Designing a novel metal-organic framework@covalent organic framework composite for the selective removal of fluoroquinolones: Adsorption behaviors and theoretical investigation

Abstract

Increasing fluoroquinolone (FQ) pollution in aquatic environments is a growing global concern owing to its extensive use as therapeutic agents and growth promoters. This study aims to present a highly selective adsorbent for the removal of FQs. A binary metal-organic framework (MOF) and covalent organic framework (COF) composite (NH2-MIL-125@TpPa-SO3H) was successfully synthesized by a solvothermal method, wherein MOF comes from the NH2-MIL-125 of the Material of Institute Lavoisier (MIL) family and COF from SO3H-anchored Schiff-base TpPa-SO3H. The adsorption outcomes showed that the as-synthesized adsorbent considerably improved the removal efficiency of FQs in water environments compared to independent MOFs or COFs. The adsorbent also displayed remarkable stability after 8 cycles of adsorption-desorption. According to the Sips isotherm model, the maximum adsorption capacities were 444.7, 455.7, 457.5, and 429.1 mg·g−1 for enoxacin, norfloxacin, ciprofloxacin, and sparfloxacin, respectively, and the adsorption kinetics were in accordance with the pseudo-second-order model. Moreover, the selective adsorption mechanism was intensively explored through characterizations and density functional theory, showing the synergistic effects of electrostatic interaction, hydrogen bonding, C–H⋯π interaction, and hydrophilic interaction. This study provides a new strategy for designing composite adsorbents for the removal of organic pollutants in the field of environmental remediation.