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Barrier performance of compacted clay liner in landfills containing fluoride: Diffusion, permeation and sorption.
| Waste Management | Wen JM, Kato T, Takai A, Katsumi T. | 28:217:115480.
Asian landfills Landfill-clay-liners Landfill-leachate Other
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Abstract

Original full-text study online at
https://www.sciencedirect.com/science/article/pii/S0956053X26001509?via%3Dihub#ab005

Highlights

  • Fluoride retention was assessed via diffusion, permeability, and sorption tests.
  • Bentonite addition increased F– sorption and reduced diffusion and permeability.
  • Around 5% bentonite showed optimal fluoride containment under low-F– leachate.

Excessive fluoride in landfill leachate has been increasingly reported across Asia due to both geogenic and anthropogenic inputs, posing long-term risks to groundwater and public health. However, the barrier performance of compacted clay liners (CCLs) against fluoride migration remains insufficiently quantified, especially under bentonite-amended conditions. This study systematically evaluated the fluoride barrier performance of compacted clay amended with 0–10% bentonite through a series of laboratory tests including Atterberg limits, standard proctor compaction, specific gravity, free swelling indices, cation exchange capacity (CEC), batch sorption, column diffusion, and hydraulic conductivity tests, complemented by analytical prediction. Results indicated that the clay specimen with 10% bentonite exhibited 64.4% liquid limit, 5.2 mL/2g free swelling index, and CEC value of 36.5 meq/100g, respectively. After adding 0–10% bentonite, the distribution coefficient of the clay specimen ranged from 2.1 to 3.4 L/kg, moreover, the hydraulic conductivity decreased from 1.4 × 10-10 to 4.9 × 10-11 m/s, and the effective diffusion coefficient decreased from 1.6 × 10-10 to 7.6 × 10-11 m2/s, respectively. Analytical modeling predicts a maximum fluoride breakthrough time of 150 years for a compacted clay liner with 10% bentonite. Mechanism interpretation indicates that increased swelling, CEC, and tortuosity jointly contribute to the improved barrier performance. Findings from this study suggest that adding approximately 5% bentonite provides a cost-effective improvement for CCLs in landfills containing low-fluoride leachate.

    Keywords: Compacted clay; Bentonite; Sorption behavior; Diffusion performance; Permeability; Analytical prediction

    EXCERPTS:

    1. Introduction

    Fluoride has emerged as a contaminant of increasing environmental concern due to its high chemical activity and potential bio-toxicity including skeletal and non-skeletal illness (Dun et al., 2026). Excessive fluoride exposure has been linked to dental and skeletal fluorosis, renal and hepatic impairment, and other systemic effects in humans and animals (Ottappilakkil et al., 2025). Geogenic processes such as volcanic activity and weathering of fluoride-bearing minerals, together with anthropogenic input activities from phosphate fertilizers, aluminum smelting, and various industrial sources, contribute to its widespread occurrence in soil and groundwater worldwide (Huang et al., 2025a, Xu et al., 2025). Most natural soils globally exhibit total fluoride concentrations in the range of approximately 20 to 500 mg/kg (Ahmad et al., 2022), reflecting geological variation (e.g., mineralogy and parent material) and soil formation processes.

    In Asian contexts, elevated fluoride concentrations have been frequently reported in landfill leachate and adjacent groundwater, posing potential risks for surrounding environments and public health, as summarized in Table S1. For example, landfill leachate and shallow groundwater near municipal and industrial sites in India (Karak et al., 2013, Choubisa et al., 2023), China (Guo et al., 2022), and Malaysia (Atta et al., 2015) have sometimes exceeded the World Health Organization’s drinking-water limit of 1.5 mg/L (Zhou et al., 2025), with reported peak fluoride concentrations reaching approximately 34 mg/L. Soil fluoride concentrations in affected regions of China have also been reported in the range of several hundred to more than a thousand mg/kg (e.g., 157.9–1076.8 mg/kg in southwestern China), often exceeding typical background values and ecological safety thresholds for crop production (Guo et al., 2023, Chen et al., 2025). These observations highlight the potential contribution of landfill leachate to elevated fluoride levels in soils and groundwater near disposal facilities, underscoring the need for effective containment strategies in Asian landfill systems.

