Bentonite transformations in strongly alkaline solutions

Geotextiles and Geomembranes 28 (2010) 219–225

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Bentonite transformations in strongly alkaline solutions W.P. Gates*, A. Bouazza 1 Monash University, Department of Civil Engineering, Building 60, Melbourne, Vic 3800, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 December 2008 Received in revised form 26 July 2009 Accepted 1 October 2009 Available online 25 November 2009

Strongly alkaline solution pH causes changes to the mineralogy of bentonites which might impact on their performance as environmental barriers. The long term effect of solution pH on the performance of bentonite barriers such as in Geosynthetic Clay Liners needs to be studied from the viewpoint of solubility and stability of the mineral phases present at extreme pH values. Changes to bentonite mineralogy brought about by extended reaction with 1 M sodium hydroxide solutions at 20–25  C reveal that certain components of bentonites, namely smectite, opaline silica and quartz, are subject to dissolution in alkaline solution. Associated with dissolution is the formation of hydrous hydroxy-aluminosilicate as well as hydrous carbonate mineral phases. It is postulated that these precipitates, formed from reaction of bentonite with alkaline leachates can result in pore filling, which is responsible for recently measured lower hydraulic conductivity of some bentonites to high pH leachates. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Alkalinity Dissolution GCLs, Hyper-salinity Mineral disequilibrium Spectroscopy

1. Introduction Large amounts of strongly alkaline waste liquors and leachates are produced annually by the mining and mineral ore processing industries in Australia. Examples include liquors used in cyanidation and Bayor processes as well as leachates from waste materials. Given the need for better containment of mining waste leachates and process liquors, questions are re-surfacing concerning the hydraulic performance of GCLs when subjected to leachates of extremely low (<4) or high (>10) pH. Typically, these liquid wastes are stored in impoundments where the aqueous phase evaporates resulting in leachates having extreme ionic strength and, depending on the geochemistry of the ore, possibly extreme pH as well. Traditionally these impoundments have been lined with compacted clayey liners (CCLs) composed of fine mineral soils ‘‘borrowed’’ from the surrounding area if available. The scarcity of suitable and economical clayey soil resources for traditional liners throughout mining localities in Australia and worldwide has resulted in recent increased interest in alternative hydraulic barrier materials, such as geosynthetic clay liners (GCLs) (Lange et al., 2007, 2009; Benson et al., 2008). Geosynthetic clay liners are comprised of a thin layer of bentonite physically contained between two layers of geotextile with the

* Corresponding author. Tel.: þ61 3 9905 4976; fax: þ61 3 9905 4944. E-mail addresses: [email protected] (W.P. Gates), malek.bouazza@ eng.monash.edu.au (A. Bouazza). 1 Tel.: þ61 3 9905 4956; fax: þ61 3 9905 4944. 0266-1144/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2009.10.010

components being held together by needle-punching or stitch bonding. The bentonite component is generally processed to impart favourable sealing characteristics. Although GCLs are widely used in landfill applications and have been subject to considerable recent research (Bouazza, 2002; Bouazza and Vangpaisal, 2004, 2006, 2007a,b; Bouazza and Rahman, 2007; Bouazza et al., 2006, 2007, 2008; Southen and Rowe, 2007; Benson et al., 2007; Meer and Benson, 2007; Mu¨ller et al., 2008; Rowe et al., 2008; Brachman and Gudina, 2008a,b; Gates et al., 2009), very few studies have focussed on mining applications, and in particular how the bentonites may respond to high pH solutions. Because smectites are important minerals controlling bentonite performance as hydraulic barriers, knowledge of smectite reactivity to leachates of extreme pH is important to improve bentonite-based barriers for containment of alkaline industrial leachates. Smectite is a 2:1 layer silicate where two silica tetrahedral sheets are bonded to either side of an aluminous octahedral (gibbsite-like) sheet. At pH > 12 smectite is unstable resulting in disintegration of the 2:1 layer structure (Claret, 2002; Ramı´rez et al., 2002; Sa´nchez et al., 2006; Taubald et al., 2000). At these pH values, the hydrated hydroxyl ion (H3O-2 or H2OOH-) strongly competes with other anions (e.g. O2-) and its reactivity drives bondbreaking and dissolution reactions of many silicate minerals (Casey, 1995). Strong concentrations of hydroxyl anion (OH-) enable many cations (e.g., Mg2þ, Fe3þ and Al3þ), all components of the smectite structure, to form hydroxylated monomers and dimmers in solution. If the pH is lowered or buffered to lower values, these can condense to form polymeric oxy-hydroxyl structures. In the

