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.