Mechanistic insights into selective adsorption and separation of multi-component anionic dyes using magnetic zeolite imidazolate framework-67 composites

Abstract The development of highly efficient magnetic adsorbents, especially those aimed at adsorption and separation of multi-component dyes, is one of the new challenges in the water treatment field. In this study, zeolite imidazolate frameworks-67 (ZIF-67) loaded on magnetic nanoparticles named MZIF-67 composites was synthesized and applied to adsorption and separation of anionic dyes mixture. Direct blue 86 (DB-86) and methyl orange (MO) were selected as model contaminants. The most intriguing feature is that MZIF-67 can successfully separate DB-86 from mixed dye molecules in aqueous solution with high effectivity and selectivity, even for DB-86 molecule with a large size. The satisfactory adsorption and separation properties (adsorption capacity, equilibrium time, adaptability to solution changes and rapid magnetic separation) indicated MZIF-67 as an efficient adsorbent. Furthermore, the MZIF-67 before and after adsorption was carefully characterized by X-ray photoelectron spectroscopy (XPS) spectra which proved the main mechanism of MZIF-67 adsorption toward anionic dyes: ZIF-67 surface provided large number of uncoordinated cobalt sites bonding with hydroxide derived from water (Co–OH) and subsequently formed complex with anionic dyes under hydrogen bonding, accompanied by the electrostatic interaction between positively charged ZIF-67 and the anionic sulfonate group. This work is expected to provide insight mechanisms of MZIF-67 adsorption toward anionic dyes and offer promising magnetic ZIFs for multi-component system separation.