Removal of fluoride from aqueous solution by natural and acid-activated diatomite and ignimbrite materials

The aim of this study was to investigate the adsorption capabilities of some natural adsorbents for the removal of fluoride from aqueous solution. The adsorption of fluoride onto diatomite and ignimbrite in aqueous solutions was studied in a batch system. Batch adsorption experiments were conducted to examine the influence of various parameters such as pH, adsorbent amount, contact time, initial fluoride concentration, and temperature. In order to increase adsorption capacity, diatomite and ignimbrite were activated by H2SO4. The fluoride adsorption capacity of acid-activated form was found to be higher than that of the natural form of adsorbents. Characterization of the acid-activated adsorbents was confirmed through scanning electron microscopy, Brunauer–Emmett–Teller, and Fourier transform infrared spectroscopy. All adsorption experimental data fitted with the Freundlich and Dubinin–Radushkevich isotherms. The maximum fluoride removal was obtained within 2 h at an optimum adsorbent amount of 0.3 g/mL. Mechanism of fluoride adsorption was found to follow a pseudo-second-order rate equation for both adsorbents. Thermodynamic parameters were calculated to explain the nature of adsorption process. Fluoride removal properties for drinking water were evaluated by acid-activated adsorbents. Overall results showed that acid-activated diatomite material shows better fluoride removal capacity than ignimbrite in drinking water. The presence of other ions in drinking water did not affect fluoride removal efficiency.