dissolution kinetics of kaolinite, illite and ... 4... · kaolinite, illite and montmorillonite...

Chapter 4: Dissolution of kaolinite, illite and montmorillonite

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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.


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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


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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


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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


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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.


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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;


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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).


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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


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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):

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+

+

=

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.


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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).


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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.


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output solution as a function of time for kaolinite dissolution experiments at pH 1–4.25 and at I =

0.01 M.


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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.


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output solution as a function of time for montmorillonite dissolution experiments at pH 1–4 and

at I = 0.25 M.


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output solution as a function of time for montmorillonite dissolution experiments at pH 1–4.25

and at I = 0.01 M.


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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.


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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).


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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).


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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).


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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.


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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.


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


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


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).


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.


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.


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


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.


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


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).


135

4.7 REFERENCES

Amonette J. E. and Templeton J. C. 1998. Improvement to the quantitative assay of

nonrefractory minerals for Fe (II) and total Fe using 1, 10 phenanthroline. Clays Clay

Miner. 46, 51-62.

Amram K. and Ganor J. 2005. The combined effect of pH and temperature on smectite

dissolution rate under acidic conditions. Geochim. Cosmochim. Acta 69, 2535-2546.

Bibi I., Singh B. and Silvester E. 2011. Dissolution of illite in saline-acidic solutions at 25°C.

Geochim. Cosmochim. Acta 75, 3237-3249.

Cama J., Ganor J., Ayora C. and Lasaga C. A. 2000. Smectite dissolution kinetics at 80°C and

pH 8.8. Geochim. Cosmochim. Acta 64, 2701-2717.

Cama J., Metz V. and Ganor J. 2002. The effect of pH and temperature on kaolinite

dissolution rate under acidic conditions. Geochim. Cosmochim. Acta 66, 3913-3926.

Carroll-Webb S. A. and Walther J. V. 1988. A surface complex reaction model for the pH-

dependence of corundum and kaolinite dissolution rates. Geochim. Cosmochim. Acta

52, 2609-2623.

Carroll S. A. and Walther J. V. 1990. Kaolinite dissolution at 25, 60 and 80°C Am. J. Sci. 290,

797-810.

Charlet L., Schindler P. W., Spadini L., Furrer G. and Zysset M. 1993. Cation adsorption on

oxides and clays: the aluminum case. Aquat. Sci. 55, 291-303.

Cicel B. and Komadel P. 1994. Structural formulae of layer silicates. In Quantitative methods

in soil mineralogy, (eds. J. E. Amonette and L. W. Zelazny), Madison, WI. pp. 114-

136.

Dent D. L. and Pons L. J. 1995. A world perspective on acid sulfate soils. Geoderma 67, 263-

276.


136

Dougan W. K. and Wilson A. L. 1974. Absorbtiometric determination of aluminium in water-

comparison of some chromogenic reagents and development of an improved method.

Analyst 99, 413-430.

Fitzpatrick R. W. and Shand P. 2008. Inland acid sulfate soils: Overview and conceptual

models. In Inland acid sulfate soil systems across Australia, (eds. R. W. Fitzpatrick

and P. Shand), Perth, Australia.

Ganor J., Mogollón J. L. and Lasaga A. C. 1995. The effect of pH on kaolinite dissolution

rates and on activation energy. Geochim. Cosmochim. Acta 59, 1037-1052.

Glover F., Whitworth K., Kappen P., Baldwin D. S., Rees G. N., Webb J. and Silvester E.

2011. Acidification and buffering mechanisms in acid sulfate soil (ASS) wetlands of

the Murray-Darling Basin, Australia. Environ. Sci. Technol. 45, 2591-2597.

Golubev S. V., Bauer A. and Pokrovsky O. S. 2006. Effect of pH and organic ligands on the

kinetics of smectite dissolution at 25°C. Geochim. Cosmochim. Acta 70, 4436-4451.

Hradil D. and Hostomsky J. 2002. Effect of composition and physical properties of natural

kaolinitic clays on their strong acid weathering rates. Catena 49, 171-181.

Huertas F. J., Chou L. and Wollast R. 1999. Mechanism of kaolinite dissolution at room

temperature and pressure Part II: Kinetic study. Geochim. Cosmochim. Acta 63, 3261-

3275.

Kalinowski B. E. and Schweda P. 1996. Kinetics of muscovite, phlogopite, and biotite

dissolution and alteration at pH 1-4, room temperature. Geochim. Cosmochim. Acta

60, 367-385.

Kalinowski B. E. and Schweda P. 2007. Rates and nonstoichiometry of vermiculite

dissolution at 22°C. Geoderma 142, 197-209.


