also contribute to
the increased RAl in the system.
Fig. 4.7 Comparison of dissolution rate (log RAl) values for montmorillonite at pH 1–4 at the
higher (I = 0.25) and lower (I = 0.01) ionic strengths.
4.6 CONCLUSIONS
This study is the first to determine the dissolution rates of phyllosilicate minerals (kaolinite
and montmorillonite) in H2SO4 solutions, the form of acidity generated in ASS, acid mine
drainage and acid rock drainage environments. The release rates of Si from kaolinite, illite
and montmorillonite dissolution were not affected by the ionic strength of the input solution,
however, Al release rates were generally greater in the higher ionic strength solutions than the
lower ionic strength solutions, particularly in the case of montmorillonite dissolution at pH 2–
4. A reduced Al release from montmorillonite dissolution in the lower ionic strength solutions
could have resulted from multiple factors including the availability of more interlayer
-14.5
-14.0
-13.5
-13.0
-12.5
-12.0
0 1 2 3 4 5
log
RA
l
pH
I = 0.25 M
I = 0.01 M
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
134
exchange sites for Al re-adsorption and a decreased cation (Na+) competition for exchange
sites in these systems. Kaolinite dissolution rates at pH 1 and 2 (H2SO4) in this study were
generally higher than the previously reported rates in HCl and HClO4 solutions (Cama et al.,
2002; Huertas et al., 1999), which was attributed to the complexation reaction of SO42– ions
with Al The greater Al release from kaolinite, illite and montmorillonite under high ionic
strength conditions will be an important factor in the ecological disturbance caused by sulfide
oxidation due to the high toxicity of Al to aquatic biota (Muniz and Leivestad, 1980).
Chapter 4: Dissolution of kaolinite, illite and montmorillonite
135
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