Cation interception and permeability characteristics of bentonite barriers exposed to NaCl and NH₄Cl solutions

Currently, urban solid waste management heavily relies on landfilling to dispose of solid wastes that cannot be recycled or incinerated. In China, approximately 79% of urban solid waste is disposed of in landfills (Havukainen et al. 2017).

High volume of leachate will be produced due to complex physical and chemical reactions during landfill operation. Na⁺ and constitute significant inorganic components, often present in high concentrations (Costa et al. 2019; Luo et al. 2020). It was found that in leachate increased over time, and the concentration can be 2,000 mg/L (Negi et al. 2020). Within 1–2 years of operation in a landfill, the concentration of can reach up to 4,000 mg/L (Sun et al. 2021). According to survey statistics, the average concentration of Na⁺ in leachate is higher than that of common inorganic components such as K⁺ and Ca²⁺ (averaging 1,675 and 400 mg/L, respectively), as well as other heavy metals (Cu²⁺, Zn²⁺, Pb²⁺, etc.). The highest concentration approaches 4,000 mg/L (3,710 mg/L) (Naveen et al. 2017). As a result, leachate resulting from MSW landfills is a typical source of excessive and Na⁺, and these should be main cations considered for landfill design. Na⁺ and have little effect on bentonite engineering properties in low concentrations (80 mM) (Setz et al. 2017), but high value will affect the performance of bentonite barriers such as its morphological structure, swelling index (SI), hydraulic conductivity, and pollution interception (Anh et al. 2017; Setz et al. 2017). It may even affect the acidity and alkalinity of the environment (Lai et al. 2023).

Geosynthetic clay liners (GCLs) are often used in landfills to prevent the migration of potential pollution due to landfill leachate (Özçoban et al. 2022). However, both Na⁺ and are monovalent cations. Generally, in comparison to divalent cations, monovalent cations possess larger diffusion coefficients (Tadimeti & Chattopadhyay 2016; Huang et al. 2017; Aryal & Ganesan 2018; Dong et al. 2020). This facilitates their movement within the landfill liner. Further investigation is warranted to elucidate the variance in pollution interception capacity of bentonite for Na⁺ and present in landfill leachate. Additionally, the impact of Na⁺ and on the permeability properties of bentonite-based engineering barriers remains a subject that requires deeper scrutiny. Clarifying the distinctions in their respective influences is imperative. This study contributes to a better understanding of the behavior of leachate pollutants within landfill barriers, particularly focusing on the Na⁺ and retention in bentonite and the permeability alterations of bentonite.

To this end, mixture soil samples are prepared using bentonite and clay in this study, and the contents of bentonite are set at 0, 5, 10, 15, 20%, respectively. NaCl and NH₄Cl solutions with four different concentrations were used to permeate bentonite–clay mixture samples under a constant pressure. The leachate concentration of Na⁺ and was obtained. Fourier transform infrared (FTIR) analysis was performed to understand the interaction between and bentonite. The effects of bentonite content and initial concentration on residual leachate concentration of Na⁺ and were investigated. The hydraulic conductivity coefficients of bentonite are obtained, and the SI of bentonite was tested. The relationship between swell potential and hydraulic conductivity coefficients of bentonite exposed to NaCl and NH₄Cl solution with different concentrations were analyzed.

Geosynthetic clay liner (GCL) bentonite (abbr. bentonite) and natural clay obtained from Shanghai (abbr. clay) were used in this study. The main clay mineral of the bentonite is montmorillonite that accounts for 45.8% of clay mineral according to X-ray diffraction analyses. The basic physical properties of the bentonite and clay were tested as per ASTM D4318-10, as shown in Table 1. The bentonite is classified as a fat clay (CH).

NaCl and NH₄Cl solutions were prepared using analytically pure (AR99.5%) NaCl and NH₄Cl powders (analytical reagent, AR) obtained from Sinopharm Chemical Reagent Co., Ltd. Based on previous tests (Lou et al. 2007; Sun et al. 2021), the concentration of NH₄Cl solution is set at 9.4, 18.7, 37.4, and 74.8 mmol/L (It is equivalent to 500, 1,000, 2,000, and 4,000 mg/L respectively), corresponding to the residual concentration after 2, 4, 6, and 8 years of operation in the landfill, respectively. It is reported that the concentrations of Na⁺ were comparable to that of in landfill leachate (Gupta & Paulraj 2017). Thus, the concentrations of NaCl were controlled to be equal to the concentrations of NH₄Cl.

