Sustainable Adsorbents for Wastewater Treatment: Template-Free Mesoporous Silica from Coal Fly Ash

The adsorption of methylene blue (MB) onto silica synthesized from coal fly ash (CFA) was investigated to evaluate its efficiency, kinetics, and thermodynamics. Adsorption studies revealed nearly 100 % MB removal under optimized conditions (50 ppm MB, pH 7, 3.1 g adsorbent dosage, and 50 min contact time). Equilibrium data fitted best with the Langmuir isotherm model (R² = 0.9881, Q_max = 32.76 mg g⁻¹), confirming monolayer adsorption and strong adsorbate–adsorbent interactions. Kinetic modeling showed that the pseudo-second-order model (R² = 0.99) best described the process, indicating chemisorption as the dominant mechanism. Thermodynamic analysis confirmed that the adsorption was spontaneous and exothermic (ΔG < 0, ΔH = − 22.84 kJ mol⁻¹), with a decrease in system entropy, suggesting an energy-efficient process that favours lower temperatures. Reusability studies demonstrated that the silica adsorbent retained 97.5% ±  0.17 % MB removal efficiency over 12 cycles, highlighting its economic and industrial feasibility. Additionally, ethanol-based desorption proved to be effective, ensuring sustainable regeneration. These findings establish silica from CFA as a cost-effective, environment friendly, and highly efficient adsorbent for wastewater treatment applications, particularly in dye-contaminant removal.