Fabrication of porous ZrO2 hollow sphere and its adsorption performance to Congo red in water

Hollow and solid ZrO2 microspheres with mesoporous structures were prepared by a hydrothermal method using zirconium oxychloride and urea as precursors. The as-prepared samples were characterized by X-ray diffraction, scanning and transmission electron microscopy and nitrogen adsorption–desorption isotherms. Adsorption performance of the as-prepared samples toward Congo red (CR) aqueous solutions was investigated and discussed. The pore structure analyses indicated that the prepared ZrO2 hollow and solid microspheres contained bimodal porous organization: small mesopores (ca. 2 nm) and large mesopores and macropores (ca. 40–70 nm). The CR equilibrium adsorption data of the as-prepared samples were fitted using Langmuir and Freundlich equations, indicating that the Langmuir equation exhibited the better correlation of the experimental data. The CR adsorption capacities were determined by the Langmuir equation and found to be 59.5, 21.4 and 4.8 mg g−1 for the ZrO2 hollow, solid microsphere and reagent samples, respectively. Adsorption data were further modeled by the pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetics equations. The results suggested that pseudo-second-order kinetic and intra-particle diffusion equations could better describe the kinetics of CR adsorption. The prepared ZrO2 hollow microsphere was found to be good adsorbent for the CR removal from water due to its high specific surface areas and bimodal porous structures.