Adsorption of heavy metal ions from aqueous solutions using metal organic frameworks: kinetic and thermodynamic study

In this work, metal-organic-frameworks were employed as adsorbents to remove cobalt and manganese from aqueous solutions. In order to achieve this goal, the MIL-53(Al), MIL-101(Cr), and Cu-BTC were synthesized by hydrothermal method. The adsorbents were characterized by Brunauer-Emmett-Teller analysis, Fourier transform infrared, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, differential thermal analysis, and thermogravimetric analysis. After that, MIL-101(Cr) as the proper adsorbent was modified by ethylenediamine at different concentrations of 2, 5, and 10 mmol. MIL-101-5 was shown to possess excellent performance which is more than two times as that of MIL-101(Cr). The effects of varying parameters such as pH, temperature, initial metal concentration, and contact time on the adsorption process were examined. The adsorption kinetics was fitted well with a pseudo-second-order model and the adsorption data correlated with Langmuir, Freundlich, Temkin, and Redlich-Peterson isotherms. Finally, the Langmuir isotherm was found to fit the equilibrium data. The thermodynamic study showed that the adsorption process was a spontaneous and exothermic process because of negative values of Delta G and Delta H at all temperatures. The most desired parameters in the adsorption process were as follows: adsorption equilibrium time of about 90 min, the maximum adsorption of ions on the MIL-101-ED-5 was observed to occur at pH = 6.5, and the Co and Mn concentration of 30 mg L-1. The efficiency of cobalt and manganese at these conditions was identified as 90.32% and 87.33%, respectively.