The bentonite and clay were dried at 105 °C for 24 h. Subsequently, the dried bentonite was thoroughly mixed with dry clay powder to prepare a homogeneous dry mixture. The dry mixture comprised varying proportions of bentonite and clay, specifically 0, 5, 10, and 15%. Deionized water (DI water) was added to the dry mixture to achieve a mass ratio of water to dry mixture of 1:5. The mixture was rigorously mixed to ensure uniformity, and the resulting mixture samples were placed in sealed bags and left to stand for 24 h at 20 °C. Then, they were compacted using a jack to form samples with a diameter of 3.8 cm, a height of 1.5 cm, a specific gravity of 2.68 (±0.01) g/cm³, and a water content of 20% (±0.5%). In the saturated permeability test, the initial moisture content of the sample has modest influence on the test results, and the initial moisture content of 20% is set to make the soil sample reach a suitable plastic state in order prepare samples conveniently. This procedure was repeated to produce a total of 36 samples with varying bentonite contents. The samples used in the outflow measurement test are summarized as shown in Table 2.

When the rate of change in permeation flux Q for three consecutive measurements is less than 1%, it is considered that permeation has reached equilibrium, and the permeability coefficient of the sample is obtained.

After obtaining stable hydraulic conductivity coefficients in the permeation tests described in Section 2.3, the leachate was obtained. The Na⁺ concentration in the leachate was determined through Inductively Coupled Plasma (ICP) testing. The concentration in the leachate was determined through Ion Chromatography (IC) testing.

Bentonite exhibits sensitive swelling behavior in cationic solutions. To conduct targeted research, the SI of bentonite was tested following ASTM D5890 standards. This test aimed to assess how the swelling potential of bentonite changes with varying Na⁺/ concentrations. Each sample was tested in duplicate. The test scheme of SI is presented in Table 3. DI water was also used for comparative purposes.

Following the procedure outlined in ASTM D5890, not more than 0.1 g increments of bentonite were removed and were evenly distributed over the water surface in a graduated cylinder within a span of approximately 30 s. Additional bentonite was added continuously at 10-min intervals, ensuring that each increment swells without entrapping air between them, until the entire bentonite sample has been added. The bentonite sample was allowed to settle for 24 h from the last addition, and the volume level in mL was recorded to the nearest 0.5 mL at the top of the settled clay mineral.

FTIR analysis was applied to identify specific functional groups in the mixture samples containing NaCl and NH₄Cl solutions with concentration of 37.4 mmol/L and DIW. Fourier infrared spectrometer Nicolet6700 was used to record the spectra ranging from 4,000 to 400 cm⁻¹ at a spectral resolution of 4 cm⁻¹. Scan repetition was 32 times. The sample used for FTIR analysis was prepared by mixing sample with KBr with a mass ratio of approximately 1–50. The sample name, bentonite content, type of solutions as well as its concentration are summarized in Table 4.

The result indicates that the concentrations in leachate significantly decrease with increasing bentonite content, indicating an effective containment of in leachate. The reduction in the concentration can be as high as 99% compared with its corresponding initial concentration. For example, as the bentonite content increases from 0 to 15%, the concentrations in leachate decreases from 5.61 to 0.48 mmol/L when mixture samples subjected to NH₄Cl solution with initial concentration of 74.8 mmol/L.

In contrast, the variation in Na⁺ concentration in leachate with bentonite content shows an opposite trend. The Na⁺ concentration in leachate increases with an increase in bentonite content. It should be noted that the Na⁺ concentration in leachate exceeds its initial concentration for mixture samples containing 15% bentonite, permeated with NaCl solutions initially at 9.4 and 18.7 mmol/L, as shown in Figure 2(a) and 2(b). The concentrations increase to 22.58 and 22.6 mmol/L, respectively. In addition, it is found that Na⁺ concentration in leachate is generally higher than concentration for a given bentonite content.

As shown in Figure 5, the band positions at 3,625 and 3,436 cm⁻¹ are due to O–H vibration of cationic binding to octahedron and H–O–H vibration of montmorillonite surface absorbing water, respectively (Kumar & Lingfa 2020). The bands at 1,086 and 1,033 cm⁻¹ are due to Si–O stretching vibration. The bands at 695–471 cm⁻¹ are assigned to Si–O–Al bending vibration.

The band at 1,434 cm⁻¹ is due to entrainment in the mixture sample (Petit et al. 2006). The band offsets to 1,428 cm⁻¹ after exposed to NH₄Cl solution is because the bending vibration at 1,427 cm⁻¹ (Gautier et al. 2010), indicating that is fixed on montmorillonite.

