Selective dye adsorption by highly water stable metal-organic framework: Long term stability analysis in aqueous media

Abstract

A highly water stable metal-organic framework (MOF) based on zirconium, i.e. UiO-66, was synthesized and then employed to adsorptive removal of an anionic dye, methyl orange (MO), and a cationic dye, methylene blue (MB), from aqueous solution. In this work, for the first time, the long term stability of UiO-66 in water was investigated for 12 months. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and N2 adsorption/desorption analysis were employed to monitor the textural alteration of UiO-66 during water aging. The results indicated that the structure of UiO-66 was mostly retained and its adsorption capacity toward dyes exhibited minor loss after long term water aging. Experimental data showed that adsorption capacity of UiO-66 toward MO was higher than that of MB, particularly at acidic and neutral environments, due to its stronger electrostatic attraction and π-π stacking. However, the adsorption capacity of UiO-66 changed with pH value of solution due to the alteration of electrostatic interactions caused by the change in zeta potential of the adsorbent. Thermodynamic analysis exhibited that the adsorption of both dyes on UiO-66 was spontaneous. Adsorption of MO on UiO-66 was dominated by enthalpy effects and exhibited stronger interaction compared with MB, while entropy effects governed the adsorption of MB. The adsorption kinetics of UiO-66 was described by pseudo-second-order and Freundlich isotherm model exhibited very good fit with adsorption data. Eventually, it was shown that UiO-66 demonstrated suitable reusability and kept mostly its initial structure as well as adsorption capacity after four consecutive adsorption-desorption experiments.