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 (gamma-Al2O3 nanoparticles and phase transformation of AIOOH into gamma-Al2O3 are reported. Hydrothermal treatment of NH3 center dot H2O-induced aluminum precipitate resulted in the formation of AlOOH, which was an intermediate product of gamma-Al2O3. AIOOH was transformed into highly crystalline 20-nm gamma-Al2O3 particles through calcination at 500 degrees C due to dehydration. The transformation was confirmed through X-ray diffraction (XRD) and thermogravimetric (TG) analyses. The resulting gamma-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 gamma-Al2O3. Thus, this study investigated crystalline phase transformation into gamma-Al2O3 and selective adsorption capacity of CR, which provides important information regarding the synthesis of crystalline gamma-Al2O3 adsorbent, with selective adsorption ability for decontamination applications. (C) 2018 Elsevier Inc. All rights reserved.