Investigating factors influencing polymer elution and the mechanism controlling the chemical compatibility of GCLs containing linear polymers

A study was conducted to investigate (1) physicochemical factors that influence polymer elution from GCLs containing a blend of bentonite and linear (water-soluble) polymer (LPB GCLs) and (2) the mechanism that controls the chemical compatibility of LPB GCLs when polymer elutes. A series of hydraulic conductivity (k), free swell and viscosity tests were performed on a commercial LPB GCL using DI water, varying concentrations of NaCl and CaCl?. Comparable tests were also performed on a conventional bentonite (CB) GCL containing the same untreated bentonite and the same physical properties as the LPB GCL. The LPB GCL showed improved swelling and hydraulic performance compared to the CB GCL when permeated with salt solutions. Total organic carbon analysis of the effluents showed that polymer eluted from the LPB GCL regardless of the permeant solution. However, the rate at which polymer eluted increased as the concentration and valence of the dominant cation increased. The rate at which polymer eluted also increased with hydraulic gradient. The mass of polymer retained inside the GCL matrix did not correlate with the k of the LPB GCL. Free swell tests coupled with chemical analysis suggest that, the improved chemical compatibility of the LPB GCL was due to the ability of the polymer to scavenge cations from the solution which allows the bentonite to undergo adequate swelling during the initial hydration period. Analogous to water-prehydrated CB GCLs, the dispersed structure of the bentonite fabric and increased adsorbed water molecules attained during initial swelling controls the k of the LPB GCL when polymer elutes.     

Hydraulic conductivity of geosynthetic clay liners to tailings impoundment solutions

The results of a comprehensive testing program conducted to evaluate the hydraulic conductivity (k) of two geosynthetic clay liners (GCLs) considered as a liner component for a tailings impoundment at a proposed zinc and copper mine are reported. The two GCLs were permeated with a relatively low ionic-strength ground water (GW) from the mine site and two electrolyte solutions, a process water (PW) and a simulated leachate (SL), with chemical compositions consistent with those expected during operation of the impoundment. A total of 22 flexible-wall tests were performed to determine the effects of prehydration with the GW, type of GCL, type of permeant liquid, and duration of the back-pressure stage of the test. The k values for both GCLs permeated with the GW were 1.7 × 10⁻⁹ cm/s, which is within the range 1–3 × 10⁻⁹ cm/s typically reported for GCLs permeated with low ionic-strength liquids, such as deionized water. However, the mean values of k based on permeation of duplicate specimens of both types of GCL with either PW or SL relative to the values of k based on permeation with GW, or k/k_w, ranged from a factor of 200 (2.3 orders of magnitude) to a factor of 7600 (3.9 orders of magnitude). Thus, both tailings impoundment solutions had significant adverse impacts on the hydraulic performance of both GCLs. Given the overall range of k/k_w values, factors such as prehydration, type of GCL, type of permeant liquid, and duration of back pressure, were relatively insignificant. The results of this study serve to emphasize the need to perform hydraulic conductivity testing using site specific materials.     

GCL收缩与搭接问题浅析

详细分析了导致GCL收缩的6个方面原因和基本机理,提出了合理的预防方法和措施。在此基础上,确定GCL搭接宽度选择的基本依据和步骤,为GCL搭接宽度的合理选择提供了有益的参考。     

Hysteresis of the water retention curves of geosynthetic clay liners in the high suction range

This paper examines two needle-punched geosynthetic clay liners' water retention behaviour at high suction ranges using the vapour equilibrium technique where super-saturated salt solutions controlled the relative humidity. This study shows that the bentonite form and its mineralogy affect the absorption/desorption of GCLs and their corresponding water retention curves. In particular, a granular bentonite-based GCL was found to absorb more and release less water than a powdered bentonite-based GCL due to its higher montmorillonite content and larger pores. The water retention curves of both GCLs exhibited very little hysteretic behaviour at high suction. Repeated wetting-drying cycles shifted the WRCs of both GCLs slightly downward with minimal impact on their degree of hysteresis.     