137

Kohler S. J., Dufaud F. and Oelkers E. H. 2003. An experimental study of illite dissolution

kinetics as a function of pH from 1.4 to 12.4 and temperature from 5 to 50°C.


Koroleff F. 1976. Determination of silicon. In Methods of sea water analysis, (eds. K.

Grasshoff, M. Ehrhardt, and K. Kremling), Verlag Chemie USA, Weinheim; Deerfield

Beach, Florida. pp. 149-158.

Lamontagne S., Hicks W. S., Fitzpatrick R. W. and Rogers S. 2006. Sulfidic materials in

dryland river wetlands. Mar. Freshwat. Res. 57, 775-788.

Metz V., Amram K. and Ganor J. 2005. Stoichiometry of smectite dissolution reaction.


Miller J. C. and Miller J. N. 1993. Errors in classical analysis-statistics of repeated

measurements. In Statistics for Analytical Chemistry, Ellis Horwood Limited, New

York. pp. 33-51.

Muniz I. P. and Leivestad H., 1980. Acidification-effects on freshwater fish. In: D. Drablos

and A. Tollan Eds.) Ecological Impact of Acid Precipitation SNSF Proceedings, Oslo,

84-92.

Nagy K. L. 1995. Dissolution and precipitation kinetics of sheet silicates. In Chemical

Weathering Rates of Silicate Minerals, (eds. A. F. White and S. L. Brantley),

Mineralogical Society of America, Washington, D. C. pp. 173-233.

Nordstrom D. K. 1982. Aqueous pyrite oxidation and the consequent formation of secondary

iron minerals. In Acid Sulfate Weathering, (eds. J. A. Kitrick, D. S. Fanning, and L.

R. Hossner), Soil Science Society of America, Madison, WI. pp. 37-56.

Norrish K. and Hutton J. T. 1977. An accurate x-ray spectrographic method for the analysis of

a wide range of geological samples. Geochim. Cosmochim. Acta 33, 431-441.


138

Norrish K. and Pickering J. G. 1983. Clay minerals. In Soils: an Australian viewpoint,

CSIRO: Melbourne/Academic Press: London Melbourne. pp. 281-309.

Oelkers E. H. 2001. General kinetic description of multioxide silicate mineral and glass

dissolution. Geochim. Cosmochim. Acta 65, 3703-3719.

Oelkers E. H., Schott J., Gauthier J. M. and Herrero-Roncal T. 2008. An experimental study

of the dissolution mechanism and rates of muscovite. Geochim. Cosmochim. Acta 72,

4948-4961.

Packter A. and Dhillon H. S. 1969. Heterogeneous reaction of gibbsite powder with aqueous

inorganic acid solutions; kinetics and mechanism. J. Chem. Soc. A, 2588-2592.

Parkhurst D. L. and Appelo C. A. J., 1999. User's guide to PHREEQC (version 2) - a

computor program for speciation, batch reaction, one dimensional transport, and

inverse geochemical calculations U. S. Geol. Surv. Water Res. Inv. Rep., 99-4259.

Ridley M. K., Wesolowski D. J., Palmer D. A., Bénézeth P. and Kettler R. M. 1997. Effect of

sulfate on the release rate of Al3+ from gibbsite in low-temperature acidic waters.

Environ. Sci. Technol. 31, 1922-1925.

Rozalen M. L., Huertas F. J., Brady P. V., Cama J., Garcia-Palma S. and Linares J. 2008.

Experimental study of the effect of pH on the kinetics of montmorillonite dissolution

at 25°C. Geochim. Cosmochim. Acta 72, 4224-4253.

Schroth B. K. and Sposito G. 1997. Surface charge properties of kaolinite. Clays Clay Miner.

45, 85-91.

Weber P. A., 2003. Geochemical investigations of neutralizing reactions associated with acid

rock drainage: Prediction, mechanisms and improved tools for management, The

University of South Australia, Mawson Lakes Campus, South Australia.


139

Wieland E. and Stumm W. 1992. Dissolution kinetics of kaolinite in acid aqueous solutions at

25°C. Geochim. Cosmochim. Acta 56, 3339-3355.

Yang L. and Steefel C. I. 2008. Kaolinite dissolution and precipitation kinetics at 22°C and

pH 4. Geochim. Cosmochim. Acta 72, 99-116.

Zysset M. and Schindler P. W. 1996. The proton promoted dissolution kinetics of K-

montmorillonite. Geochim. Cosmochim. Acta 60, 921-931.

dissolution kinetics of kaolinite, illite and ... 4... · kaolinite, illite and montmorillonite...

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