Hierarchically mesostructured MIL-101 metal-organic frameworks with different mineralizing agents for adsorptive removal of methyl orange and methylene blue from aqueous solution

Hierarchically mesostructured MIL-101 metal-organic frameworks (MOFs) were synthesized using anhydrous sodium acetate or hydrofluoric acid as a mineralizing agent and cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. These new materials were then used to adsorb and remove the anionic dye methyl orange (MO) and the cationic dye methylene blue (MB) from contaminated water. The adsorption performance was systematically investigated and discussed. Experimental results revealed that the amount of MB adsorbed on MIL-101 that was prepared using sodium acetate is about 3.2 times the amount adsorbed on MIL-101 prepared using hydrofluoric acid, but quantities of adsorbed MO by the two adsorbents were nearly the same. Adsorbents synthesized with higher CTAB content had a higher content of the meso- and macro-scale pores, which favored the MB adsorption but impeded the MO adsorption. Dye adsorption is partially governed by the electrostatic interaction between the dye and the adsorbent, because the mineralizing agent can vary the charge carried by framework in a dye solution. The MB and MO adsorption kinetics followed a pseudo-second-order model, and the equilibrium dye adsorption data provided a good Langmuir model.