Technical Note
An investigation into the use of blends of two bentonites for geosynthetic clay liners

https://doi.org/10.1016/j.geotexmem.2008.01.001Get rights and content

Abstract

Enhancement of geotechnical parameters of Fe-rich bentonite of lower smectite content (L: Lieskovec, Slovakia; 28–56% montmorillonite) by addition of an Al-rich bentonite of higher smectite content (JP: Jelšový potok, Slovakia; 64% montmorillonite) is discussed. The blends of different L and JP contents (L:JP 85:15, 75:25, and 65:35 mass%) were investigated. Addition of more expensive Ca2+-JP to Ca2+-L bentonite caused increase in the geotechnical properties (liquid limit wL, plasticity limit wP, water adsorption by Enslin test ES) compared to raw Ca2+-L samples, but the parameters were still insufficient for sealing purposes of Ca2+-blends. After natrification of the blends with soda ash, the geotechnical parameters markedly improved. Permeability coefficients of all the blends were satisfactorily low, of the order of 10−11 m s−1. The liquid limit and water absorption values of the blend containing 65 mass% of Na+-L and 35 mass% of Na+-JP bentonites meets the requirements on geotechnical parameters of bentonites used in GCLs. Smectite content in the blends is the dominant factor affecting their properties.

Introduction

In recent years, the community increasingly focuses on broader uses of raw materials, occurring in relatively sufficient amounts. Among them, bentonites play an important role in the environmental applications. Results presented herein are related to the yet comprehensively unexplored Fe-rich bentonite from Lieskovec deposit (Slovakia).

One of the main problems in the environmental field is the intrusion of toxic contaminants from waste disposal and other sources into the subsoil and underlying ground water supply. Bentonite is commonly used for its low permeability characteristics for (a) a barrier in landfills (Browning, 1998; Bouazza, 2002; Janotka et al., 2002; Malusis et al., 2003; Montes-H et al., 2005; Touze-Foltz et al., 2006), where the adsorption ability, the ion exchange capacity and the swelling behavior of bentonite is important; (b) cut-off walls, where bentonite controls rheological behavior of the slurry and is responsible for the low permeability of the hardened bentonite–cement wall (Loxham and Westrate, 1995; Garvin and Hayles, 1999; Koch, 2002) and (c) nuclear fuel waste disposal sites where bentonites act as buffers to control the spread of radioactive materials into the ground and to protect the integrity of the canister against small rock movements on fractures (Güven, 1990; Cheung, 1994; Lingnau et al., 1996; Choi et al., 2001; McKinley et al., 2006).

Bentonite is being used in combination with geosynthetics to form a composite commonly known as a geosynthetic clay liner (GCL), which has been in use in the landfill construction since 1988 (Koerner, 1999). GCLs are used as a hydraulic barrier and/or contaminant layer for leachate, either in place of a composite layer or in addition to other layers in bottom landfill lining system (e.g., Bouazza, 2002; Southen and Rowe, 2005; Barroso et al., 2006; Bouazza et al., 2007). Bentonite used in GCLs is mainly a sodium bentonite, where sodium ions are located in the interlayer space, between clay platelets and on the external surfaces. A fully hydrated sodium bentonite layer has a hydraulic conductivity of approximately 100 times lower than a typical compacted clay liner (CCL). A single GCL of less than 10 mm provides superior hydraulic performance than 1 m of typical compacted clay. The increase in hydraulic conductivity of bentonite is caused either by aggressive leachates containing high amounts of divalent or higher valence cations, especially in landfills subjected to high percolation or in covers also, where the impact of cation exchange can be taken into the consideration. Na+-bentonite in contact with landfill leachate will exchange its sodium for other cations (such Ca2+, Mg2+, K+, heavy metals, etc.), which are present in the leachate in abundance. This exchange has a detrimental influence on the GCL hydraulic conductivity (it increases). Laboratory measurements by Ruhl and Daniel (1997) suggest that the influence can be minimized if the first liquid to permeate the GCL is water; these observations need to be investigated further in order to better understand the underlying mechanisms and establish whether this behavior lasts over time (Guyonnet et al., 2001). Thus, the hydraulic conductivity of bentonites is particularly sensitive to changes in the composition of the pore fluid that influences the thickness of the adsorbed layer. High concentrations of monovalent cations (e.g., 0.6 M NaCl) as well as low concentrations of divalent cations (e.g., 0.0125 M CaCl2) can cause a significant increase in hydraulic conductivity provided the test is performed sufficiently long to allow for exchange of adsorbed cations (Shackelford et al., 2000).

