Solvent-free hydrothermal synthesis of gamma-aluminum oxide nanoparticles with selective adsorption of Congo red

Aluminum hydroxide and oxide have been widely used for decontamination due to their environmentally friendly nature and cost effectiveness. Aluminum (hydro) oxides are the main phases of aluminum-derived environment materials. Herein, the solvent-free hydrothermal synthesis of gamma-aluminum oxide (γ-Al₂O₃) nanoparticles and phase transformation of AlOOH into γ-Al₂O₃ are reported. Hydrothermal treatment of NH₃·H₂O-induced aluminum precipitate resulted in the formation of AlOOH, which was an intermediate product of γ-Al₂O₃. AlOOH was transformed into highly crystalline 20-nm γ-Al₂O₃ particles through calcination at 500 °C due to dehydration. The transformation was confirmed through X-ray diffraction (XRD) and thermogravimetric (TG) analyses. The resulting γ-Al₂O₃ had superior adsorption ability for the anionic Congo red (CR) dye than for the cationic methylene blue (MB) and malachite green (MG) dyes. The selective adsorption ability of CR instead of MB was attributed to the electrostatic attraction and hydrogen bonds between the amino group and azo double bond of CR, and between the amino group and hydroxyl group in γ-Al₂O₃. Thus, this study investigated crystalline phase transformation into γ-Al₂O₃ and selective adsorption capacity of CR, which provides important information regarding the synthesis of crystalline γ-Al₂O₃ adsorbent, with selective adsorption ability for decontamination applications.