Understanding the adsorption of sulfonamide antibiotics on MIL-53s: Metal dependence of breathing effect and adsorptive performance in aqueous solution.

This investigation is based on experimental data to deeply understand the unusual adsorption behavior of the flexible MIL-53s in aqueous solution. In contrast to the strongly flexible MIL-53(Cr) and MIL-53(Al) with large pore form (lp) in water and in their anhydrous state, MIL-53(Fe) exhibits narrow pore form (np) or very narrow pore form (vnp), indicating that breathing effect depends on the nature of the metal. Sulfamethoxazole (SMZ) adsorption results demonstrated that the maximum adsorption capacities predicted by Langmuir model were 1.85, 1.78 and 0.314 mmol/g for MIL-53(Cr), MIL-53(Al), and MIL-53(Fe), respectively. The adsorption equilibrium was rapidly reached within 60 min and the kinetic data best fitted with the pseudo second order model. The lp form of MIL-53(Cr) and MIL-53(Al) in aqueous solution provided the easy entrance for contaminants, lead to lower binding energy and caused modifications of the hydrophobic/hydrophilic character, which all enhanced their adsorption capacities for SMZ. However, the np form of MIL-53(Fe) with small inner pores and hydrophilicity compromised its adsorption capacity for SMZ. The experimental results revealed electrostatic interactions, hydrogen bonding, and π-π interaction/stacking contributed to the adsorption of SMZ on MIL-53s as well. In summary, the complexation of different metal nodes to MOFs is accompanied by the diversity of properties, which significantly affect their adsorptive performance.

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.