Kinetic and thermodynamic studies of neutral dye removal from water using zirconium metal-organic framework analogues

A series of isostructural water stable UiO-66-X (X = H, NH2, NO2), one of the most well-known metal-organic framework (MOF) materials, was synthesized by solvothermal synthesis. Their dye adsorption capacity toward a neutral dye, Phenol Red (PR), was investigated for the first time in this research in order to elucidate how the functional group of the organic linker contribute to the adsorption capacity. The operational parameters, i.e. pH, adsorbent concentration, initial dye concentration, contact time and temperature were monitored. The maximum adsorption capacities of UiO-66, UiO-66-NO2 and UiO-66-NH2 were measured to be 19 mg/g, 11 mg/g and 27 mg/g, respectively. The adsorption kinetics of all three adsorbents were similar and fitted best with pseudo-second-order kinetics model. Adsorption isotherms of the adsorbents can be explained by Freundlich model which suggests that the adsorption of PR on the adsorbents can be both physisorption and chemisorption. Thermodynamic studies indicated that the adsorption process in all studied adsorbents was spontaneous, endothermic and entropic-driven which was confirmed by the obtained thermodynamic variables. Functional group of the organic ligand was found to contribute significantly to the adsorption capacity of MOFs. The adsorbent with a functional group that can create more intermolecular forces will enhance the adsorption capacity. Experimentally, UiO-66-NH2 showed the highest adsorption capacity when compared to UiO-66 and UiO-66-NO2.