Porous NiCo-LDH microspheres obtained by freeze-drying for efficient dye and Cr(VI) adsorption

Hierarchical NiCo-LDH microspheres (NiCo-LDHM) were synthesized via a simple one-step hydrothermal process in the presence of 2-methylimidazole, and a freeze-drying technique was subsequently used to further adjust its microstructure. The obtained porous hierarchical microspheres are about 5 μm in average diameter, and consist of ultrathin nanosheets with an average thickness of 5 nm. The freeze-drying endows LDH microspheres with higher surface area and improved pore structure. The unique hierarchical porous structure, high specific surface area and positive surface charge together contribute NiCo-LDHM’ excellent capability for anionic dyes and heavy metal ions removal, its theoretical adsorption capacities for Congo red (CR), acid fuchsin (AF) and Cr(VI) are 2120, 1176 and 123 mg/g at room temperature respectively. The adsorption of pollutant molecules/ions onto NiCo-LDH microspheres conforms to the quasi-secondary kinetics model. Isotherm studies suggest that the CR adsorption onto adsorbent fits Langmiur model, while AF/Cr(VI) adsorption fits Freundlich model, indicating that the size and morphology of pollutant molecules influence the adsorption process. The adsorption is a spontaneous endothermic process and NiCo-LDHM demonstrates good recyclability, the removal rates of CR, AF and Cr(VI) can still reach 90.7%, 84.7% and 82% after five cycles, respectively. In addition, NiCo-LDH hierarchical microspheres also exhibit good anionic selective adsorption capacity in mixed-dye systems, displaying great potential for water restoration and dye recycling.