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presence of dissolved silica, such structures can form the template for precipitation and growth of alkaline-stable hydroxy-aluminosilicates and other minerals (Ramı´rez et al., 2002; Sa´nchez et al., 2006). Bauer and Velde (1999) observed smectite dissolution in concentrated (up to 4 M) KOH to be essentially congruent (ie stoichiometric) until illite layers formed which interstratified with remaining smectite layers. In the final stages (up to 2 months), zeolites and quartz precipitated. Under conditions of strong alkalinity (pH > 10), incongruent dissolution reactions might serve to counter the adverse effects of ionic strength through promotion of pore filling precipitates. Detecting subtle changes in mineralogy under less severe, but more realistic ‘‘surface-environment’’ conditions are difficult and can be easily missed using only a single technique, especially at low temperature. Strongly alkaline pH solutions also typically have high ionic strengths, perhaps exceeding 1 M. Ionic strength can exert simultaneous influence on two effects: (1) the pore structure and fabric of the bentonite; and (2) the activity of water. Together these reactions can be expected to result in a collapsed fabric with greater hydraulic conductivity. Ionic strength will often have a greater and more detrimental effect on interparticle dispersion due to compression of the diffuse electric double layer (Likos et al., 2010). The effect of ionic strength on the activity of water can also be detrimental to clay fabric in that water is withdrawn from the clay interlayer spaces and results in chemical desiccation. If accompanied by high Kþ concentrations as well as high pH, desiccation and dissolution can result in formation of illite or interstratifications of non-expanding layers with unreacted smectite (Eberl et al., 1993) and potentially diminish swelling characteristics (Jozefaciuk and Matyka-Sarzynska 2006). The purpose of this study was two-fold: first, to demonstrate that strongly alkaline solutions can cause dissolution of some mineral components of a bentonite commonly used in geosynthetic clay liners, and also to demonstrate that these reactions are accompanied by the precipitation of hydrous and hydroxyl-rich aluminosilicate and carbonate phases. Secondly, the aim is to show that because many of the mineralogical effects are subtle, multiple methods need to be applied to develop a full understanding of the reactions that take place when bentonite is subjected to strong thermodynamic disequilibrium. 2. Materials and methods A bentonite (Miles Queensland, Australia) typically used in locally manufactured GCLs, was used as received. The water-soluble sodium content of the bulk was determined on a 10:1 water:soil extract by atomic absorption. The exchange cation composition of the bulk material was determined on an ammonium acetate extract (pH 7) by inductively coupled plasma (ICP) spectrometry. Batch digestions (in duplicate) containing 200 ml of 1 M NaOH added to 50 g of bentonite were allowed to react (covered) under ambient (20–25  C) conditions with minimal disturbance for six months. After 6 weeks and 6 months, 5  0.1 g sub-samples (and 20  1 ml solution) were removed for analysis. Solid phase subsamples were allowed to air dry without washing, ground and examined by X-ray diffraction (XRD), infrared spectroscopy (IR), X-ray absorption spectroscopy (XAS) and thermal gravimetric analysis (TGA). The liquid fraction was tested for soluble and exchangeable major cations by ICP. The initial pH of the NaOH solution was measured as 13.8. Dried materials were finely ground using an agate mortar and pestle until all grittiness was destroyed (about 10 min). Random powdered samples were back-pressed into aluminium sample supports onto frosted glass slides to minimise any preferred

