Hydraulic performance of geosynthetic clay liners to sulfuric acid solutions

https://doi.org/10.1016/j.geotexmem.2014.11.004Get rights and content

Abstract

The ability of geosynthetic clay liners (GCLs) to contain acidic mining leachates is examined. The results of saturated hydraulic conductivity (k) of two GCLs permeated with sulfuric acid solutions (H2SO4) at 0.015 M, 0.125 M and 0.5 M concentrations are reported. Also, the saturated k values of consolidated (35 kPa) bentonite cakes made from sodium bentonite extracted from both GCLs were compared to a commonly used magnesium–sodium form bentonite. Chemical compatibility and effects of pre-hydration and effective stress were assessed as part of this study. Results indicated that an increased acid concentration (ionic strength) increased the k of all tested specimens. The ratio of the k0.5 values for non-prehydrated specimens permeated with 0.5 M H2SO4 to the kw values for specimens permeated with deionized (DI) water (k0.5/kw) ranged from 10 to 110. Pre-hydration (50–140% water content) and increased effective stress (35–200 kPa) improved the performance of GCLs (lower k). Strong correlations were observed between k and liquid limit and swell index parameters independent of pre-hydration and effective stress in this study. However, care should still be taken when using these correlations to evaluate hydraulic performance because the intrinsic micro-structure properties of bentonite, such as porosity, should also be considered. This work showed that, for example, high SI of bentonite does not translate necessarily to a better hydraulic performance of GCLs.

Introduction

Geosynthetic clay liners (GCLs) are low permeable materials increasingly considered for deployment as hydraulic barriers in mining facilities (e.g., mill facility liners, brine evaporation ponds, heap leach pads, waste rock dumps, etc.) and other industrial facilities (Hornsey et al., 2010). As a result, there is a high probability that they may come into contact with aggressive leachates (e.g. leachates having non-neutral pH or excessive salinity) (Shackelford et al., 2000, Shackelford et al., 2010, Gates et al., 2009, Benson et al., 2010, Gates and Bouazza, 2010, Mazzieri et al., 2013, Liu et al., 2013, Liu et al., 2014, Bouazza and Gates, 2014).

Many investigations have focused on the hydraulic conductivity, k, of GCLs with non-standard liquids, consisting of either saline solutions (Petrov et al., 1997, Petrov and Rowe, 1997, Shackelford et al., 2000, Simpson, 2000, Jo et al., 2001, Jo et al., 2005, Katsumi et al., 2001, Kolstad et al., 2004a) or non-neutral pH solutions (Kashir and Yanful, 2001, Jozefaciuk and Matyka-Sarzynska, 2006, Benson et al., 2008, Benson et al., 2010, Benson and Ören, 2009, Gates and Bouazza, 2010, Shackelford et al., 2010). These studies have shown that these types of permeant solutions tend to cause increased k in GCLs relative to water. Bouazza and Gates (2014) reported that the elevated ionic strength of saline liquids may influence sealing performance by opening the pore structure of the bentonite component, thereby increasing the proportion of hydraulically active pores. In addition, hydration water that is normally strongly bound to the smectite mineral surface may be removed by the high osmotic potential of non-standard leachates, resulting potentially in the collapse of smectite double layers and decreased bentonite swelling, and eventually leading to an increased k (Liu et al., 2013, Liu et al., 2014). Possible cation exchange reactions between the leachates and the bentonite can also directly affect both bulk (Kelessidis et al., 2007) and crystalline swelling (Laird, 1999). Chemical desiccation and cation exchange can thus result in a deterioration of the performance of GCLs as hydraulic barriers.

Exposure to acidic solutions can cause a decrease in bentonite swelling and therefore an increase in the hydraulically active void ratio of the bentonite. Such changes are expected to produce a negative effect on the hydraulic performance of GCLs, as has been shown by Petrov et al., 1997, Shackelford et al., 2000, Shackelford et al., 2010, Kolstad et al., 2004b, Fall et al., 2009, and Liu et al. (2013). However, few studies have focused on extreme acidic conditions (i.e. pH < 3). Shackelford et al. (2010) reported an increase in hydraulic conductivity, k, from 1.7 × 10−11 m/s (with water) to 2.7–3.9 × 10−8 m/s when a GCL was permeated with an acidic solution of pH = 2.5. Similar results were observed by Kolstad et al. (2004b), where measured k increased from 1.2 × 10−11 m/s (with water) to 1.5 × 10−7 m/s (pH = 1.2). Liu et al. (2013) reported decreases in geotechnical properties (liquid limit, LL, and swell index, SI) of bentonite when exposed to acidic solutions with extreme pH varying from −0.3 to 1.5.

The objective of this research was to examine the hydraulic performance of two GCLs and three bentonite cakes to acidic solutions having an extremely low pH. These experiments excluded the influence of dissolved salts other than possibly from partial dissolution of bentonite and focused solely on the influence of pH alone. The assessment of the hydraulic performance was based on the measurement of k of the same specimens when exposed to various concentrations of H2SO4. The influence of experimental factors such as effective stress, pre-hydration on the k of the GCLs, and bentonite cakes were investigated.

Section snippets

Materials

Two commercially available needle-punched and thermally treated GCLs, referred to herein as GCL1 and GCL2, and three bentonite cakes, referred to as B1, B2 and B3, were examined. Their properties are provided in Table 1, Table 2, the Mg/Na bentonite (B3) had more smectite (99%) than the Na-bentonites in GCL1 (78%) and GCL2 (84%), B1 and B2 respectively. GCL1 contained a minimum of 5.26 kg/m2activated (undisclosed beneficiation) powdered sodium bentonite (B1, origin Ipswich, Queensland,

Hydraulic conductivity to water

The measured data from k tests with DI water are shown in Table 5. The k values of GCL1 (k = 1.1 × 10−11 m/s) and GCL2 (k = 1.8 × 10−11 m/s) at 35 kPa are within the range reported in literature (Bouazza, 2002; Rowe et al., 2006, Gates et al., 2009). The bentonite cake made from B3 had the highest k values in DI water (2–5 times higher than GCL1 and GCL2) regardless of the effective stresses applied (Table 5). The higher k values determined for B3 compared to those observed for the GCL

Conclusions

The hydraulic performance of GCLs to acidic solutions was evaluated through a series of k tests. Results indicated that an increased acid concentration (ionic strength) resulted in an increase in k for all tested specimens. The hydraulic conductivity of the bentonite cake prepared from B3 had the highest overall k, but was the least affected when exposed to acidic solutions. An increase in k of ∼10× was observed in B3 at 0.5 M which could be attributed to the chemical composition of

Acknowledgments

The technical support provided by the Department of Civil Engineering, Monash University for this project 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 GCL manufacturers for providing bentonites and GCL specimens for this project. The anonymous reviewers made many constructive comments and valuable suggestions. These comments and efforts associated with the

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