Imidazole/pyridine-based ionic liquids modified metal-organic frameworks for efficient adsorption of Congo red in water

In this work, a metal-organic framework (MOF), {[Ba2L(NO3) (DMF)2] (DMF·H2O)}n, denoted as HBU-167, was synthesized under solvothermal conditions. HBU-167 obtained a 3D layer framework comprising the imidazole carboxylic acid ligand and Ba2+ as nodes. It was demonstrated to have an adsorption capacity for Congo red (CR), with an adsorption capacity of 407.37 mg•g−1 and a removal rate of 65.46%. To improve the adsorption capacity of HBU-167, simply impregnated three imidazole/pyridine-based ionic liquids (ILs) onto HBU-167 to obtain IL@HBU-167 for efficient CR adsorption. The results of Powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) revealed that ILs successfully bonded to HBU-167. Adsorption experiments on CR revealed that IL@HBU-167 exhibited increased stability as well as improved adsorption capacity. The adsorption capabilities of the IL@HBU-167 were 440.70 mg•g−1, 491.23 mg•g−1, and 544.56 mg•g−1, respectively, while the removal rates were 72.96%, 75.00%, and 83.28%, respectively. Batch experiments were applied to analyze the impact of factors such as contact time, initial concentration, pH, temperature, and ionic strength on the adsorption of CR by IL@HBU-167. The maximum adsorption capacity was observed at 25°C and pH = 6. The kinetic data indicated that the adsorption conformed to the pseudo-second-order kinetic model. The Langmuir isothermal model accurately described isothermal data. Furthermore, the adsorption capacity of IL@HBU-167 was investigated in real water samples from various sources. The results showed that IL@HBU-167 can be used as an innovative and inexpensive adsorbent for the removal of CR from water. The adsorption capacity was mainly derived from π-π, electrostatic interactions, and hydrogen bonding between IL@HBU-167 and CR.