Dissolution kinetics of kaolinite, illite and montmorillonite under acid-sulfate conditions: a comparative study 1 ABSTRACT

Transcription

1 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. 105

2 compositions similar to the inland ASS solutions were used in these experiments. Flowthrough 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 (R Al ) 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 R Al 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 (SO 2 4 ) 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 (H 2 SO 4 ), and 106

3 in soils with very little or no carbonate buffering, can result in the dissolution of phyllosilicates in soils and release of hydrolysable (Al 3+, Fe 3+ ) and base (K +, Na +, Mg 2+, Ca 2+ ) 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 HClO 4 ) 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 HClO 4 ) and at temperatures between 25 and 70 C. In experiments designed to determine the effect of ionic strength, NaClO 4 was used to adjust the ionic strength of the solution. The authors proposed two independent and parallel reaction mechanisms for 107

4 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 HNO 3 ), and temperature range 25 to 70 C, with NaNO 3 used to adjust the ionic strength. Smectite dissolution rates were found to increase with decreasing ph, following a rate law with reaction order of 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 HNO 3 ) experiments; KNO 3 was used as background electrolyte in concentrations of 0.01 to 0.1 mol/l (batch) and 0.01 to 0.05 mol/l (flowthrough). In this work stoichiometric dissolution of montmorillonite was reported at ph < 4.5 with a proton reaction order of 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 108

5 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 H 2 SO 4. 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 (H 2 SO 4 ) 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 (H 2 SO 4 ). 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 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. 109

6 The mineralogy of the clay samples was determined by X-ray diffraction (XRD). The XRD (GBC MMA: CoKα radiation, λ= 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 θ 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 (TiO 2 ) 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 N 2 adsorption isotherms and the Brunauer-Emmett-Teller (BET) method. Specific surface area values for kaolinite and montmorillonite were 21 and 37 m 2 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 ; 110

7 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 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 using AR-grade H 2 SO 4 (2 M) 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). 111

8 4.3 CALCULATIONS Dissolution rate calculations In acidic solutions, the dissolution reactions of kaolinite and montmorillonite can be expressed as follows: (Al 1.91 Fe 0.04 Ti 0.05 )(Si 1.93 Al 0.07 )O 5 (OH) H Al Fe Ti H 4 SiO H 2 O (1) K 0.01 Na 0.56 (Al 3.13 Mg 0.45 Fe Fe Ti 0.01 )[Si 7.98 Al 0.02 ]O 20 (OH) H H 2 O 0.01K Na Al Fe Fe Mg Ti H 4 SiO 4 (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): R j j ( C ) 1 F = j (3) V SM where V j 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 (m 2 /g), M is the mineral mass (g) of the mineral and C j 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 112

9 with respect to the relevant minerals was calculated in terms of Gibbs free energy of reaction, G r (kcal mol 1 ): IAP G r = RTln (4) Keq where R is the gas constant, T is the absolute temperature, IAP is the ion activity product and K eq 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): σ R R = σ C C 2 σ F + F 2 2 σ S + S (5) where σ R is the uncertainty in the calculated rate, σ C is the uncertainty in Al or Si concentration of the output solution and σ F and σ S are the uncertainties associated with the fluid flow rate and the mineral surface area, respectively. 4.4 RESULTS 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. 113

10 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). 114

11 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. 115

12 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 and at I = 0.01 M. 116

13 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. 117

14 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. 118

15 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 and at I = 0.01 M. 119

16 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 Montmorillonite Illite 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. 120

17 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 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 Montmorillonite Illite This data has been presented in our earlier study (Bibi et al., 2011) (Chapter 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 R Si and R Al ) 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 R Si values are similar at both ionic strengths across the ph range studied, whereas R Al is significantly greater at ph 2 4 at the higher ionic strength (Table 4.3). 121

18 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 rate c Si Al Fe Mg a R Si I = 0.25 M R Al a R Si % b R Al % b Kaol Mont I = 0.01 M Kaol Mont Kaol. = Kaolinite; Mont. = Montmorillonite; (a) Log R Si and log R Al (mol m -2 s -1 ) are the dissolution rates calculated from the release rates of Si and Al, respectively;(b) R Si and R Al are the errors in the calculated R Si and R Al, respectively; (c) Fluid flow rate (ml/min). 122