    Compacted clay liners (CCLs) are widely used as engineered barriers in municipal landfills because of their low permeability, material availability, and compatibility with conventional construction (Jayawardane et al., 2025, Qi et al., 2025). Although soil-bentonite cutoff walls and geosynthetic clay liners provide excellent hydraulic performance (Di Emidio et al., 2015, Zhuang et al., 2025), their application is often limited by higher cost, construction complexity, and restricted availability in many regions (Yang et al., 2018a). In contrast, ordinary CCLs amended with moderate bentonite contents offers a practical compromise by enhancing barrier performance while maintaining economic feasibility and construction flexibility (Dhar et al., 2023, Huang et al., 2025b, Malusis et al., 2021). Increasing bentonite beyond moderate levels yields diminishing hydraulic benefits but disproportionately increases cost and construction difficulty (Bi et al., 2025), highlighting the need to identify an optimal amendment level. However, clay minerals exhibit limited affinity for anions such as fluoride due to electrostatic repulsion, particularly in smectite-rich systems (Lai et al., 2025, Li et al., 2025, Lin et al., 2025, Shah et al., 2026). As a result, fluoride advection, diffusion, and sorption in bentonite-amended CCLs under landfill-relevant conditions remain insufficiently quantified. A mechanistic understanding of fluoride transport is essential for developing cost-effective liner design criteria, especially for Asian landfills where ordinary clay remains dominant and fluoride-bearing leachate is increasingly reported.

    Previous studies have pointed out that the migration of contaminants in CCLs is governed by diffusion, advection, and sorption (Rosanne et al., 2003, Sharma and Reddy, 2004), whereas mechanical dispersion is negligible under the low hydraulic conductivity conditions typically required for engineered barriers (<1 × 10-9 m/s) (U.S. Environmental Protection Agency. , 1993, Rowe et al., 2004). Diffusion becomes dominant when the Darcy velocity falls below approximately 1 × 10-11 m/s (Rowe et al., 2004), making it a critical mechanism for solute transport under realistic landfill conditions. In addition, sorption also plays an important role in retarding reactive contaminants such as fluoride (Kau et al., 1999). Therefore, a holistic understanding of fluoride advection–diffusion-sorption behavior in bentonite-amended clays is essential for predicting long-term migration in landfill liner systems.

    Although numerous studies have investigated fluoride sorption by clayey materials (e.g., Kato et al., 2021, Zhang, 2024, Han et al., 2025, Liu et al., 2025a, Liu et al., 2025b), experimental quantification of fluoride diffusion and hydraulic conductivity in CCLs remains limited. Hamdi and Srasra (2013) reported hydraulic conductivity values of 8.5 × 10-10, 2.1 × 10-9, and 6.8 × 10-10 m/s for palygorskitic, smectitic, and illito-kaolinitic soils, respectively, under non-standard compaction and exposure to industrial wastewater containing extremely high fluoride (2360 mg/L) and phosphate concentrations (1500 mg/L). For diffusion behavior, Kau et al. (1999) observed effective diffusion coefficients of 3.5 × 10-10–7.0 × 10-10 m2/s for kaolin and 4.8 × 10-11–1.4 × 10-10 m2/s for bentonite using 100 mg/L fluoride (F–) solution, in which sodium fluoride was employed as the source. These studies primarily focused on specialized clay minerals or high-fluoride industrial effluents, which differ substantially from municipal landfill environments where fluoride concentrations are typically much lower.

    In contrast, the present study targets landfill-relevant conditions by adopting a fluoride concentration of 33 mg/L, corresponding to the upper range reported for landfill leachate and groundwater in Asia (Choubisa et al., 2023), and employs ordinary compacted clay commonly used in municipal landfills systematically amended with 0–10% bentonite, with the upper bound selected based on reported bentonite contents in Japanese landfill liner systems (International Geosynthetics Society Japan Chapter, 2000). Despite the practical importance of bentonite-amended CCLs, systematic evaluations of fluoride transport behavior across controlled bentonite gradients under landfill-relevant concentrations remain scarce. Although co-existing ions may influence fluoride migration in CCLs (Sahoo et al., 2022, Yin et al., 2015), this study intentionally focuses on single-anion transport to establish baseline behavior of fluoride in bentonite-amended compacted clay, thereby providing a reference framework for future investigations involving more complex leachate chemistries.

