Adsorption, diffusion, and hydraulic performance of geopolymer from fly ash waste–bentonite clay in ammonium solution

Sanitary landfills necessitate careful design and construction of liner systems to prevent leachate migration into surrounding areas. This study investigates the barrier performance of a geopolymer made from fly ash waste mixed with natural bentonite clay at ratios of 10% (FB1), 30% (FB3), and 50% (FB5) in ammonium (NH₄⁺) solution. The performance of the bottom liner application is characterized by adsorption, diffusion, and permeability. The results demonstrate that FB3 effectively represents the Langmuir model and exhibits the highest NH₄⁺ uptake capacity, achieving a maximum adsorption capacity of 24.58 mg/g. The diffusion coefficient of FB1–FB5 varies from 5.0 × 10^–10 m²/s to 1.0 × 10^–9 m²/s. The optimal 10% content meets the regulations for liner materials with a hydraulic conductivity of 8.46 × 10^–10 m/s in ammonium solution (1000 mg/L NH₄⁺-N); however, increasing bentonite clay beyond 30% raises hydraulic conductivity. Thus, these findings suggest that the proposed geopolymer made from bentonite clay and fly ash could serve as a promising liner material for NH₄⁺ ions.