Facile and template-free fabrication of hollow spherical AlOOH and Al2O3 from the waste aluminum residue: Growth mechanism and fast removal of Congo red

Organic dye pollution has attracted considerable concern, and the adsorption technology using inorganic nanostructures has become prevalent. In this work, hollow spherical AlOOH (HSHA) was successfully fabricated via an one-pot and template-free hydrothermal method. After calcination at 550 ​°C for 1 ​h, hollow spherical Al2O3 (HSAO) with the same morphology was obtained. The hollow spherical structures comprised of plenty of uniform nanoflakes possessed the high specific surface areas and total pore volumes. HSHA and HSAO exhibit excellent adsorption performance for Congo red (CR) with the maximal adsorption capacities of 277.00 and 478.47 ​mg ​g−1, respectively. The adsorption almost reached the equilibrium within 10 ​min. The pseudo-second-order model fitted the adsorption process well. The adsorption was spontaneous. Both the Langmuir and Freundlich models fitted the adsorption well, but the Freundlich model was slightly better. HSHA and HSAO are the potential and qualified candidates for the dye removal.