    The objective of this study is to quantify fluoride migration in compacted clay amended with varying bentonite contents through integrated evaluation of its diffusion, permeability, and sorption characteristics. Column diffusion tests, hydraulic conductivity tests and batch sorption tests were performed to determine the key transport parameters including permeability (k), effective diffusion coefficient (D*) and sorption distribution coefficient (Kd). These experimental results were incorporated into analytical modelling to predict long-term fluoride breakthrough behavior in CCLs. Finally, the underlying mechanisms governing fluoride migration and its interactions with bentonite-amended clay were examined and discussed.

    4. Discussion

    4.1. Migration of fluoride in compacted clay amended with bentonite

    The enhancement in fluoride barrier performance of compacted clay specimens after bentonite addition is governed by the combined effects of sorption, permeability, and diffusion behaviors. Regarding sorption, fluoride retention is controlled by both physical and chemical sorption mechanisms. Under the experimental pH of approximately 6.72 (See Fig. 3), partial protonation of clay mineral surfaces is expected to occur, which reduces negative surface charge and promotes anion sorption through ligand exchange with surface hydroxyl groups (Kau et al., 1997, Kau et al., 1998, Mitchell and Soga, 2005). The incorporation of bentonite substantially increases the specific surface area, thereby strengthening physical sorption. In addition, bentonite exhibits a markedly higher Al2O3 content (17.8%) and lower Fe2O3 content (1.8%) compared with the base clay (8.2% Al2O3 and 4.5% Fe2O3) (See Table 1). The resulting increase in Al3+-associated hydroxyl sites, together with the relative reduction in Fe oxides, favors fluoride complexation via Al–OH surface groups, which are known to exhibit stronger affinity for fluoride than Fe-based sites under near-neutral pH conditions (Kau et al., 1997). These combined mineralogical and surface-chemical effects account for the observed, albeit moderate, enhancement in fluoride sorption capacity with increasing bentonite content (See Fig. 3, Fig. 4).

    The addition of bentonite exerted a pronounced influence on the hydraulic behavior of the compacted clay specimens. The reduction in hydraulic conductivity with increasing bentonite content (See Fig. 6(f)) can primarily be attributed to pore filling by finer bentonite particles and their subsequent swelling upon hydration (Zeng et al., 2025), which collectively constrict pore spaces and increase flow-path tortuosity. Migration and accumulation of fine bentonite particles can locally block pore throats and form low-permeability layers, leading to further decreases in hydraulic conductivity (Lee et al., 2016, Du et al., 2021). Moreover, the increased cation exchange capacity (Kumar et al., 2026) associated with bentonite addition enhances physicochemical interactions between pore fluid and clay minerals, which may further modify pore structure and promote membrane behavior, thereby reducing seepage (Shi et al., 2014). It should be noted that, for specimens KK-2, KK-3, and KK-4, hydraulic conductivity values were obtained prior to satisfying the chemical equilibrium criteria defined in ASTM D7100 (ASTM, 2020) and thus represent short-term performance. Given that continued bentonite hydration and physicochemical equilibration would be expected to further decrease hydraulic conductivity with time, the values adopted in subsequent analyses are conservative and consistent with the engineering practice of adopting unfavorable hydraulic parameters for barrier performance assessment. In practical landfill liner applications, such short-term laboratory values therefore represent a conservative estimate of field performance, as continued hydration and physicochemical equilibration are expected to further reduce hydraulic conductivity over service time.

    The addition of bentonite also significantly mitigates fluoride diffusion in compacted clay specimens. The effective diffusion coefficient is governed by both pore tortuosity and membrane behavior (Rowe and Booker, 1984). Although the free diffusion coefficient of fluoride remains unchanged, bentonite swelling (Zeng et al., 2025) increases tortuosity by obstructing pore connectivity, thereby reducing effective diffusivity. Moreover, bentonite addition enhances the cation exchange capacity of the compacted clay, strengthening its semipermeable membrane characteristics (Fritz, 1986, Malusis et al., 2001). The overlapping diffuse double layers between adjacent clay particles restrict anion mobility and contribute to chemico-osmotic effects, which further impede fluoride transport across the compacted matrix. Together with the improved sorption capacity, these mechanisms collectively suppress fluoride migration under diffusive conditions.

