Superior removal of arsenic and lead from water using zinc based metal organic framework ZN-MOF-74.

Abstract

This study investigates and compares the removal of As(V) and Pb(II) from aqueous media using the water-stable zinc-metal organic frameworks (Zn-MOF-74) prepared via room-temperature precipitation (RT-Zn-MOF-74) and solvothermal procedure (HT-Zn-MOF-74). The Zn-MOF-74 crystals possess an average particle size of 66 nm and 144 µm for RT-Zn-MOF-74 and HT-Zn-MOF-74, respectively. Moreover, the nano-sized RT-Zn-MOF-74 exhibited superior performance to that of HT-Zn-MOF-74. While the BET surface area of the RT-Zn-MOF-74 was smaller than the HT-Zn-MOF-74, higher adsorption took place onto the room temperature synthesized ones, owing to their small particle size and better dispersion. Adsorption isotherm studies showed that Langmuir isotherm was effective for the adsorption of As(V) onto RT-Zn-MOF-74 and HT-Zn-MOF-74 with a maximum adsorption capacity (qmax) value of 99.0 mg g−1 and 48.7 mg g−1, respectively. Also Langmuir model was able to describe the data of Pb(II) removal using both RT and HT Zn-MOF-74 with adsorption capacities capacity (qmax) value of 487 mg g-1 and 329.13 mg g-1 respectively. These values exceed most reported maximum adsorption capacities at neutral pH and in the case of Pb(II) removal, 487 mg g-1 is the highest ever reported lead adsorption capacity in the literature. The thermodynamics of As(V) adsorption revealed a spontaneous highly endothermic process (ΔH=46.2 KJ mol-1) that is due to the substitution of adsorbed water molecules by arsenate in the pores of the MOF crystal resulting in a chemical adsorption process. For the removal of Pb(II) ions, the process was also found to be spontaneous and slightly endothermic (ΔH=20.9 KJ mol-1) and the main force controlling the interaction between Pb2+ and Zn-MOF-74 is Van der Waals resulting in a physical adsorption process. This was further investigated using plane-wave density functional theory (DFT) calculations. This study constitutes a direct evidence for the importance of tuning the size of MOF crystals to enhanc