Waste Beneficiated Kaolin Filter Cake: A Sustainable Adsorbent for Efficient Fluoride Removal from Contaminated Water

Résumé

Abstract Fluoride contamination in water poses severe threats to human health, and low-cost, sustainable adsorbents are needed for efficient fluoride removal in developing nations. Here in, a facilely prepared beneficiated kaolin filter cake (BKF) was developed and applied as an effective fluoride adsorbent in batch adsorption experiments. A thirty-batch adsorption experimental design was conducted to optimize fluoride adsorption performance, with initial fluoride concentration, contact time, adsorbent dosage, and solution pH as independent variables. The variables interaction was identified using the central composite design with response surface method. The response surface methodology predicted an optimal fluoride removal efficiency of 92% at 0.1 g of adsorbent, an equilibrium contact time of 105 minutes, an initial fluoride concentration of 7 mg/L, and a solution pH of 4.8. This prediction was validated by actual water samples under the same conditions, and the BKF removed 87% of fluoride. Adsorption kinetic and equilibrium isotherm modeling verified that fluoride removal follows the pseudo-second-order kinetic model (R² = 0.99), and fits both the multilayer Freundlich isotherm (R² = 0.98) and monolayer Langmuir isotherm (R² = 0.97), indicating coexisting single- and multi-layer adsorption pathways on BKF surfaces. Thermodynamic behaviour analysis shows –4.91 kJ/mol of standard Gibbs free energy change at room temperature and a standard enthalpy change of 24.3 kJ/mol. Fluoride adsorption by the BKF monolayer and multilayer proceeded endothermically via spontaneous weak surface ligand exchange and a dominant physisorption (electrostatic attraction) mechanism. More importantly, the regeneration experiment confirms that the BKF adsorbent is stable and retains 87.5% of its initial efficiency in the third-cycle measurement. BKF is therefore a cost-effective, and environmentally sustainable fluoride adsorbent for large-scale use.

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