Preparation of the Carbonized Zif-8@PAN Nanofiber Membrane for Cadmium Ion Adsorption

The zeolitic imidazolate framework (ZIF-8)@polyacrylonitrile (PAN) nanofiber membrane was prepared and carbonized for heavy metal cadmium ion (Cd²⁺) adsorption in aqueous medium. Zinc oxide (ZnO) was first sputtered onto the surface of the PAN electrospun nanofiber membrane to provide a metal ion source. Then, the ZIF-8@PAN nanofiber membrane was prepared via in situ solvothermal reaction and carbonized in a tube furnace at 900 °C under a N₂ atmosphere to enhance adsorption performance. The synthesized ZIF-8 particles with polyhedral structure were uniformly immobilized on the surface of the PAN electrospun nanofiber membrane. After being heated at 900 °C, the polygonal ZIF-8 shrank, and the carbonized ZIF-8@PAN nanofiber membrane was obtained. Compared with the nanofiber membrane without being carbonized, the adsorption capacity of the carbonized ZIF-8@PAN nanofiber membrane reached 102 mg L^-1, and its Cd²⁺ adsorption efficiency could be more than 90% under the adsorption temperature of 35 °C and solution of pH = 7.5 conditions. According to the adsorption thermodynamics analysis, the Cd²⁺ adsorption process of the carbonized ZIF-8@PAN nanofiber membrane was spontaneous. The whole Cd²⁺ adsorption process was more suitably described by the pseudo second-order adsorption kinetics model, indicating that there exists a chemical adsorption mechanism besides physical adsorption.