Selective and Competitive Adsorption of Azo Dyes on the Metal–Organic Framework ZIF-67

Zeolitic imidazole frameworks (ZIFs), a new adsorbent with a high chemical and thermal stability and a high adsorption capacity, are used for adsorptive removal of azo dyes. The synthesized ZIF-67 was characterized with Fourier transform infrared spectroscopy (FTIR), thermogravimetric–differential thermal analysis (TG–DTA) and zeta potential instrument. The adsorption of some azo dyes on ZIF-67 in the single dye systems showed that the removal efficiencies are congo red > methyl orange > methyl red > methyl blue. The highest adsorption capacity of congo red and methyl orange were 3900 and 1340 mg/g, respectively. In a binary dye system the adsorption capacity of congo red decreased, while the removal efficiency of methyl orange increased in comparison with the single systems, indicating that a competitive adsorption of congo red and methyl orange over the ZIF-67 occurred. The experimental data indicate that the electrostatic attraction between ZIF-67 and congo red is the major driving force and the π – π stacking is also responsible for dye adsorption. After 5 cycles of ZIF-67 adsorption and desorption, the congo red removal efficiency maintained more than 95%. The adsorption of Congo red and Methyl orange on the ZIF-67