orientation effects (Moore and Reynolds, 1989). Powder XRD analysis was performed using a Phillips 1710 diffractometer modified with an electronic stepper motor and Cu Ka radiation source. X-ray traces were collected within the range of 2–65  2q using step size of 0.02  2q and a scan rate of 0.6  2q per min. Mineral phases present in the traces were identified from autoselected references in the international crystallographic diffraction database (ICDD) using the program XPLOT (CSIRO). Criteria for mineral phase selection consisted of matching d-space values (within 0.05 °2q) of the most intense reflections for a given phase. Infrared spectra were collected using a Merlin Digilab Excalibur FTS3000mx spectrometer employing a glow bar radiation source and fitted with an extended KBr beamsplitter, MCT detector and a diffuse reflectance (DRIFT) sampling assembly. The sample space was continuously purged with dry nitrogen. Dried powdered samples were mixed at 5  0.1 wt% into dry sodium chloride (NaCl) powder and front packed into an anodised aluminium sample holder. All spectra were ratioed against NaCl prior to transforming the reflectance data to Kubelka–Munk absorbance units. Major absorbance features of bentonite were assigned according to existing literature (Farmer, 1974; Gates, 2005; Gates et al., 2002). X-ray absorption spectroscopy was performed at the Australian Synchrotron as part of the commissioning of the ‘‘soft X-ray beamline’’ (14ID-01). Total electron yield (measured as drain current) Al K near edge X-ray absorption fine structure (NEXAFS) spectra, in the energy range of 1550–1650 eV, were collected on undiluted powdered samples pressed onto carbon tape. Steps of 0.1 eV and acquisition times of one sec were used. A minimum of 2 scans were co-added after adjusting for instrumental energy drift. Spectra were processed following typical procedures (eg see Gates, 2006) and compared to reference minerals and from the survey work of Li et al. (1995) and Ildefonse et al. (1998). Thermal gravimetric analysis was conducted using a Perkin-Elmer Pyris TFA under dry nitrogen purge (40 mL/min) on approximately 15 mg of powdered sample loaded onto a platinum sample pan and stirrup. Samples were held isothermally at 60  C for 10 min prior to heating to 800  C at a rate of 5  C min1. Plots of percentage mass loss, determined from the sample mass at the end of the isothermal stage, were differentiated as a function of sample temperature to locate the onset and cessation of mass loss associated with particular events. Differences in mass between the onset and cessation temperatures were taken as the percentage mass loss associated with that particular event (e.g. dehydroxylation). 3. Results and discussion 3.1. Initial bentonite composition Table 1 shows the soluble sodium and exchangeable cation composition of the initial bentonite and Fig. 1 displays the XRD

Table 1 Soluble and exchange cations of untreated bentonite using ammonium acetate extract (pH 7). Solublea

Exchangeable cations

Naþ

Naþ

108

Ca2þ

Mg2þ

Kþ

30

9.9

16.8

1.2

% 52 83

17 6

29 10

2 1

cmol/kg Total Cations Percent Exchangeable Percent Total a

NaHCO3 is added to improve dispersion.

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AHC

SAS

AHC

Counts

6 months

CAS, SAS, CAS, CSH, SAS T CAS, T CAS, T T CSH

6 weeks Q

S SQ O Untreated S

10

F

20

FF

S

30

Q

Q Q Q

40

Q Q

50

Q S Q

Q S

60

2θ (Cu K α radiation)

o

Fig. 1. X-ray powder diffraction patterns depicting changes to the bentonite with time of reaction with 1 M NaOH. Letters above the untreated sample plot indicate reflections associated with major mineral constituents: S ¼ smectite; Q ¼ quartz; O ¼ opal; F ¼ feldspar. Letters above the 6 Month plot indicate reflections of new mineral phases: SAS ¼ sodium aluminium silicate hydrate; AHC ¼ alumohydrocalcite; T ¼ trona; CAS ¼ calcium aluminium silicate hydrate.

traces of the unreacted bentonite. The bentonite is dominated by sodium, both as an exchange cation and present as a soluble salt. Sodium bicarbonate is added to the bentonite to assist in dispersion and to increase the Na-saturation of the exchange complex (Murray, 1995). Analysis indicates the initial bentonite is composed of 70% smectite, 15% quartz, 8% feldspars and 7% opaline silica, as well as traces (<1%) of illite/mica, results entirely consistent with those of Gates et al. (2002) and Benson et al. (2010). Note that in those studies Co Ka radiation was used for quantification, and traces (<1%) of zeolite were also identified by Gates et al. (2002). Specific peaks are marked in Fig. 1 depicting the four main mineral constituents of the Miles bentonite. 3.2. Reaction with 1 M NaOH Fig. 1 displays the XRD traces of the bentonite reacted in 1 M NaOH for 6 weeks and 6 months while Table 2 displays the changes in the solution chemistry. The most obvious changes in the XRD are the loss of w40% of relative intensity of the d(001) reflection for smectite at w1.20 nm (w6  2q in Fig. 1) and shift to w1.55 nm after 6 weeks reaction in 1 M NaOH. The decreased intensity is a result of both delamination as also indicated by loss of the 002 reflection near 14.4  2q. The shift to 1.55 nm indicates that interlayer exchange (e.g. Naþ for Ca2þ or Mg2þ) and interlayer hydration have occurred (Likos et al., 2010). Continued reaction for 6 months resulted in a slight shift of this reflection to 1.4 nm and slightly increased intensity over the 6 week sample (compare the

