Chapter 4: Dissolution of kaolinite, illite and montmorillonite 105 4 Dissolution kinetics of kaolinite, illite and montmorillonite under acid-sulfate conditions: a comparative study 1 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 (H 2 SO 4 ). 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, HNO 3 and HClO 4 ); 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 H 2 SO 4 (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.
35
Embed
Dissolution kinetics of kaolinite, illite and ... 4... · kaolinite, illite and montmorillonite under acid-sulfate conditions’. Authors are Irshad Bibi, Balwant Singh and Ewen Silvester.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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
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