Katsumi et al. (2008) concluded that bentonites exhibit remarkable swelling and chemical resistances, respectively, to electrolytic solutions of NaCl and CaCl2 with a molar concentration of ⩽1.0 M.

The Na+- to Ca2+-smectite conversion is the most significant chemical transformation of smectite considered to occur during life span of a GCL. Artificially natrificated smectite could be reversibly transformed to origin Ca2+-form with storage time. This cation exchange in MX80 (natural Na+-bentonite, Wyoming) treated in 1 M solution of calcium chloride has been also confirmed. A simplified method based on volume balance has shown that swelling capacity of this bentonite would be affected after 1000 years of diffusion reaction (Montes-H et al., 2005). Time required establishing completion of cation exchange between the permeant liquid and thin layers of bentonite simulating GCLs is affected by (1) the rate at which adsorbing cation is delivered to the pore space (affected by seepage velocity and influent concentration); (2) the rate of mass transfer between the mobile and immobile liquid phases (controlled primarily by grain size of the bentonite) and (3) the number of sites available for sorption (controlled by CEC and the dry density of the bentonite) (Young Jo et al., 2006). It is shown that for Atterberg limit tests, even using 0.125 M calcium chloride solution full calcium exchange could be not achieved as the amount of liquid added to bring the bentonite to the liquid or plastic limit did not contain sufficient calcium ion for full exchange (Bouazza et al., 2007).

Substantial number of papers oriented to the effects of inorganic salts on the behavior of bentonite were published. However, the test with natrified Lieskovec/Jelšový potok blend have been just commenced and the investigation of Na+ to Ca2+ conversion in terms of estimating the degree of sodium–calcium exchange has been not carried out until now.

The purpose of the present study was to evaluate the geochemical and geotechnical parameters of Ca2+-bentonite vs. Na+-bentonite from the Lieskovec deposit and of the blends of Fe-rich Lieskovec and Al-rich Jelšový potok bentonites in various ratios to examine the possibility for application of Lieskovec bentonite in GCLs.

Section snippets

Materials

Raw Ca2+-bentonite samples (Ca2+-L4, Ca2+-L5, Ca2+-L11, Ca2+-L15) from Lieskovec (L) deposit (Central Slovakia) were chosen according to various montmorillonite contents (Andrejkovičová, 2006, Andrejkovičová, 2008). The samples were chosen by the previous geological survey. Individual sampling sites are illustrated in Fig. 1.

Ca2+-L4 and Ca2+-L5 (samples with the lowest Fe-rich montmorillonite content of 29% and 31%, respectively) belong to the 30 samples obtained from the Lieskovec deposit,

Results and discussion

Recent information on geotechnical properties of selected Lieskovec bentonite samples (Ca2+-L4, Ca2+-L5, Ca2+-L11, Ca2+-L15) is provided by Andrejkovičová et al. (2008). The permeability coefficients of the order of 10−11 m s−1 suggest possible suitability of this Ca2+-bentonite for GCLs; however, other geotechnical properties like liquid limit and water adsorption by Enslin test provide insufficient values when compared to industrially made GCL Tatrabent (Janotka et al., 2002). On that account,

Conclusions

FTIR reports that AlAlOH bending vibrations of smectite are typical for Ca2+-JP. Therefore, Ca2+-JP bentonite is Al-rich. The presence of Fe(III) is confirmed by AlFeOH bending vibrations in Ca2+-L bentonites indicating Fe-rich smectite. Differences in the content of iron in the structure of smectite are confirmed by chemical analysis; Fe2O3 in Ca2+-JP is 2.3% opposite to 6.1–7.1% occurring in Ca2+-L samples.

Soda ash treatment of the blends of an Al-rich (JP) bentonite of higher smectite

Acknowledgments

The authors are grateful to Mariana Rebelo (University of Aveiro, Portugal) for helpful discussions and technical assistance. Financial support from the Slovak Grant Agencies (projects APVT-51-018502, VEGA 2/6108/27 and VEGA 2/6177/26) is appreciated.

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