Mechanistic insight into the interaction and adsorption of Cr(VI) with zeolitic imidazolate framework-67 microcrystals from aqueous solution

In this study, we report on the synthesis of the well-defined rhombic dodecahedral cobalt-based zeolitic imidazolate framework-67 (ZIF-67) and demonstrate its fast kinetics and high adsorption efficiency towards chromium removal in water for the first time. The ZIF-67 shows good performance for the Cr(VI) removal from aqueous solution. The time needed to reach the equilibrium depends on the relevant initial concentrations, ranging from approximately 20 min for a low initial concentration of 6 mg L−1 to more than 60 min for a high initial concentration of 15 mg L−1. The equilibrium adsorption capacities are 5.88, 9.32, and 13.34 mg g−1 for 6, 10, and 15 mg L−1 of initial Cr(VI) solutions, respectively. The Cr(VI) removal process nicely fits the Langmuir isotherm model and its kinetics follows pseudo-second-order rate equation. The Cr(VI)-adsorbed ZIF-67 is carefully examined by energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), which suggest that the Cr(VI) removal mechanism could be attributed to the electrostatic adsorption of Cr(VI) anions by positively charged ZIF-67 and ion-exchange between the surface hydroxyl groups and Cr(VI), accompanying partial reduction of Cr(VI) to Cr(III) by the reactive framework on the ZIF-67. This work could provide a promising metal–organic framework for the Cr(VI) decontamination in water.