Experimental study on the permeability and self-healing capacity of geosynthetic clay liners in heavy metal solutions

Geosynthetic clay liners (GCLs), which have a very low permeability to water and a considerably high self-healing capacity, are widely used in liner systems of landfills. In this study, a series of experimental tests were carried out under complex conditions on typical commercial GCLs from China. In particular, the effects of pH values and lead ions (Pb²⁺) were tested in addition to other factors. The swelling properties of natural bentonite encapsulated between geotextile components in the GCLs were tested first. The swelling capacity was reduced rapidly at pH values < 3 and concentrations of Pb²⁺ >40 mM. Permeability tests on GCLs with different concentrations of lead ions were then performed by using the self-developed multi-link flexible wall permeameter, and data showed that increases in lead ion concentrations greatly improved the permeability. Finally, self-healing capacity tests were conducted on needle-punched GCLs under different levels of damage. Results showed that the GCLs have a good self-healing capacity with small diameter damage holes (2 mm, close to three times the original aperture), but with a damage aperture larger than 15% of the sample area, the self-healing capacity could not prevent leakage; hence, in certain situations it will be necessary to repair the damage to meet the anti-seepage requirement.     

Effect of ammonium on the hydraulic conductivity of geosynthetic clay liners

Hydraulic conductivity and swell index tests were conducted on a conventional geosynthetic clay liner (GCL) containing sodium-bentonite (Na-B) using 5, 50, 100, 500, and 1000 mM ammonium acetate (NH₄OAc) solutions to investigate how NH₄⁺ accumulation in leachates in bioreactor and recirculation landfills may affect GCLs. Control tests were conducted with deionized (DI) water. Swell index of the Na-B was 27.7 mL/2 g in 5 mM NH₄⁺ solution and decreased to 5.0 mL/2 g in 1000 mM NH₄⁺ solution, whereas the swell index of Na-B in DI water was 28.0 mL/2 g. Hydraulic conductivity of the Na-B GCL to 5, 50, and 100 mM NH₄⁺ was low, ranging from 1.6–5.9 × 10^?11 m/s, which is comparable to the hydraulic conductivity to DI water (2.1 × 10^?11 m/s). Hydraulic conductivities of the Na-B GCL permeated with 500 and 1000 mM NH₄⁺ solutions were much higher (e.g., 1.6–5.2 × 10^?6 m/s) due to suppression of osmotic swelling. NH₄⁺ replaced native Na⁺, K⁺, Ca²⁺, and Mg²⁺ in the exchange complex of the Na-B during permeation with all NH₄⁺ solutions, with the NH₄⁺ fraction in the exchange complex increasing from 0.24 to 0.83 as the NH₄⁺ concentration increased from 5 to 1000 mM. A Na-B GCL specimen permeated with 1000 mM NH₄⁺ solution to chemical equilibrium was subsequently permeated with DI water. Permeation with the NH₄⁺ converted the Na-B to “NH₄-bentonite” with more than 80% of the exchange complex occupied by NH₄⁺. Hydraulic conductivity of this GCL specimen decreased from 5.9 × 10^?6 m/s to 2.9 × 10^?11 m/s during permeation with DI water, indicating that “NH₄-bentonite” can swell and have low hydraulic conductivity, and that the impact of more concentrated NH₄⁺ solutions on swelling and hydraulic conductivity is reversible.     

The impact of mining solutions/liquors on geosynthetics

Mine owners and operators are presented today with a diverse range of geosynthetic products which all appear to provide similar benefits. Key factors in selecting geosynthetics for use in the mining industry include construction and operational durability issues such as slope stability, puncture resistance and resistance to weathering; but also their chemical resistance when they come into contact with the extreme liquors present on many mining operations and processes. The long-term performance of the geosynthetic depends largely on the type of polymer used in the manufacture, or in the case of geosynthetic clay liners (GCLs), also on the mineralogy and chemical make of the bentonite present in the GCL. This paper provides a guide to the characteristics of the leachates/liquors likely to be generated for a given mining process and the likely effect it will have on the performance of a given geosynthetic.     

Effect of water salinity on the water retention curve of geosynthetic clay liners

The effect of water salinity on the water retention curve of geosynthetic clay liners (GCLs), under constant volume condition is examined. The results indicate that at a constant gravimetric moisture content the total suction increases as the salinity of the wetting liquid increases. Furthermore, the difference in total suction between the GCL hydrated by saline water and distilled water is greater than the difference in the osmotic potential of the wetting water. This behaviour is possibly caused by the matric suction being affected by the expected chemically induced pore size change of the bentonite component of the GCL.     