Table 2 Elemental (ICP) analysis of leachates following reaction of sodium bentonite in 1 M NaOH. Values reported are average of triplicate analyses, and in brackets one standard deviation. Al3þ

Fe3þ

Mg2þ

Ca2þ

Naþ

pH

mg/kg Untreateda 6 weeks 6 months a

nil <1 <1

nil <0.2 0.6

Extracted in deionised water.

<0.1 <0.3 <0.3

<0.1 <0.1 <0.1

45.9 1020 1400

12.7 11.9

221

two traces for 6 weeks and 6 months in Fig. 1), indicating that the layer structure was largely preserved under the conditions of this study. The reflection for quartz near 22  2q (labelled Q in Fig. 1) lost substantially more intensity relative to the reflection near 20  2q (labelled S in Fig. 1), which was used to monitor smectite loss due to dissolution. The reflection for opal CT near 21 2q (peak labelled O in Fig. 1) substantially disappeared after 6 weeks reaction in 1 M NaOH and continued reaction for 6 months resulted in a loss of w75% intensity of this reflection. Note that losses of silica observed here differ from the results of Vlasova et al. (2007), who observed no loss in cristobalite from RT reaction with pH ¼ 12 NaOH. However, the pH of their system was strongly buffered by the presence of gypsum (CaSO4), which probably minimised dissolution of silicates. According to losses of the solid phases, opal was more reactive than smectite under the conditions of this study. However, given the overall amount of quartz in the bentonite, its losses during the reaction may also be important in providing silica in solution. The XRD results for samples reacted in 1 M NaOH for 6 months enable possible assignment of new mineral phases accumulating in the samples. The XRD trace for the 6 month reacted sample (Fig. 1) shows reflections that cannot be assigned to any of the original mineral phases; the most important being hydrous calcium and sodium silicates and calcium aluminium silicates (Table 3). Sufficient Ca and Al are likely released from smectite dissolution to enable formation of these phases with Si released from opal. A similar finding was observed by Benson et al. (2010). It is noteworthy that these represent the only silicate phases found to fit the peaks of the new phases in the experimental XRD pattern, and may contain significant aluminium. However, the percentages of these phases are very low, estimated to be less than 1–2% by weight. Other possible mineral phases identified include trona, and alumohydrocalcite (AHC), both of which are hydrous carbonates. Alumohydrocalcite does not ideally fit the expected chemistry of a calcite-like phase from the Miles mineralogy, as it requires no sodium. Thus the hydrous carbonate phase formed is most similar to trona (hydrous sodium carbonate bicarbonate), a typical calcareous brine evaporite. Carbonate phases may be artefacts of the experimental conditions (eg exposure to atmosphere during drying), but they nonetheless can be expected to occur under environmental conditions as well.

Table 3 X-ray diffraction peak positions of various mineral phases possibly present as new phases in bentonite following reaction ith 1 M NaOH for 6 months. SAS ¼ sodium aluminium silicate hydrate; CAS ¼ calcium aluminium silicate hydrate; CSH calcium silicate hydrate; Trona ¼ sodium carbonate hydrate; AHC alumohydrocarbonate. AHC

13.7 17.7 21.7 22.5 24.4 24.8 27.8 28.9 29.2 29.8 31.6 33.8 34.9 35.8 37.2

SAS 

Observed peak position

CAS

CSH

Trona

2q (Cu Ka-radiation)

