Enhanced adsorptive removal of anionic and cationic dyes from single or mixed dye solutions using MOF PCN-222

Metal-organic frameworks (MOFs) are attractive for the removal of industrial dyes from aqueous pollution. However, effective simultaneous removal of oppositely charged dye ions is still a challenge. We find that zirconium-metalloporphyrin mesoMOF (PCN-222/MOF-545) exhibits excellent adsorption/removal capacities for numerous anionic and cationic dyes individually and together in solution. PCN-222 is fabricated by solvothermal synthesis and characterized by powder XRD and FT-IR methods to confirm its structure. N-2 adsorption/desorption indicates PCN-222 has a large pore size of 3.2 nm and a surface area of 2336 m(2) g(-1). The zeta potential measurement shows that PCN-222 has a slight basic isoelectric point at pH 8 with appropriate potentials of 23.5 and -13.6 mV in the range of pH 3-10. These features facilitate the dual-function of PCN-222 toward anionic and cationic dyes, or even their complexes. The results show that PCN-222 has a maximum loading efficacy of 906 mg g(-1) for anionic methylene blue (MB) and 589 mg g(-1) for cationic methyl orange (MO) in a single dye system. Interestingly, when MB and MO co-exist in solution, they mutually enhance the capability by 36.8% (1239 mg g (-1)) for MB and 73.5% (1022 mg g(-1)) for MO. The adsorption kinetics, isotherms, stability and reusability of PCN-222 are reported. The results reveal that no significant changes of either crystal structure or loading capacity are observed up to eight recycles during the removal of MO and MB in solution. The adsorption efficacies of eight representative dyes are presented. Compared with previously published data, the capacity of PCN222 ranks at the top. We prepared a facile adsorption chromatography column to demonstrate the broad application potentials of PCN-222 in dye removal from aqueous pollution. Finally, we propose a pushpull mechanism to explain the mutual enhancement of adsorption in a mixture dye solution.