Elsevier

Geotextiles and Geomembranes

Volume 36, February 2013, Pages 71-80
Geotextiles and Geomembranes

Acid induced degradation of the bentonite component used in geosynthetic clay liners

https://doi.org/10.1016/j.geotexmem.2012.10.011Get rights and content

Abstract

Bentonite is a natural clay mineral widely used in the mining and solid waste containment industry, for example, as a soil mixture for the construction of seepage barriers, or as a component of geosynthetic clay liners (GCLs), to provide low hydraulic conductivity. However, degradation of bentonites generally occurs when permeated with acid solutions, such as encountered in mining applications, which may influence physical properties, and particularly, the hydraulic performance of geosynthetic clay liners.

In this paper, properties such as Atterberg limits, free swell index, and fluid loss of three bentonites were measured with different concentrations of sulphuric acid solutions. These properties were found to deteriorate even with low (0.015 M) sulphuric acid solutions; higher concentrations (up to 1 M) resulted in larger degradation. X-ray diffraction and infrared spectroscopy were used to monitor the change of bentonites after interaction with the acid solutions. Acid leachates in general result in the overall degradation of the hydraulic performance of geosynthetic clay liners and potentially, any bentonite-soil mixture.

Introduction

Bentonites are naturally occurring soft rocks with a high content of the clay mineral montmorillonite. Montmorillonite is capable of swelling during uptake of water. In addition, montmorillonite has a large specific surface area, excellent plasticity, and low hydraulic conductivity. Because of these properties, hydrated bentonites are of great significance for industrial, environmental, and civil engineering activities requiring sealants, absorbents, and hydraulic barriers. Bentonite is the key component in geosynthetic clay liners (GCLs) which are low permeable materials increasingly used in barriers, or components of barriers, for a wide variety of hydraulic and gas containment applications in mining waste containment facilities. (Kashir and Yanful, 2001; Bouazza, 2002; Vangpaisal and Bouazza, 2004; Bouazza et al., 2006; Bouazza and Rahman, 2007; Gates et al., 2009; Lange et al., 2007, 2009; Benson et al., 2010; Hornsey et al., 2010; Gates and Bouazza, 2010; Shackelford et al., 2010).

Bentonites can be used as originally mined or after some physico-chemical treatments. A common chemical modification of bentonite is acid activation, usually with HCl or H2SO4 (e.g. see Komadel and Madejová, 2006) to enhance catalytic and absorptive properties (e.g. see Wallis et al., 2007). The treatment results in an increase in specific surface area, porosity and surface acidity (Komadel, 2003). The increase in specific surface area can improve the performance of bentonites in applications, for example, as catalyst (Mokaya and Jones, 1994; Bovey and Jones, 1995), bleaching earth (Siddiqui, 1968; Wu et al., 2006), and as a component in carbonless copying papers (Fahn and Fenderl, 1983). However, the increase in porosity caused by acid is expected to produce a negative effect on the hydraulic performance of bentonite, as has been shown for GCLs (Petrov et al., 1997; Shackelford et al., 2000, 2010). Interaction with acidic solutions can result in increased porosity of bentonite and decreased swelling thus causing increased hydraulic conductivity of the GCL (Kolstad et al., 2004a; Fall et al., 2009; Shackelford et al., 2010). Kolstad et al. (2004b) reported a drop in swell index of a granular bentonite GCL from 35.5 mL/2 g (water) to 18 mL/2 g (acidic solution, pH = 1.2) and an increase in hydraulic conductivity from 1.2 × 10−11 m/s (to water) to 1.5 × 10−7 m/s when permeated with the acidic solution. Similarly, Shackelford et al. (2010) noted an increase from 1.7 × 10−11 m/s (to water) to 2.7–3.9 × 10−8 m/s when a GCL was permeated with an acidic solution of pH = 2.5.

The scarcity of suitable and economical clayey soil resources for traditional liners (i.e. compacted clay liners) throughout mining localities has resulted in recent increased interest in alternative hydraulic barrier materials, such as GCLs (Fourie et al., 2010; Bouazza, 2010). As a result, the bentonite component has a high probability to coming into contact with acids in mining projects, e.g., in heap leach containment systems (Hornsey et al., 2010) or the acidic leachate caused by the oxidation of impounded tailings (Shackelford et al., 2010).

A wide variety of research has been conducted to analyze the degree of decomposition of clays by acids, changes of surface acidity, cation exchange capacity, and catalytic power, most focused on the physico-chemical properties of the resulting acid-activated material (Breen et al., 1995a; Gates et al., 2002; Önal, 2007; Önal and SarIkaya, 2007; Wallis et al., 2007). However, few studies have focused directly on how some of the engineering index properties (Atterberg limits, swelling index, and fluid loss etc.) of bentonites may degrade when subjected to acid solutions. These parameters have close relationships with hydraulic conductivity and could be used as indicators of potential changes to the hydraulic conductivity (Jo et al., 2001; Lee et al., 2005; Mishra et al., 2011).

Thus, the purpose of this study is to clarify the degradation in Atterberg limits, free swelling and fluid loss properties of bentonites when subjected to different concentrations of sulphuric acid solutions to evaluate the potential deterioration in hydraulic performance of geosynthetic clay liners in light of studies by Kolstad et al. (2004a) and Singh and Prasad (2007).

Section snippets

Materials

Three bentonites were used in this study. Bentonite 1 (B1) was a powdered sodium-magnesium bentonite from Australia having low swell index (SI), generally ∼11 mL/2 g, and which had received no beneficiation other than drying and grinding at the plant. Bentonite 2 (B2) was an activated powdered sodium bentonite (undisclosed beneficiation besides drying and grinding) from Australia with a SI of ∼23 mL/2 g. Bentonite 3 (B3) was a powdered natural sodium bentonite from South Africa with a SI

Mineralogy analysis

The X-ray diffraction (XRD) results are given in Fig. 1 for the three bentonites. The XRD results illustrate that the predominant mineralogy of the three bentonites is smectite, while the position of the d-(060) reflection at ∼62°/2θ (d ≈ 1.5 Å) indicates it is dioctahedral. For untreated B1, the intense peak of d-(001) registers at 5.82°, which corresponds to an interlamellar distance of 15.2 Å, while the main corresponding reflection in B2 and B3 registers at interlamellar distances of 15.4 Å

Conclusions

Results of laboratory tests show changes of Atterberg limits, free swell values, and fluid loss of three different bentonites with various concentrations of sulphuric acid solution. Liquid limit values generally decreased with increasing acid concentration, while only slight changes occurred in plastic liquid values, the resulting plasticity index mirrored the changes in the liquid limit. B1 had the lowest liquid limit values but was relatively stable to the influence of low pH solutions. The

Acknowledgment

The technical support provided by the Department of Civil Engineering, Monash University is gratefully acknowledged. The first author is grateful for the funding provided by the China Scholarship Council to support his PhD studies. We also thank the bentonite producers and the GCL manufacturers for providing bentonites and GCL samples for this project. The anonymous reviewers made many constructive comments and valuable suggestions and their efforts associated are greatly appreciated by the

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