Highly efficient and selective removal of anionic dyes from aqueous solution by using a protonated metal-organic framework

Abstract

Hydrothermal reaction of Zn(NO3)2·6H2O with a flexible bipyridyl ligand, (pyridin-4-yl)methyl 3-(2-(4-((pyridin-4-yl)methoxy)phenyl)diazenyl)benzoate (L) and 1,3-benzenedicarboxylic acid (1,3-H2BDC) gave rise to a novel metal-organic framework (MOF), {[Zn (1,3-BDC)L]∙H2O}n. The as-synthesized MOF has been characterized by Fourier transformed infrared (FT-IR), powder X-ray diffraction, single-crystal X-ray diffraction and scanning electron microscopy (SEM). By introduction of the ligand L containing ester, diazene and ether groups, a large number of oxygen and nitrogen sites that can be readily protonated in acidic conditions were successfully decorated on the surfaces of the predesigned MOF. The as-prepared protonated Zn-MOF with abundant positive charges (−NNH+−, –OH+−, –COOH+−) was applied as an adsorbent to remove anionic dyes from aqueous solutions. Such an adsorbent exhibited ultrahigh uptake capacities for different sized anionic dyes of Amido Black 10 B (AB, 2402.82 mg g−1), Methyl Orange (MO, 744.02 mg g−1), Orange ΙΙ (OII, 522.83 mg g−1) and Direct Red 80 (DR, 1496.34 mg g−1). The anionic dyes uptakes of the adsorbent are obviously higher than that of most reported adsorbents. Moreover, the adsorbent showed excellent selectivity in the adsorption of anionic dyes. The adsorption process followed the pseudo-second-order kinetics. The equilibrium adsorption isotherm data were well fitted by Langmuir and Sips models. FT-IR, zeta potential analysis and SEM investigation revealed that the interactions between the protonated Zn-MOF and the anionic dyes are mainly dominated by electrostatic interaction and surface adsorption. This work may provide a new perspective for the development of functional materials on the removal of contaminants from water environment.