Insights into methyl orange adsorption behavior on a cadmium zeolitic-imidazolate framework Cd-ZIF-8: A joint experimental and theoretical study

Abstract

Dyeing wastewater from textile industries is often contaminated with chemical pollutants, which causes severe water pollution problems that are challenging to treat. Recently, Zeolitic imidazolate frameworks (ZIFs) have attracted extensive research interest due to their potential applications in the removal of pollutants from wastewater. Herein, a Cd- zeolitic-imidazolate framework (Cd-ZIF-8) was synthesized and used as an adsorbent for the removal of methyl orange (MO) at different operating conditions. The prepared adsorbent was characterized using a variety of techniques, including X-ray diffraction (XRD), the Brunauer Emmett Teller (BET) method, FT-IR and scanning electron microscope (SEM). Its stability after adsorption was confirmed by XRD and pH tests. The effects of different variables, such as adsorbent dose, initial concentration, pH and temperature, were investigated to optimize the adsorption process. Adsorption parameters were determined by applying Langmuir and Freundlich models, and good fitting was observed for the Langmuir isotherm. Maximum MO adsorption capacities on Cd-ZIF-8 were 93.24 and 145.41 mg g−1 at 30° and 50°, respectively. Possible controlling mechanism and the potential rate-limiting steps were analyzed using Lagergren's pseudo-first-order and pseudo-second-order models, and the data were found to follow the pseudo-second-order equation. The prepared material was found to have good stability after the adsorption with no release of Cd ions over a wide range of pH. For a further explanation of the adsorption mechanism, interactions of MO molecules with the Cd and organic linkers of ZIF-8 were investigated using Density Functional Theory (DFT) cluster calculations.