Hydraulic conductivity of two geosynthetic clay liners permeated with a hyperalkaline solution

GCLs containing powdered Na-bentonite treated with different dosages of a proprietary additive intended to reduce the impacts of chemical interactions were permeated with three solutions: a hyperalkaline solution (1 M NaOH and 1.3 mM CsCl) having similar pH to aluminum refining leachate, a 1.3 mM CsCl solution (no NaOH), and DI water. For a given permeant solution, the hydraulic conductivity of both GCLs was similar. Thus, the higher additive dosage had no measureable impact on hydraulic conductivity. Hydraulic conductivity of both GCLs decreased by a factor of approximately 1.5–1.8 during permeation with CsCl in response to osmotic swelling induced by the low ionic strength of the CsCl solution entering the pore space. In contrast, permeation with the NaOH–CsCl solution caused the hydraulic conductivity of both GCLs to increase modestly (<50 times the hydraulic conductivity to DI water), and then level out (or decrease slightly) as a result of reduced osmotic swelling in the interlayer combined with dissolution of the mineral. For the tests conducted with CsCl solution, nearly all of the Cs was adsorbed by the bentonite. In contrast, Cs broke through readily when the NaOH–CsCl solution was used as the permeant solution. Permeation with the NaOH–CsCl solution also increased the sodicity of the bentonite by replacing bound K, Ca, and Mg on the mineral surface.     

Heat mitigation in geosynthetic composite liners exposed to elevated temperatures

The hydrothermal behaviour of single and double composite liners subjected to elevated temperatures is examined. Particular interest is given to the effect of the presence of wrinkles in the geomembrane (GMB) as well as defects, and the existence of a gap between the primary and the secondary liners caused by the presence of a leak detection system. Heat flow resulting from elevated temperature was found to be mainly influenced by the size of the air-filled gaps present within the composite lining systems. The larger the air-filled gap size, the lower was the heat flow through a barrier system. The presence of a leak detection layer (i.e., large air-filled gap) and GMB layers were found to be the primary factors to reduce heat flow substantially through the lining systems. Therefore, the presence of a leak detection layer combined with a secondary GMB can improve the overall thermal insulation capacity of a double liner system, minimise heat flow through the secondary liner and offer the possibility of protecting the GCL (if present) and the subgrade from possible heat induced drying/desiccation. A leak in the geomembrane can minimise the gain in thermal insulation. However, this effect can be reduced if the liquid is regularly pumped out.     

Hydraulic conductivity of bentonite-polymer composite geosynthetic clay liners permeated with bauxite liquor

The high ionic strength of the porewater in red mud (bauxite liquor from digestion) can suppress swelling of montmorillonite, resulting in geosynthetic clay liners (GCLs) that are too permeable to be effective as liners in red mud disposal facilities. Bentonite-polymer composite GCLs (BPC GCLs) have been developed as more resilient lining materials, and some BPC GCLs have been shown to have very low hydraulic conductivity to bauxite liquors that have extreme ionic strength and pH. In this study, a nationwide investigation was conducted in China to evaluate the characteristics of bauxite liquor in Chinese impoundments, and to evaluate the suitability of GCLs containing granular sodium bentonite or BPCs for containment. Hydraulic conductivity tests were conducted on six BPC GCLs with two characteristic Chinese bauxite liquors that are hyperalkaline (pH > 12) and had ionic strengths of 76.9 mM and 620.3 mM. The BPC GCLs had hydraulic conductivity ranging from 10^?8-10^?12 m/s, which is higher than the hydraulic conductivity of BPC GCLs to deionized water (10^?12-10^?13 m/s), but lower than the hydraulic conductivity of conventional GCLs with granular sodium bentonite GCLs to the same liquors (10^?7-10^?8 m/s). The hydraulic conductivity of the BPC GCLs depends on the chemical properties of the leachate, the polymer loading, and the type of polymer. Microstructural analysis by scanning electron microscopy (SEM) suggests that the hydraulic conductivity of BPC GCLs is controlled by pore-blocking by polymer hydrogel, which is affected by the bauxite liquor.