Efficient removal of copper and silver ions in electroplating wastewater by magnetic-MOF-based hydrogel and a reuse case for photocatalytic application

Abstract

Direct discharge of electroplating wastewater containing hazardous metal ions such as Cu²⁺ and Ag ⁺ results in environmental pollution. In this study, we rationally prepare a magnetic composite hydrogel consisted of Fe₃O₄, UiO-66-NH₂, chitosan (CTS) and polyethyleneimine (PEI), namely Fe₃O₄@UiO-66-NH₂/CTS-PEI. Thanks to the strong attraction between the amino group and metal cations, the Fe₃O₄@UiO-66-NH₂/CTS-PEI hydrogel shows the maximum adsorption capacities of 321.67 mg g^-1 for Cu²⁺ ions and 226.88 mg g^-1 for Ag ⁺ ions within 120 min. As real scenario, the Fe₃O₄@UiO-66-NH₂/CTS-PEI hydrogel exhibits excellent removal efficiencies for metallic ions even in the complicated media of actual electroplating wastewater. In addition, we explore the competitive adsorption order of metal cations by using experimental characterization and theoretical calculations. The optimal configuration of CTS-PEI is also discovered with the density functional theory, and the water retention within hydrogel is simulated through molecular dynamics modeling. We find that the Fe₃O₄@UiO-66-NH₂/CTS-PEI hydrogel could be reused and after 5 cycles of adsorption-desorption, removal efficiency could maintain 80%. Finally, the Ag⁺ accumulated by hydrogel are reduced to generate a photocatalyst for efficient degradation of Rhodamine B. The novel magnetic hydrogel paves a promising path for efficient removal of heavy metal ions in wastewater and further resource utilization as photocatalysts.

Keywords: Adsorption; Heavy metal; Magnetic hydrogel; Photocatalytic; Resource utilization.