Al2O3–Fe3O4–expanded graphite nano-sandwich structure for fluoride removal from aqueous solution

Abstract

In this study, a novel Al₂O₃–Fe₃O₄–expanded graphite nano-sandwich adsorbent was prepared to remove fluoride from aqueous solutions. Field emission scanning electron microscope examination was carried out to characterize its microstructure. Fe₃O₄ cubes, with the size of 60–80 nm, and pea-shaped Al₂O₃ particles, with the length of 20–50 nm, were observed on the surface of the expanded graphite. Fourier transform infrared spectra results indicated that new functional groups had occurred on the sandwich surface following the adsorption process. Identified X-ray powder diffraction curves determined that Al₂O₃ was amorphous, while Fe₃O₄ and expanded graphite were crystalline. Vibrating sample magnetometer experiments revealed that the saturation magnetization of the nano-sandwich was 3.7 emu g⁻¹, which was strong enough to separate the sorbents from the solution. The adsorption kinetics followed the pseudo-second-order rate equation with intra-particle diffusion as the rate determining step. The adsorption pattern of the nano-sandwich followed the Langmuir isotherm better than the Freundlich isotherm and the adsorption capacity increased by rising temperature. Under optimized conditions, fluoride removal efficiency reached 96.8%. After two cycles of regeneration with 0.1 mol L⁻¹ NaOH, fluoride removal efficiency could still reach 91.4% and the residual fluoride concentration was lower than 1.5 mg L⁻¹. The sorbents demonstrated great potential in fluoride removal, in terms of higher adsorption efficiency, wider pH adsorption range, good regeneration as well as convenient solid–liquid separation characteristics.