High-density loading of bis Schiffs bases containing nitrogen, oxygen and sulphur active sites in UiO-66 for efficient and selective removal of heavy metals from water

Abstract

Although Schiff base ligand incorporation in UiO-66-NH₂ through post-synthesis modification has demonstrated potential for enhancing heavy metal adsorption selectivity, its application has been limited by low ligand loading efficiency and inadequate adsorption capacity. This study addressed these challenges by developing a methanol reflux pretreatment strategy to activate amino groups (increasing amino content from 57.72 % to 99.02 %) combined with bis-Schiff base structural design. This synergistic approach enabled a controlled high-density distribution of Schiff base groups and functional adsorption sites (hydroxyl/mercapto/amino) within the UiO-66 framework in order to obtain HD-UiO-66-AHB/AMB/DB. Characterization techniques confirmed the successful integration of multiple active sites and uniform ligand distribution, which established a synergistic adsorption network that significantly enhanced heavy metal adsorption performance. By comparison, HD-UiO-66-AMB has shown higher adsorption capacity for Pb and Cd with 340.16 and 155.28 mg/g at 298 K, respectively, while HD-UiO-66-AHB was better suited to adsorption of Hg (300.33 mg/g at 298 K), which was much more than other Schiff base-based adsorbents currently reported. All adsorption processes followed pseudo second-order kinetics and the Langmuir model. Furthermore, the coordination of heavy metal ions with the nitrogen atoms in the bis-Schiff base and the doped O/S/N atoms forms bidentate and tridentate complexes, endowing the material with a highly selective adsorption capability for target metal ions under the coexistence conditions of ten coexisting cations. After 5 adsorption-desorption cycles, the materials still possessed good adsorption effects, proving the excellent cyclic stability of materials. This study offers new perspectives on the application of Zr-MOFs in the wastewater.