Structural modification of aluminum oxides for removing fluoride in water: crystal forms and metal ion doping

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.