Preparation of hollow hierarchical porous CoMgAl-borate LDH ball-flower and its calcinated product with extraordinary adsorption capacity for Congo red and methyl orange

Three-dimensionally ordered hollow borate anions intercalated CoMgAl-LDH ball-flower with hierarchical porous structure have been prepared by using zeolitic imidazolate framework-67 (ZIF-67) as a self-sacrificing template. After calcination it, the obtained CoMgAl-borate LDO can inherit the hollow hierarchical porous structure. Its unique structure has the merits of low density, large specific surface area, large pore size distribution and short diffusion pathway, which should be beneficial to the adsorption of larger dye molecules. The maximum uptake capacity of CoMgAl-borate LDO can even reach as high as 1493.3 mg g−1 for Congo red (CR) and 990.1 mg g−1 for methyl orange (MO) owing to the hierarchical hollow porous structure, the “memory effect”, electrostatic interactions, hydrogen bond and ion exchange between anionic dye and cationic adsorbent surface, surpassing those of most previously reported LDH-based adsorbents. The entire adsorption processes for CR and MO follow Langmuir isotherm and pseudo-second-order kinetics model. Moreover, the removal efficiency of CoMgAl-borate LDO can still reach 86.5% for CR and 84.3% for MO after recycling five times, indicating that CoMgAl-borate LDO is a very promising high-efficiency adsorbent. This work not only demonstrates that CoMgAl-borate LDO is a very green and efficient adsorbent for the removal of anionic dyes from wastewater, but also provides a strategy for the rational design and development of hollow nanostructures.