Tris(2-benzimidazolyl)amine (NTB)-Modified Metal-Organic Framework: Preparation, Characterization, and Mercury Ion Removal Studies

Heavy metal ions (HMIs) are exceedingly hazardous to both humans and the environment, and the necessity to eliminate them from aqueous systems prompted the development of novel materials. In this study, tris(2-benzimidazolylmethyl)amine (NTB) was impregnated into MIL-101-(Cr) metal-organic framework using an incipient wetness impregnation approach, and the ability of the composite material to adsorb Hg2+ ions from the water was examined. The synthesized materials were analyzed with several physico-chemical techniques such as powder X-ray diffraction, elemental analysis, scanning electron microscopy, thermogravimetric analysis, nitrogen sorption isotherms at 77 K, and X-ray photoelectron spectrometry. MIL-101-NTB quickly adsorbs 93.9% of Hg2+ ions within 10 min from a 10.0 ppm single ion solution. A better fit of the kinetic data to a pseudo-second-order model validated the chemisorption of Hg2+ ions on MIL-101-NTB. The experimental data fitted well with the Langmuir isotherm model, and the maximum adsorption capacity obtained at 125 ppm initial concentration was 111.03 mg/g. Despite the presence of other competing ions (Cu2+, Pb2+, and Cd2+), high Hg2+ ions removal efficiency (99.6%, 1.0 ppm initial concentration) was maintained in the diverse ion batch adsorption studies. A 0.2 M EDTA solution could desorb the Hg2+ ions, and cyclic Hg2+ ions sorption studies indicated that MIL-101-NTB might have a high Hg2+ ions removal efficiency for at least five consecutive cycles. Based on the FTIR and XPS analyses, Hg2+ ions chelation by NTB molecules and electrostatic interactions between Hg2+ ions and carboxylate groups in MIL-101-NTB are plausible mechanisms for Hg2+ ions adsorption.