Adsorptive removal of fluoride from water by activated carbon derived from CaCl₂-modified Crocus sativus leaves: Equilibrium adsorption isotherms, optimization, and influence of anions

A central composite design using response surface methodology was applied for the experimental design and optimization of fluoride adsorption on an activated carbon derived from calcium chloride-modified Crocus sativus leaves (AC-CMCSL). Fluoride removal efficiency as function of independent variables, such as initial fluoride concentration, pH, adsorbent dose, and time has been investigated. The maximum percentage removal of fluoride at optimum conditions (initial fluoride concentration = 6.5 mg L⁻¹, pH = 4.5, adsorbent dose = 15 g L⁻¹ and time = 70 min) was 85.43%. By comparing adsorption isotherm, the Freundlich model provided the best correlation (R² = 0.99) for the adsorption of fluoride on AC-CMCSL. The maximum adsorption capacity from the Langmuir model (q_max) was 2.01 mg g⁻¹. The influence of the co-existing anions on fluoride adsorption was in the following order: PO₄ ³⁻ > SO₄ ²⁻ > Cl⁻ > NO₃ ⁻. The results of the present study showed that activated carbon derived from the leaves of calcium chloride-modified Crocus sativus has a good potential for fluoride removal from aqueous solution.