Self-propelled nanomotors based on hierarchical metal-organic framework composites for the removal of heavy metal ions

The outstanding performance efficiency for the removal of heavy metal ions in solution is governed by various factors: (a) sufficient contact probability between heavy metal ions and the adsorbent, (b) convenient diffusion/accessibility of heavy metal ions to the surface and the interior of the adsorbent, and (c) abundant binding sites for heavy metal ions on the adsorbent. We designed an efficient MnFe2O4 @MIL-53 @UiO-66 @MnO2 adsorbent for Pb(II) and Cd(II) removal. The adsorbents were fabricated by merging self-propelled nanomotors, exploiting hierarchical structure, and using a metal-organic framework (MOF) composite to simultaneously meet the three requirements. The sufficient contact probability between Pb(II)/Cd(II) and MnFe2O4 @MIL-53 @UiO-66 @MnO2 was achieved via the self-propelled movement of MnFe2O4 @MIL-53 @UiO-66 @MnO2 which was induced by the catalytic decomposition of H2O2 by MnO2. The convenient diffusion/accessibility of Pb(II)/Cd(II) on the surface and interior of MnFe2O4 @MIL-53 @UiO-66 @MnO2 was achieved by exploiting the properties of the hierarchical structure of MnFe2O4 @MIL-53 @UiO-66 @MnO2. Abundant binding sites (-COOH) on MIL-53 and UiO-66 composites were present for the binding of the Pb(II)/Cd(II) ions to the adsorbent. The adsorption capacities of the nanomotor adsorbent for Pb(II) and Cd(II) were 1018 and 440.8 mg g-1 at 25 °C, respectively. Additionally, the complex formed of MnFe2O4 and MIL-53 endowed the adsorbent with easy-recyclable properties under the influence of an external magnet. The nanomotors exhibit satisfactory removal performances for Pb(II) and Cd(II).