In this paper, the effect of different crystal forms of Al2O3 on fluoride removal was studied. All crystal forms of Al2O3 were based on the same boehmite precursor and were obtained using a hydrothermal and calcination method. γ-Al2O3 had higher fluoride removal performance (52.15 mg/g) compared with θ-Al2O3 and α-Al2O3. Density functional theory (DFT) calculations confirmed that fluoride removal was greatest for γ-Al2O3, followed by θ-Al2O3 and α-Al2O3, and γ-Al2O3 possessed the strongest fluoride binding energy (-3.93 eV). The typical adsorption behaviour was consistent with the Langmuir model and pseudo-second-order model, indicating chemical and monolayer adsorption. Different metal ions were used to modify γ-Al2O3, and lanthanum had the best effect. Lanthanum oxide was shown to play an important role in fluoride removal. The best La/Al doping ratio was 20 At%. The adsorption process of the composite was also consistent with chemical and monolayer adsorption. When the La/Al doping rate was 20%, the adsorption capacity reached 94.64 mg/g. Compared with γ-Al2O3 (1.39 × 10-7 m/s), the adsorption rate of 20La-Al2O3 was 3.93 × 10-7 m/s according to the mass transfer model. Furthermore, DFT was used to provide insight into the adsorption mechanism, which was mainly driven by electrostatic attraction and ion exchange.
Keywords: Aluminium oxide; Lanthanmu; crystal structure; density functional theory; fluoride.