A comparative study of rigid and flexible MOFs for the adsorption of pharmaceuticals: Kinetics, isotherms and mechanisms

Recently metal-organic frameworks (MOFs) have attracted great attention in the field of environmental remediation. In this article, rigid MIL-101(Cr) and flexible MIL-53(Cr) were synthesized and used for the adsorption of two typical pharmaceuticals, clofibric acid (CA) and carbamazepine (CBZ), from water. The adsorption equilibrium was rapidly reached within 60 min and the kinetics best fitted with the pseudo-second-order kinetic model. There was no significant difference in the maximum adsorption capacity of CA on MIL-101(Cr) and MIL-53(Cr), and electrostatic interaction was suggested to be the main factor in the adsorption processes. However, for the removal of CBZ, MIL-53(Cr) showed much better adsorptive performance (0.428 mmol/g) than MIL-101(Cr) (0.0570 mmol/g), indicating the adsorption of CBZ on MOFs is affected by the structural property. The Powder X-ray diffraction analysis revealed that MIL-53(Cr) was transformed into large pore form, leading to variations in cell volume up to 33%, lower binding energy and crucial modifications of the hydrophobicity/hydrophilicity. This unusual behavior enhanced its adsorption capacity for CBZ. Moreover, hydrogen bonding and π-π interactions/stacking also contributed to the adsorption of pharmaceuticals on the MOFs. The excellent adsorptive performance of MIL-53(Cr) and its structure/property switching might lead to the applications in water treatment.