13.7 18.0 21.8

18.1 22.2

22.7 24.5 24.9 27.9

27.9 28.9 29.4

33.5 34.7 35.6

29.8 32.1 34.1

29.1

32.1 33.8

35.6 36.8

222

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3638 1048

3480 3050 3399 3255 1475

1430

6 months 3636 3025

3480 3410 3255

1034

799

6 weeks

778

3636 3430

916 881 848

3150 1490

3005 Untreated

3850

3450

3050

2650

1500

Wavenumbers (cm-1)

1300

1100

900

700

Wavenumbers (cm-1)

Fig. 2. Infrared spectra detailing development of hydroxylated and carboxylated mineral phases in bentonite reacted in 1 M NaOH for 6 weeks and 6 months, compared to the unreacted bentonite. See Table 4 for decomposition results of the OH stretching region. Downward pointing arrow indicates loss of AlFeOH and AlMgOH bending modes while the upward pointing arrow indicates formation of carbonate deformation near 852 cm1.

Fig. 2 shows the IR spectra of unreacted bentonite and bentonite reacted in 1 M NaOH for 6 weeks and 6 months. Bands associated with the development of new phases are observed and the duration of reaction resulted in stronger signals associated with these new phases (Table 4). The most obvious difference is the changes to the OeH stretching region between w2450 and 3600 cm1, development of carbonate bands near 1490 cm1 and changes to the OH bending region between w800 and 950 cm1. For the untreated bentonite, the spectrum in the OH stretching region is typical of this smectite (initial water content of w10% by weight). The strong band at 3636 cm1 is assigned to OH stretching dipoles where the hydroxyl is bonded to two octahedrally coordinated cations, such as Al or Mg, present in the montmorillonite structure (Madejova´ et al., 1994; Gates, 2005). The broad Table 4 Results from decomposition of the OH stretching region of the infrared spectrum. Position Untreated

6 weeks

6 months

Assigment

Intensity %

Intensity %

Intensity %

3697 3638

0.17

1.2

0.21

0.26 4.30

3636 3480 3430

5.26

36.2 5.43 0.85 31.4

3410 3399 3255 3150 3050 3025 3005 2880 2800

4.57

3.06

1.0

26.7 4.2 2.19

3.72

18.3

0.97

4.8

6.18

30.4

2.97

14.6

1.32 2.51

21.1 4.18

1.46

10.1 3.28

1.4 Smectite OH stretch 23.8 Smectite OH stretch Smectite OH stretch

overlapping bands near 3430, 3150 and 3000 cm1 are HeOeH stretching modes associated with water adsorbed to the smectite surfaces. After 6 weeks reaction with 1 M NaOH, these broad overlapping bands substantially disappear or shift to lower wavenumber, centred near 3025 cm1, typical of water that is strongly held within the interlayer spaces of clay. Following 6 months reaction, the adsorbed OeH stretching bands reveal a substantial shift to higher wavenumbers 3050 cm1, but also several unique bands form near 2848, 3255, 3399 and 3480 cm1. The broadness

B

C

D

A

Normalised Intensity

3.3. Infrared spectroscopy

6 Months

E

12.1 Silicate OH stretch Smectite water HeOeH Smectite water H–O–H 7.3 Silicate OH stretch 13.9 Silicate OH stretch Smectite water HeOeH 23.2 Silicate water HeOeH stretch Smectite water HeOeH Silicate water HeOeH 18.2 stretch

Untreated

1560

1570

1580

1590

1600

Energy (eV) Fig. 3. Al K NEXAFS spectra of untreated bentonite and bentonite reacted for 6 months in 1 M NaOH. Peak labels for decomposed spectra are explained in text.

