Superior adsorptive removal of azo dyes from aqueous solution by a Ni(II)-doped metal–organic framework

Abstract

Toxic azo dyes pose a significant threat to all living organisms when they are directly discharged into aqueous environments. Therefore, new adsorbents with excellent adsorptive efficiencies are urgently required for the removal of azo dyes. Herein, the adsorption space, surface charge, morphological and structural defects, and mesostructure of MIL-101(Cr) were adjusted by Ni(II) doping to understand the effects of these factors on the adsorptive removal of anionic azo dyes such as Congo red (CR), methyl orange (MO), and acid chrome blue K (AC) from aqueous solution. The adsorption uptakes of Ni(II)-doped MIL-101(Cr) toward CR (1607.4 mg g−1) and MO (651.2 mg g−1) exceeded those of undoped MIL-101(Cr) by 31.4% and 23.4%, respectively, whereas Ni(II) doping decreased the adsorption uptake toward AC (161.0 mg g−1) by 50.2%. Ni(II) doping produced morphological and structural defects, resulting in a new mesoporous structure, which played a more prominent role than electrostatic interactions and adsorption space in increasing the adsorption uptake toward linear anionic azo dyes, such as CR and MO. Despite these effects, for nonlinear azo dyes such as AC the adsorption uptake was decreased by steric hindrance between the Ni(II)-doped MIL-101(Cr) adsorbent and the dye. Thus, the adsorption performance of Ni(II)-doped MIL-101(Cr) was found to be controlled by the synergetic interplay among electrostatic interactions, π–π interactions, steric hindrance, and the morphological and structural defects, pore volume, and mesoporous structure of the adsorbent.