19 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 G r for both kaolinite and montmorillonite increased with increasing ph as did the G r values for Fe and Mg oxide minerals. Amorphous SiO 2 and quartz showed the opposite trend, with decreasing G r 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(SO 4 ) +, whereas at ph 3 4, Al 3+ 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). 123

20 Table 4.4 Saturation state of the steady state solutions for kaolinite dissolution experiments with respect to selected minerals. Output ph Kaolinite SiO 2 (am) Quartz Gibbsite I = I = Table 4.5 Saturation state of the steady state solutions for montmorillonite dissolution experiments with respect to selected minerals. Output ph I = 0.25 Mont SiO 2 (am) Qtz Gibb Kaol Bruc Akag Goe Hem I = *Mont = montmorillonite; SiO 2 (am) = amorphous silica; Qtz = quartz; Gibb = gibbsite; Kaol = kaolinite; Bruc = brucite; Akag = akaganéite; Goe = goethite; Hem = hematite. 124

21 4.5 DISCUSSION 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 H 2 SO 4 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. 125

22 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 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

23 (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 > ph PZC ; the dissolution of mineral is promoted by the adsorption of OH ions, whereas at ph < ph PZC 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 127

24 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 (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 Mg 2+ 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 ph dependence Fig. 4.5 shows a plot of log R Si versus ph for kaolinite and montmorillonite from the current study and log R Si 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). 128

25 Fig. 4.5 (a) The plot of log R Si versus ph for kaolinite, illite and montmorillonite at the higher ionic strength (0.25). The regression equations for kaolinite, illite and montmorillonite are: log R Si = -0.72pH ; log R Si = -0.32pH-12.47; and log R Si = -0.22pH-12.55, respectively. (b) The plot of log R Si versus ph for kaolinite, illite and montmorillonite at the lower ionic strength (0.01); the regression equations for kaolinite, illite and montmorillonite are: log R Si = ph ; log R Si = ph ; and log R Si = ph , respectively. Kaolinite dissolution data at ph 4 at both ionic strengths was not included in the regression analysis. 129

26 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) Mineral dissolution rates Fig. 4.6 shows a comparison of kaolinite dissolution rates (log R Si (Fig. 4.6a) and log R Al (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 CH 3 COOH. Also shown are values from Cama et al. (2002), who conducted dissolution experiments on Georgia kaolinite (KGa-2) in flow-through reactors with HClO 4 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. 130

27 Fig. 4.6 A comparison of kaolinite (KGa-2) dissolution rates logr Si and logr Al 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 HClO 4, 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 131

28 dissolution study, it was reported that the total Al released in H 2 SO 4 -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(SO 4 )) +, species dominated the H 2 SO 4 -NaCl solutions (Ridley et al., 1997). Gibbsite dissolution rates have also been reported to be times greater in H 2 SO 4 solutions compared to HClO 4 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 SO 2 4 ions the studied system. As discussed previously, the dominant Al species in the steady state solutions were Al 3+ and Al(SO 4 ) +, with a greater proportion present as Al(SO 4 ) + at ph 1 and 2 (Appendix 2). Fig. 4.7 shows a comparison of R Al values for montmorillonite dissolution at the higher (Fig. 4.7a) and lower ionic strengths (Fig. 4.7b). While similar R Al values are obtained at ph 1, with increasing ph the difference between the R Al values obtained at the two ionic strengths increases, with higher values obtained at the higher ionic strength. R Si 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 R Al values are due to Al re-adsorption at the lower ionic strength rather than enhanced Al dissolution at the higher ionic strength. The increased R Al 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. 132

29 Complexation of Al 3+ with Cl ions in the high ionic strength solution may also contribute to the increased R Al in the system I = 0.25 M I = 0.01 M log R Al ph Fig. 4.7 Comparison of dissolution rate (log R Al ) 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 H 2 SO 4 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 133

30 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 (H 2 SO 4 ) in this study were generally higher than the previously reported rates in HCl and HClO 4 solutions (Cama et al., 2002; Huertas et al., 1999), which was attributed to the complexation reaction of SO 2 4 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). 134

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