A novel approach to hierarchical sphere-like ZnAl-layered double hydroxides and their enhanced adsorption capability

A simple route has been developed to prepare hierarchical sphere-like zinc–aluminum layered double hydroxides (ZnAl-LDHs). The whole process involved the use of a simple hydrothermal method, in which a precursor mixture was prepared and calcined to obtain ZnAl-layered double oxides (LDOs), which were then transformed into ZnAl-LDHs by reaction with carbonate solution. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2 adsorption, from which a possible mechanism of formation of ZnAl-LDHs is described. It was found that the morphology of ZnAl-LDHs can be tuned from hollow flower-like spheres to porous microspheres by simply varying the molar ratio of urea to metallic cations. With relatively high specific surface areas, LDOs and ZnAl-LDHs with 3D hierarchical nanostructures were tested for their adsorption capacity for methyl orange (MO) in waste water. The results indicate that the as-prepared LDOs show excellent adsorption capacity for MO, which is much better than that for ZnAl-LDHs. The adsorption isotherms and kinetics for MO onto LDOs and ZnAl-LDHs were investigated as well. Our work proposes a new avenue to fabricate hierarchical ZnAl-LDHs microspheres and demonstrates that they possess excellent adsorption capabilities that could be very useful for wastewater treatment.