Barrier performance of compacted clay liner in landfills containing fluoride: Diffusion, permeation and sorption

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.