Preparation of tourmaline-supported MIL-125-NH₂@UiO-66-NH₂ composite and its norfloxacin adsorption performance

To address the demand for efficient removal of norfloxacin (NOR), a typical fluoroquinolone antibiotic prevalent in aqueous environments, natural tourmaline (TM) was used as substrate, and a two-step solvothermal route with surfactant assistance was applied to successfully synthesize a MOF-on-MOF composite adsorbent TM/UM-NH₂ with MIL-125-NH₂@UiO-66-NH₂ supported on tourmaline. The structural properties of TM/UM-NH₂ were systematically characterized via XRD, FT-IR, SEM, XPS and N₂ adsorption-desorption isotherms. Meanwhile, the adsorption performance of TM/UM-NH₂ toward NOR was comprehensively evaluated via batch static adsorption experiments. The results showed that TM/UM-NH₂ exhibited significantly higher specific pore size and pore volume than that of the single-component MOFs. Its adsorption capacity (210.08 mg/g) for 50 mg/L NOR solution was 1.3 times that of TM/MIL-125-NH₂ (164.27 mg/g) and 1.7 times that of pristine UiO-66-NH₂ (120.78 mg/g). Such enhanced adsorption performance can be attributed to the dual-MOF composite structure, which provided more abundant active adsorption sites and a more developed pore network. The adsorption kinetic process was highly consistent with the pseudo-second-order kinetic model. The adsorption isotherms were well fitted by both the Langmuir and Freundlich models, and the adsorption capacity increased with the rising system temperature, confirming that the adsorption process was endothermic. Hence, the adsorption of NOR onto TM/UM-NH₂ was dominated by chemisorption with mixed monolayer-multilayer behavior, mainly driven by electrostatic interaction synergistically regulated by hydrogen bonding and π-π stacking. This study not only provides a novel high-performance adsorption material for the efficient removal of NOR from water, but also offers theoretical insights for the design, fabrication, and environmental applications of MOF-on-MOF heterogeneous structures.