    Overall, the coupled enhancement of sorption, reduction in hydraulic conductivity, and suppression of diffusion following bentonite incorporation substantially improves the fluoride barrier performance of compacted clay specimens.

    4.2. Environmentally safe CCLs design against fluoride contamination

    The experimental results provide quantitative guidance for the design of compacted clay liners in landfills where fluoride-bearing leachate may occur. Both laboratory measurements and analytical modeling indicate that bentonite amendment markedly improves liner performance by reducing hydraulic conductivity and effective diffusion coefficient while slightly enhancing fluoride sorption. In particular, the analytical predictions show that a CCL incorporating 5% bentonite can delay fluoride breakthrough for more than 100 years, whereas further increasing bentonite content to 10% yields only marginal additional improvement. This diminishing return is consistent with the measured trends in k and D* (See Fig. 5(b) and 6(f)), suggesting that approximately 5% bentonite represents an engineering-optimal dosage that balances barrier performance and material cost.

    Beyond bentonite content, the measured basic parameters provide important insights for practical liner construction. The specimens in this study were compacted to 90% of maximum dry density, which is commonly achievable in engineering practice. Although higher compaction degrees may further reduce hydraulic conductivity, they also increase construction difficulty and cost. The present results indicate that 90% compaction degree combined with modest bentonite amendment is sufficient to meet low-fluoride containment requirements. The achieved degree of saturation exceeded 90%, which is representative of initial Sr of well-compacted CCLs (Rowe et al., 2004). Under such near-saturated conditions, effective diffusion coefficients are substantially higher than in unsaturated states, whereas relatively low degrees of saturation may lead to hydraulic conductivities exceeding 1 × 10-9 m/s. These findings emphasize that liner performance is governed not only by bentonite content but also by strict control of compaction quality, and saturation during construction.

    It should also be noted that the hydraulic conductivities adopted for specimens with higher bentonite contents were obtained prior to full chemical equilibrium and therefore represent short-term values. As continued bentonite hydration and physicochemical equilibration would be expected to further reduce hydraulic conductivity, the analytical predictions based on these parameters likely overestimate fluoride migration rates, thereby providing conservative breakthrough estimates consistent with unfavorable-condition design principles. Although the present study focuses on a single contaminant, the quantified diffusion-permeation-sorption framework offers a practical basis for risk-informed liner design, optimization of bentonite dosage, and long-term performance assessment. Future studies incorporating co-existing ions and coupled transport processes will further enhance the applicability of these recommendations.

    5. Conclusions

    This study conducted a series of experimental tests and analytical predictions using MATLAB software on compacted clay specimens with various bentonite contents to comprehensively evaluate their fluoride barrier performance. The underlying improvement mechanisms of the distribution coefficient, hydraulic coefficient and effective diffusion coefficient were also illustrated. The main conclusions are summarized as follows.

    The increase in bentonite content leads to a decrease in maximum dry density and increase in liquid limit, optimum water content, free swelling index, and CEC value. The specimen with 10% bentonite exhibited a minimum ?dmax of 1.57 g/cm3, maximum liquid limit, optimum water content, free swelling index, and CEC value of 64.4%, 20.2%, 5.2 mL/2g, and 36.5 meq/100 g, respectively, reflecting the influence of adding bentonite on material properties. After adding bentonite, the specific surface area and Al2O3 content were significantly increased, while the Fe2O3 content decreased slightly, contributing to the slightly enhanced fluoride sorption capacity of the clay specimen. Results showed that the distribution coefficient (Kd) of specimens with 0%, 2.5%, 5.0%, 7.5% and 10% bentonite was 2.1, 2.8, 3.0, 3.2, and 3.4 L/kg, corresponding to Rd of 8.5, 10.1, 11.6, 12.8 and 13.4, respectively.

