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

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.