Selective Removal of Anionic Dyes by Metal–Organic Framework-Anchored CoAl-Layered Double Hydroxide Nanosheets

A copper terephthalate metal organic framework-anchored CoAl-layered double hydroxide nanocomposite [CoAl-LDH/Cu-(tpa) MOF] was synthesized by in situ nucleation and growth of crystalline Cu-(tpa) MOF on hexagonal LDH sheets. Successive ion exchange of carbonate-intercalated CoAl-LDH by chloride and terephthalate anions yielded CoAl-(tpa) LDH. The “tpa” ions de-intercalated and coordinated with the Cu2+ ions for in situ growth of Cu-(tpa) MOF on LDH nanosheets retaining crystallinity, uniformity, and platelet morphology. The structure and properties of the synthesized materials were studied by powder X-ray diffraction analysis, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, optical spectroscopy, thermal gravimetric analysis, X-ray photoelectron spectroscopy, and N2 adsorption–desorption isotherms analysis. The CoAl-LDH/Cu-(tpa) MOF nanocomposite exhibited selective adsorption of anionic dyes. The dye adsorption performance of the nanocomposite was investigated under various parameters such as initial dye concentration, adsorbent dosage, pH of dye solution, temperature of dye solution, and contact time. Adsorption kinetics followed a pseudo first order kinetic model, and the equilibrium adsorption data fitted well to the Langmuir isotherm model with a maximum dye adsorption capacity of 666 mg g–1 for the anionic methyl orange dye. The endothermic nature of the adsorption process was elucidated from a thermodynamic study, and the resulting energy of adsorption was 19.37 kJ mol–1. Furthermore, the CoAl-LDH/Cu-(tpa) MOF nanocomposite adsorbent could be regenerated for six successive cycles with no significant loss in efficiency and exploited as the potential candidate for selective removal of anionic dyes in the field of wastewater treatment.