    Finer bentonite particle with superior swelling property and CEC value could fill in the inter-pores and flow pathways in compacted specimens, resulting in a filter cake effect and decreased hydraulic conductivity. Results indicated that the hydraulic conductivity of specimens with 0 and 2.5% bentonite after reaching chemical equilibrium was 1.4 × 10-10 and 8.8 × 10-11 m/s, while the short-term hydraulic conductivities of specimens with 5.0, 7.5, and 10% bentonite were 5.3 × 10-11, 5.1 × 10-11, and 4.9 × 10-11 m/s. The use of short-term hydraulic conductivity for higher bentonite contents may lead to conservative estimates of solute breakthrough, and long-term permeation behavior warrants further investigation. Increasing tortuosity, membrane property, and fluoride sorption capacity significantly mitigated the diffusion behavior. The ?Qt/?t values of compacted clay-bentonite mixture with 0, 2.5, 5.0, 7.5 and 10% bentonite was 13.6, 9.7, 7.8, 6.9 and 6.9 mg/(m2·d), and the effective diffusion coefficients of specimens with 0, 2.5, 5.0, 7.5 and 10% bentonite were 1.6 × 10?10, 1.1 × 10?10, 8.6 × 10?11, 7.7 × 10?11 and 7.6 × 10?11 m2/s, respectively. Analytical modeling predicted the breakthrough time of the clay-bentonite mixture with 0, 2.5, 5.0, 7.5, and 10% bentonite to be 45, 78, 118, 145, and 150 years, respectively, when the F? concentration and hydraulic gradient (i) were 33 mg/L and 1. Combining experimental results, affordability and analytical predictions, adding approximately 5% bentonite is economical and effective, and it is recommended for CCLs in landfills containing low-fluoride leachate.

    Nevertheless, several limitations of this study should be acknowledged. First, for specimens with bentonite contents > 5%, hydraulic conductivity tests were terminated prior to full chemical equilibrium, and the reported values therefore represent short-term performance. Continued physicochemical equilibration and bentonite hydration may further reduce hydraulic conductivity over time. Consequently, the transport parameters adopted in the analytical prediction are likely conservative, potentially leading to underestimated breakthrough times. From an engineering perspective, such conservative estimates are consistent with unfavorable-condition design principles. However, future long-term permeation tests are required to quantify equilibrium hydraulic conductivity and refine long-term performance predictions.

    Second, only sodium fluoride solution was employed as the permeant, whereas real landfill leachate typically contains a complex mixture of inorganic and organic constituents (Karak et al., 2013, Atta et al., 2015, Guo et al., 2022, Choubisa et al., 2023, Bi et al., 2026). Previous studies (Sahoo et al., 2022, Liu et al., 2025a, Liu et al., 2025b) have shown that co-existing anions such as carbonate, sulfate, and phosphate may compete with fluoride for sorption sites and alter transport behavior in clay-based barriers. Although the present study intentionally focuses on single-anion transport to establish baseline fluoride migration mechanisms in bentonite-amended compacted clay, the obtained parameters may differ under multi-ion conditions. Therefore, future investigations should incorporate representative landfill leachate chemistries to evaluate competitive adsorption, coupled diffusion-permeation processes, and their implications for liner performance under realistic field environments.

    Despite these limitations, the present work provides a systematic experimental and analytical framework for assessing fluoride migration in bentonite-amended CCLs under landfill-relevant concentrations, offering a reference basis for subsequent studies involving more complex geochemical systems.

    CRediT authorship contribution statement

    Jia-Ming Wen: Writing – review & editing, Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Conceptualization. Tomohiro Kato: Writing – review & editing, Writing – original draft, Software, Resources, Investigation, Formal analysis. Atsushi Takai: Writing – review & editing, Writing – original draft, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization. Takeshi Katsumi: Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Funding acquisition.

    Declaration of competing interest

    The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

    Acknowledgments

    The authors are grateful for the financial support provided by JSPS KAKENHI 21H01426 and 22H00227.

    Appendix A. Supplementary data

    The following are the Supplementary data to this article:Download: Download Word document (1MB)Supplementary Data 1.

    Data availability

    Data will be made available on request.

    References