Response surface methodology for optimization of ZIF-8 Synthesis conditions to enhance its removing capability for Pb(Ⅱ) in aqueous solutions

摘要

The synthesis conditions of zeolitic imidazolate framework-8 (ZIF-8) have been optimized by a response surface method using central composite design (CCD) to obtain the high adsorption capability for the Pb(II) removal in aqueous solution. The physic-chemical properties of the as-prepared ZIF-8, such as morphology, structure and chemical composition, were characterized by field emission scanning electron microscopy (FE-SEM), powder X-ray diffraction (XRD) and energy-dispersive X-ray photoelectron spectroscopy (XPS), respectively. The adsorption capability, influence factors, isotherms, thermodynamics, kinetics and mechanism of Pb(II) in aqueous solutions were investigated. The optimal parameters of which have the highest adsorption capability for Pb(II) were 200 mL of water as the solvent, and the molar mass ratio of 2-methylimidazole and Zn(NO3)(2)center dot 6H(2)O was 45.63. At this condition, the as-prepared ZIF-8 was a pure hexagonal three-dimensional crystal structure. The equilibrium adsorption capacity up to 668 mg/g for 300 mg/L of initial Pb(II) solutions and the adsorption was a spontaneous process. The largest adsorption capacity from Langmuir isotherm model was 686.25 mg/g. The adsorption kinetics followed the pseudo-second-order model. The equilibrium adsorption time and the optimal adsorption pH was 4 h and 6.0-7.0, respectively. The ZIF-8 still kept well adsorption effect after three times of usage. Therefore, this study showed that ZIF-8 is a promising sorbent for removing environmental pollutants.