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223

including layer silicates and Ildefonse et al. (1998) extended this to study of smectites. Accordingly, the spectra displayed in Fig. 3 are dominated by the aluminium sites of smectite, which are typified by a doublet absorption with maximum intensities near 1567.9 and 1571.2 eV (peaks B and C in Fig. 3). These two peaks correspond to 1s / t and 1s / e (3 p-like) transitions (Li et al., 1995) of two distorted octahedral Al sites and are typical of edge sharing Al octahedra (Ildefonse et al., 1998). The presence of tetrahedral Al (1s / a 3s-like transition) is indicated as a weak shoulder near 1566.2 eV (peak A). Peak D is related to multiple scattering from higher energies and peak E is related to symmetry forbidden 1s / t transitions, but is also affected by multiple scattering from the local crystal structural environment. According to Gates et al. (2002) the Miles bentonite contains w8% tetrahedral Al, a result entirely consistent with the fitting of the Al NEXAFS spectra for the untreated sample (Fig. 3). With reaction in 1 M NaOH for 6 months, the most obvious difference is a slight increase in the relative intensity of peak C due to broadening and thus a decrease in the ratio of peak B and peak C compared to the untreated bentonite. The full width at half maximum of this doublet increases by w20%, and the tetrahedral Al estimate decreases by 20%, to a value of 6.5%. According to Ildefonse et al. (1998), the hydrous aluminium oxides typically display broadening of the high energy side of the peak

and position of the latter three bands would indicate the development of structural OH associated with hydrous mineral phases (Table 4). These interpretations are consistent with the continued development of hydrous silicate/aluminosilicate minerals such as identified by XRD following 6 months reaction compared to the samples reacted for 6 weeks. The OH bending bands of the bentonite (848 and 881 cm1) associated with OH sharing octahedrally coordinated Al–Mg and Al–Fe pairs (Gates, 2008), respectively, largely disappear, and indicate the loss of smectite structural integrity (downward pointing arrow in Fig. 2). The strong Si–O bending mode near 1034 cm1, typical of layer silicates, shifts to 1048 cm1, more typical of non-layer silicates (Farmer, 1974). A band near 1460 cm1 is observed in the sample reacted for 6 weeks, that shifted from 1490 cm1 in the original material. This band resolves to two bands near 1475 and 1430 cm1 following 6 month reaction, and is accompanied by a band at 852 cm1 (upward pointing arrow in Fig. 2), all consistent with the formation of carbonate minerals (Adler and Kerr, 1963). 3.4. X-Ray absorption spectroscopy Results from Al K NEXAFS spectroscopy are displayed in Fig. 3. Li et al. (1995) surveyed the Al K NEXAFS spectra of several silicates,

Weight (%)

0 Untreated -2 -4 -6

Differential

-8

288 (2.4%)

489 (12.4%)

92

600

647 (35.1%)

75 (50.1%) 200

400

600

800

Temperature

Weight (%)

0 6 weeks -2 -4 -6 -8

Differential

-10 288 (1.9%)

525 (22.1%)

615 (6.4%)

89 (69.6%) 200

400

600

800

Temperature Fig. 4. Differences in thermal gravimetry of untreated bentonite and bentonite reacted for 6 weeks in 1 M NaOH. Decomposition results are detailed in Table 5.

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near 1571 eV compared to aluminosilicates, and the broadening here could be related to the presence of an aluminium silicate phase with increased AleO bond distances compared to montmorillonite (Li et al. 1995). The changes to the Al NEXAFS spectra are interpreted as representing either a loss of tetrahedral Al from the smectite or formation of new octahedral Al environments within hydroxylrich silicate phases. Any loss of tetrahedral Al would appear inconsistent with past studies of alkaline leachate interaction with smectites, as most have observed increased proportion of illites, where illite-smectite phases were present (e.g., Eberl et al., 1993; Claret, 2002). Illite has a greater tetrahedral Al content than smectite. Thus, we consider the latter interpretation, that is the formation of new octahedral Al environments with greater line broadening as being consistent with the changes quantified in the XRD and IR.

3.5. Thermal gravimetry analysis Fig. 4 shows thermal gravimetric weight loss as well as the differential weight loss for the untreated bentonite and the bentonite reacted for 6 weeks in 1 M NaOH. The results displayed in Table 5 indicate that the dehydration of smectite (weight loss events at 75  C) accounted for 46% of total weight loss, and those associated with dehydroxylation (events between 489 and 647  C) accounted for 48%. Loss of carbon, present as sodium bicarbonate additives in the bentonite, accounted for w3% of sample mass. Following 6 weeks reaction with 1 M NaOH the thermal gravimetry and differential curves changed remarkably. The maximum temperature associated with dehydration shifted to 89  C increased to 70% of the total weight loss. Most notable, however was the loss of 70% of the intensity of the large dehydroxylation peak near 650  C, and development of a broad weight loss event centred near 525  C, which constituted 22% of the total weight loss (Table 5), essentially equal to the change in the higher temperature dehydroxylation event. The higher temperature dehydroxylation event was hardly resolved and accounted for only 6% of weight loss. About the same amount of decarboxylation was observed, however the weight loss events at higher temperature appear to initiate at about 300  C. Based on total absolute weight loss in the two samples, the dehydroxylation events for the untreated smectite amounted to 3.8%, which is typical of the hydroxyl content of smectite (Frost et al., 2000). This decreased by w50% to 1.5% following reaction, indicating significant loss of hydroxyls having a stable smectite-like local environment. In comparison, the IR data (Fig. 2) shows that the structural OH stretch decreased in intensity by only w27% and the OH bending modes for AlFeOH near 880 cm1 lose more than

