Study on the mechanism of removing Pb (II) and Cd (II) from industrial wastewater by copper based MOF modified with ethylenediamine

In this study, we synthesized ethylenediamine modified copper-based organometallic framework by hydrothermal method. The materials were characterized by SEM, XRD, FTIR, XPS and BET. The adsorption properties of the materials for Cu (II) and Pb (II) were studied in depth by varying the adsorbate concentration, ionic strength, contact time, adsorption temperature and pH of the solutions. At a pH value of 5.2, the maximum adsorption capacities for Cd (II) and Pb (II) were 909.09 and 12.85 mg·g−1 within 20 min, respectively. The adsorption process was fitted with pseudo second order kinetics and Langmuir isotherm model. Thermodynamic experiments showed that an appropriate temperature reduction was helpful to promote Cd (II) and Pb (II) removal. Given that the underlying chemical properties of the materials are unknown, the factors leading to affecting the selectivity of heavy metals remain unclear. To reveal key molecular level factors, we performed state-of-the-art computational simulations combining high-density functional theory (DFT), semi empirical calculations, and configurational sampling of metal ion MOF binding modes in aqueous solution. And the intermolecular interactions, absorption patterns, and most remarkable structural features were determined to improve the selectivity against heavy metals. The mechanism of heavy metals separation by ethylenediamine grafted MOFs was rationalized using quantum mechanical and electrostatic principles. Density functional theory calculations showed that the chelating effect of amino groups, ion-exchange effect and electrostatic interaction of hydroxyl groups were the main factors to improve the removal efficiency of Cd (II) and Pb (II). These results provide a new method for the preparation of MOF based adsorbents and demonstrate the great potential of ethylenediamine modified copper based metal organic frameworks for high-performance environmental remediation.