Hierarchical Reduced Graphene Oxide-MnO₂@ Polypyrrole Coaxial Nanotube Composite Hydrogel as a Potential Adsorbent for Cr(VI) Removal

A hierarchical reduced graphene oxide-MnO₂@polypyrrole coaxial nanotube composite hydrogel was prepared via oxidative polymerization of pyrrole in the presence of MnO₂ nanotubes, followed by the hydrothermal treatment of graphene oxide and MnO₂@polypyrrole coaxial nanotubes. The stable composite hydrogel with a hierarchical network was composed of one-dimensional MnO₂@polypyrrole coaxial nanotube and two-dimensional graphene nanosheet and characterized by scanning electron microscope, Fourier transform infrared spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller surface, and X-ray photoelectron spectroscopy measurements. The composite hydrogel can be used as an efficient adsorbent for Cr(VI) removal due to the synergistic interaction between graphene and MnO₂@polypyrrole and the hierarchical structure of the hydrogel. Moreover, the composite hydrogel is easily separated because of its stable monolith, and it is reusable (76.8% of removal ability remaining after five adsorption-desorption cycles). The simple fabrication and cost-effective separation process together with the excellent absorption performance endow the composite hydrogel with great potential for practical wastewater treatment.