Removal performance and adsorption kinetics of dyes by a Co-based metal organic framework

A succession of Zr-based porphyrinic metal-organic frameworks (Zr-MOFs) are synthesized with metalation and utilized as adsorbents for removal of the cationic dye malachite green (MG) and the anionic dye methyl orange (MO). Results show the optimal adsorbent is MOF-525(Co) and the removal efficiencies for MG and MO by MOF-525(Co) are 99.0% and 90.5% at 60 min as the initial dye concentration is 100 mg L−1. The N2 adsorption/desorption results indicate the high surface area, micropore of MOF-525(Co); the FT-IR spectra and adsorption thermodynamics results demonstrate that the adsorption mechanisms of both MG and MO by MOF-525(Co) belong to the π-π stacking interactions of physical adsorption, and the hydrogen bonds and Co–O bond of chemical adsorption. Additionally, the optimal pH of dye adsorption is 7.0 for MG and 3.0 for MO, and the adsorption capacities of MG and MO decrease in saline solution. The adsorption kinetics reveal that the pseudo-second-order kinetic model is better for illustrating the adsorption performances of both dyes by MOF-525(Co); the adsorption isotherm models suggest that the Freundlich model is fitted for MG adsorption but the Langmuir is fitted for MO. Furthermore, the adsorption processes can be demonstrated as spontaneous, exothermic for MG with negative entropy but endothermic for MO with positive entropy by thermodynamics data. In conclusion, MOF-525(Co) possesses a significant adsorption ability and excellent reusability for MG, and it could be a promising adsorbent for MG removal.