Green and facile synthesis of hierarchical ZnOHF microspheres for rapid and selective adsorption of cationic dyes

' Employing DL-Alanine (DL-Ala) as the structure-directing agent, hierarchical microspheres of ZnOHF have been designed with optimized geometric features in a facile one-step hydrothermal approach. Morphological characterizations demonstrated that the obtained hierarchical microspheres were formed by numerous nanotubes with outer diameter of 10 nm and wall thickness about 3.5 nm. A series of controllable experiments proved that DL-Ala and reaction time played vital roles in the formation of nanotubes-assembled hierarchical microspheres. By taking advantage of the optimized geometric features and large specific surface areas (271.06 m(2)/g), the obtained ZnOHF nanotubes-assembled microspheres were evaluated as adsorbents for the removal of various dyes in aqueous solution. Adsorption behavior conducted by batch mode experiments indicated that the adsorption was well described by Langmuir isotherm and pseudo-second-order model. The obtained hierarchical ZnOHF microspheres exhibited effective adsorption and selectively properties towards cationic dyes. For example, the maximum adsorption capacity of ZnOHF microspheres was 140.4 mg/g for malachite green (MG) at 298 K. Analysis of thermodynamic data suggested that it was an endothermic and spontaneous physical adsorption process. The fabrication strategy may provide new insights for designing other high-performance materials, and the novel hierarchical ZnOHF microspheres are promising adsorbents for wastewater purification. (C) 2021 Elsevier B.V. All rights reserved.