In the residual leachate concentration tests, the concentration of was significantly lower than that of Na⁺ (as discussed in Figure 2). The decrease in concentration of leachate is attributed to the cation exchange between the pollutants and bentonite. The method of ion exchange has been used to remove metals from aqueous solutions (Hussain & Ali 2021). The interlayer cations in the montmorillonite mainly consist Na⁺, K⁺, Ca²⁺ and Mg²⁺ (Sun et al. 2013). It is reported that the cation replaceability order of bentonite can be as follows: > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺ (Ye et al. 2017; Xiang et al. 2020). This indicates that in leachate can be fixed more easily due to exchange cation than Na⁺.

The increase in hydraulic conductivity coefficient is due to the reduction in the double-layer thickness of montmorillonite particles caused by Na⁺ and ⁠. Positively charged cations appear between the montmorillonite layers, weakening the repulsion between the layers and the negative charges. This causes the microstructure to transition from a dispersed state to an aggregated state, leading to layer compression (Akinwunmi et al. 2020). It leads to an increase in the gaps between particles, reducing water flow resistance, and consequently increasing the hydraulic conductivity. The experimental results of the SI in Section 3.3 also support this view.

In this study, the SI of the mixture sample exposed to both NaCl and NH₄Cl solutions should theoretically be similar for a given concentration, as both Na⁺ and are monovalent cations. However, noticeable differences were observed, as shown in Figure 4. The lower SI of the mixture sample exposed to NH₄Cl solution is attributed to the lower hydration enthalpy of compared to that of Na⁺ (Peng et al. 2020; Morida et al. 2023). Hydration enthalpy serves as a measure of the interaction strength between a cation and water. Bentonite typically exhibits smaller swelling deformation in solutions with cations that have lower hydration enthalpy (Xiang et al. 2020). This results in differences in the swelling of bentonite in NaCl and NH₄Cl solutions. This indicates that the bentonite barrier may experience increased leachate percolation in NH₄Cl solution, potentially posing a risk of leakage for contaminants not adsorbed by the bentonite.

Permeation tests, concentration analysis, SI tests, and FTIR spectroscopy analysis were conducted to understand the pollutant interception and permeation characteristics of bentonite–natural clay mixtures in NaCl and NH₄Cl solutions. It can be concluded from the following points.

• (1) The concentrations in leachate significantly decrease with increasing bentonite content. The reduction in the concentration can be as high as 99% compared with initial concentration, indicating an effective containment of in leachate. In the residual leachate concentration, the Na⁺ concentration is higher than the concentration, with the difference reaching up to 1,000 times at certain points. The results indicate that the bentonite–clay mixture intercepts more effectively than Na⁺.

• (2) The hydraulic conductivity coefficient of mixed soil samples increases with the concentration of NaCl and NH₄Cl solutions. The mixed soil samples demonstrate different hydraulic conductivity characteristics in NaCl and NH₄Cl solutions. Compared to NaCl solution, the samples have a lower SI and a higher hydraulic conductivity coefficient in NH4Cl solution. When a solution with a concentration of 74.8 mmol/L was used to permeate a mixed sample with 5% bentonite content, the difference in permeability coefficient reached 1.5 times.

• (3) is more readily intercepted in the interlayers of montmorillonite through cation exchange than Na⁺. Additionally, the smaller hydration radius of and the formation of hydrogen bonds contribute to the lower concentration of compared to Na⁺ in the leachate. This indicates that the bentonite barrier exhibits better interception of ⁠.

• (4) The variation in hydraulic conductivity coefficient can be explained by the double-layer theory. Positively charged cations weaken the repulsion between the negative charges in the layers, causing layer compression. This leads to an increase in the gaps between particles, consequently increasing the hydraulic conductivity of the samples. has a lower hydration enthalpy than Na⁺, and bentonite typically exhibits smaller swelling deformation in solutions with cations of lower hydration enthalpy. This results in differences in the hydraulic conductivity coefficient of the samples in NaCl and NH₄Cl solutions.

We acknowledge the constructive feedback and suggestions provided by our colleagues and professionals, which helped improve the quality of this manuscript.

This research was financially supported by the National Natural Science Foundation of China (Grant No. 42372308, 51908121), the Natural Science Foundation of Shanghai Province (No. 22ZR1401800), and the Fundamental Research Funds for the Central Universities (No. 2232024A-06), the Open Funds of Hubei Key Laboratory of Disaster Prevention and Mitigation (No. 2022KJZ01) and the Open Funds of Engineering Research Center of Eco environment in Three Gorges Reservoir Region, Ministry of Education (No. KF2023–06).

Free and informed consent of the participants or their legal representatives was obtained and the study protocol was approved by the Donghua University.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.