Application of NH₂-MIL-101(Al) as a new adsorbent for defluoridation of aqueous solutions: isotherms, kinetics, and thermodynamics

Fluoride is a potentially toxic element that its uptake through drinking may cause adverse impacts on human health. The present study aims to investigate the efficiency of a metal-organic framework (MOF, NH₂-MIL-101(Al)) as a novel adsorbent for defluoridation of experimental and real solution samples. The effects of important controlling parameters (i.e. solution pH and temperature, duration of interaction, dose of the adsorbent, and initial content of F⁻ ion) on the removal efficiency (R_t) and adsorption capacity (q_e) were studied. The maximum adsorption capacity (q_max, 120.48 mg/g) occurred at T = 298 °K, pH = 5.9, the initial concentration of 5.00 mg/L F⁻, and the adsorbent dose of 30 mg. The highest correlation coefficient was obtained for the Freundlich adsorption isotherm, and the obtained data was well-adapted to the pseudo-second-order model. Based on the thermodynamic factors, the adsorption mechanism was endothermic and spontaneous, with an enhanced disorder during the process. The R_t of F⁻ ions was not influenced by the presence of K⁺, NO₃⁻, and Cl⁻ ions in concentrations up to 0.04 M; however, SO₄²⁻, HCO₃⁻, H₂PO₄⁻, and Br⁻ caused a decline in F⁻ ions adsorption efficiency. Desorption experiments indicated that the highest value of F⁻ R_t is related to the adsorbent recovered by ethanol. The experimental data indicated that NH₂-MIL-101(Al) MOF is also a useful adsorbent for real water defluoridation (R_t = 57.8%). In conclusion, NH₂-MIL101(Al) promises to be a proper adsorbent with high q_e to treat high fluoride waters.