Table 5 Results from thermal gravimetric analysis. Temperature ( C)

Untreated

6 weeks

Assignment

% of total weight loss 75 89 92 288 489 525 605 615 647

45.2 69.6 4.9 2.4 12.4

2.9 21.6

16.4 5.9 18.7

Dehydration Dehydration Dehydration decarboxylation dehydroxylation dehydroxylation dehydroxylation dehydroxylation dehydroxylation

70% of their initial intensity, but the amount of surface bound HeOeH stretching increased significantly (Table 4). 3.6. Implications for barrier performance Claret (2002) observed that exposure of an illite-smectite sediment to pH > 13 solutions led to the preferential dissolution of the discrete smectite phases and retention of illite. Ramı´rez et al. (2002) and Sa´nchez et al. (2006) observed alteration of bentonite in the presence of high pH and portlandite resulted in formation of alkaline-stable calcium silicate hydrate (CSH) phases, zeolites and magnesium enriched smectites (ie saponites). In addition, they showed that elevated temperatures increased dissolution even for 0.1 M NaOH and promoted the formation of more stable zeolite structures. Interestingly, in reactions of an opal-rich smectite with simulated dissolved concrete solution (pH 13.2, and high dissolved Ca and Si levels) Taubald et al. (2000) observed no changes in mineralogy after 18-months according to X-ray diffraction. Further analysis by scanning electron microscopy, however, revealed the formation of CaAlSi hydroxide secondary cements, which caused the ksat to remain an order of magnitude less than the original value for water. Reaction of the Miles bentonite with 1 M NaOH in batch (this study) and during hydraulic conductivity measurements (Benson et al., 2010) results in production of small amounts of hydrated calcium silicates and calcium aluminium silicates. Benson et al. (2010) also reported development of pore filling hydrated phases in SEM images of a GCL composed of Miles bentonite permeated with 1 M NaOH, but the EDX returned pointed to it being more akin to a magnesium silicate carbonate. In the batch studies detailed here, hydrated carbonates were identified, but are believed to be mostly associated with drying of the sample, which did not occur in the study reported by Benson et al. (2010). The possible phases observed to precipitate from the incongruent dissolution of smectite in 1 M NaOH in this study appear to be implicated in stabilising the Miles bentonite to hyper-alkaline pH (Benson et al., 2010). In that study, the hydraulic conductivity of 1 M NaOH (þ1.3 mM CsCl) in a GCL composed of the same bentonite as used here (GCL1) was observed to increase slightly from a very low value of 8  1012 m s1–w3  1011 m s1. While only 10 pore volumes of flow transmitted through the GCL, the hydraulic conductivity did stabilize after 8 pore volumes and then a decrease was observed. We interpret this as evidence of pore filling by mineral phases such as described here. 4. Conclusions The results of four physical methods – X-ray diffraction, diffuse reflectance infrared and Al X-ray absorption spectroscopy and thermal gravimetric analysis, provide strong evidence that moderately strong alkaline pH solutions at 20–25  C cause differential degradation of the mineral components of bentonite. Namely,  Changes to bentonite chemistry indicate that Na, Mg, Ca and Al are available either from exchange or from dissolution.  Powder X-ray diffraction points toward formation of calcium silicate hydrates, calcium aluminium silicate hydrates, hydrous sodium carbonates and sodium aluminium carbonates.  Infrared reveals formation of hydrous and hydroxylated carbonate and silicate phases as well as selective dissolution of AlFeOH and AlMgOH groups within the smectite.  Thermal analysis reveals increased amount of adsorbed water present in the reacted material, and a decrease in the amount and temperature of the dehydroxylation events associated with smectite.

