Optimization of fluoride removal by activated clays using response surface methodology: Box–Behnken design, kinetic and isotherm studies

In numerous regions globally, particularly in Tunisia, the fluoride concentration in industrial wastewater surpasses regulatory thresholds, which posing environmental challenges. Therefore, this research investigates the efficacy of two clays sourced from Jebel Haidoudi Gabes–Tunisia (east longitude 9° 42′ 51.5″ (9.7143°); north latitude 34° 2′ 9.7″ (34.036°)) and Douiret–Tunisia (east longitude 10° 17′ 18.28 (10.28°); north latitude 32° 51′ 24.36″ (32.85°) as adsorbents and compared their performance in removing fluoride. These clays were used as adsorbents after activation with acid for 3.5 and 4.5 h, respectively. The design of Box–Behnken was applied to enhance the influence of initial fluoride concentration (50–300 mg/g), pH (2–6), and contact time (2–5 h). The results highlight the following order of significance for these factors: initial fluoride concentration > contact time > pH. The empirical model predicted that a maximum amounts adsorbed of fluoride with activated clays were obtained at fluoride concentration of 300 mg/L at a contact time of 5 h and pH = 2. A comparative analysis of activated clays from Jebel Haidoudi (142.56 mg/g) and Douiret (136.24 mg/g) elucidates their differential performance in fluoride removal. Results show that the experimental data fit well the pseudo-second-order equation, with R² close to 1 (0.999) and χ² low (1 ≥); the isotherm studies illustrate that the most excellent liner fits were obtained use the Langmuir model with highest R² (0.986 and 0.983) and lowest χ² (41.07 ad 43.56). These results demonstrate that fluoride removal might be mostly a chemisorption and a monolayer process. Finally, both activated clays could be used as efficient and inexpensive adsorbents. This work offers practical insights for optimizing treatment processes in regions grappling with elevated fluoride concentrations in industrial effluents.