Fast and Selective Heavy Metal Removal by a Novel Metal‐Organic Framework Designed with In‐Situ Ligand Building Block Fabrication Bearing Free Nitrogen

Abstract

Fast and effective adsorbents for the selective removal of Hg and Pb ions were prepared by the reaction of Zn(NO ) ⋅6H O, H BDC, and N ,N -bis(pyridin-4-ylmethylene) ethane-1,2-diamine (L) that yields an unprecedented two-dimensional layer-based supramolecular framework, {Zn(BDC)(L*)}⋅DMF (TMU-40), by solvothermal reaction. The formation of this framework involved an in situ C=C coupling of L to L* [L*=5,6-di(pyridin-4-yl)-1,2,3,4-tetrahydropyrazine]. As L* contains free nitrogen atoms, direct reaction of L* and metals led to metallated products. Post-synthetic modification of this novel MOF (TMU-40) with H O gives a new framework (O-TMU-40) by same structure and different ligand, which also bears free nitrogen atoms. FTIR spectra, TGA analysis, X-ray diffraction, Zeta Potential analysis and H NMR spectroscopy were used to characterize the prepared frameworks. The TMU-40 and O-TMU-40 frameworks were used for heavy-metal removal from aqueous solutions. Maximum adsorption values of 269 mg g for Hg with TMU-40 and 215 mg g for Pb with O-TMU-40 were achieved in 10 min at 298 Kwithout changes in the pH of the adsorption medium with pseudo-second order kinetics based on the Langmuir model. The extremely fast kinetics of TMU-40 means this adsorbent can reduce heavily contaminated water containing Hg concentrations of 40 ppm down to the acceptable limit of 2 ppb.

Recent Progress in the Removal of Heavy Metal Ions from Water Using Metal‐Organic Frameworks

“…Two‐dimensional MOFs, formulated as [Zn(BDC)(L 7 )] (TMU‐40) [BDC=1,4‐benzene dicarboxylate; L 7 =5,6‐di(pyridine‐4‐yl)‐1,2,3,4‐tetrahydropyrazine], was prepared to check selective removal of Hg II and Pb II ions from wastewater . Morsali et al.…”

“… The suggestion mechanism for C=C coupling and cyclization reaction of L to L**. Adapted with permission from Ref . Copyright 2018, Wiley.…”

Goal‐Directed Design of Metal–Organic Frameworks for HgII and PbII Adsorption from Aqueous Solutions

“…The results demonstrated that the maximum values of 269 mg g −1 for Hg II were acquired in just less than 10 min without changes in the pH of the adsorption medium. These extremely fast kinetics, which were observed for TMU‐40, are able to reduce heavily contaminated water with the Hg II concentration of 40 ppm to the acceptable limit of 2 ppb for drinking water …”

“…For instance, intra‐particle diffusion models based on the experimental data for TMU‐40 indicate that the adsorption process occurs in three different steps. The first step, with a high slope, shows the very fast adsorption of Hg II ions on the surface of the MOF, the second step is attributed to the entrance of metal ion into the pores, and the third step represents the equilibrium level …”

“…These extremely fast kinetics, which were observed for TMU-40, are able to reduce heavilyc ontaminated water with the Hg II concentration of 40 ppm to the acceptable limit of 2ppb for drinking water. [32] Hou et al,h as used aM OF decorated with O À groups to adsorb Pb II .T he authors believe that the negative charge of the oxygen leads to the electrostatic interaction with Pb II , which is the driving force in the adsorption process. The authors concluded that the high adsorption capacity (616.64 mg g À1 )a nd the rapid kinetics of the structure are due to the proper electrostatic interaction of negative charges of the O À groups with Pb II and acceptanceo fo xygen electrons pairs by Pb II as aL ewis acid.…”