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 Al K NeXAFS points to the introduction of a solid phase with longer octahedral AleO bonds than in the original bentonite. All methods indicate that highly hydroxylated silicate and aluminosilicate mineral phases precipitate as a result of the reactions. Hydrous carbonate minerals also formed, probably resulting from drying of samples for analysis. While our study was limited in scope, it can be expected that the above reactions upon exposure to high pH solutions, particularly the formation of hydrous silicates and aluminium silicates might result in changes in barrier performance over time. The extent of the reactions and therefore how they may impact barrier performance will depend on the strength of alkalinity, ionic strength of solutions, temperature of reaction as well as the concentrations and identity of dissolved cations and anions present. Acknowledgements We thank the editor and two anonymous reviewers for comments to improve our manuscript. This research was supported by the Monash University Faculty of Engineering’s New Staff Grant Program. Part of this research was undertaken on beamline 14ID-01 at the Australian Synchrotron, Victoria Australia. The views expressed are those of the authors and not necessarily those of the owner or operator of the Australian Synchrotron. B Cowie, L Thompsen, R Hocking, A Koo and J Cashion all kindly provided assistance. References Adler, H.H., Kerr, P.F., 1963. Infrared spectra, symmetry and structure relations of some carbonate minerals. American Mineralogist 48, 839–853. Bauer, A., Velde, B., 1999. Smectite transformation in high molar KOH solutions. Clay Minerals 34, 259–273. Benson, C.G., Thorstad, P.A., Jo, Y.Y., Rock, S.A., 2007. Hydraulic performance of geosynthetic clay liners in a landfill final cover. Journal of Geotechnical and Geoenvironmental Engineering 133 (7), 814–827. Benson, C., Wang, X., Gassner, F.W., Foo, D.C.F., 2008. Hydraulic conductivity of two geosynthetic clay liners permeated with an alumina residue leachate. In: Proceedings 1st Pan-American Conference on Geosynthetics. Cancun. Mexico (CD-Rom). Benson, C., Oren, A.H., Gates, W.P., 2010. Hydraulic conductivity of two geosynthetic clay liners perrmeated with a hyperalkaline solution. Geotextiles and Geomembranes 28 (2), 206–218. Bouazza, A., 2002. Geosynthetic clay liners. Geotextiles and Geomembranes 20 (1), 3–17. Bouazza, A., Vangpaisal, T., 2004. Effect of straining on gas advective flow of a needle punched GCL. Geosynthetics International 11 (4), 287–295. Bouazza, A., Vangpaisal, T., 2006. Laboratory investigation of gas leakage rate through a GM/GCL composite liner due to a circular defect in the geomembrane. Geotextiles and Geomembranes 24 (2), 110–115. Bouazza, A., Rahman, F., 2007. Oxygen diffusion through partially hydrated geosynthetic clay liners. Ge´otechnique 57 (9), 767–772. Bouazza, A., Vangpaisal, T., 2007a. Gas permeability of GCLs: effect of poor distribution of needle punched fibres. Geosynthetics International 14 (4), 248–252. Bouazza, A., Vangpaisal, T., 2007b. Gas transmissivity at the interface of a geomembrane and the geotextile cover of a partially hydrated geosynthetic clay liner. Geosynthetics International 14 (5), 316–319. Bouazza, A., Vangpaisal, T., Jefferis, S., 2006. Effect of wet dry cycles and cation exchange on gas permeability of geosynthetic clay liners. Journal of Geotechnical and Geoenvironmental Engineering 132 (8), 1011–1018. Bouazza, A., Jefferis, S., Vangpaisal, T., 2007. Analysis of degree of ion exchange on Atterberg limits and swelling of geosynthetic clay liners. Geotextiles and Geomembranes 25 (3), 170–185. Bouazza, A., Vangpaisal, T., Abuel-Naga, H.M., Kodikara, J., 2008. Analytical modelling of gas leakage rate through a geosynthetic clay liner-geomembrane composite liner due to a circular defect in the geomembrane. Geotextiles and Geomembranes 26 (2), 109–204.

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