Three-dimensional macroporous graphene-wrapped zero-valent copper nanoparticles as efficient micro-electrolysis-promoted Fenton-like catalysts for metronidazole removal

Highlights: • Metronidazole was removed using 3D-GN@Cu⁰ as the catalyst in air-saturated solutions. • 3D-GN@Cu⁰ activated O₂ to generate H₂O₂ further forming ·OH. • Surface-bounded ·OH_ads was mainly responsible for the removal of metronidazole. • A micro-electrolysis-promoted Fenton-like reaction mechanism was proposed. -- Abstract: Three-dimensional macroporous graphene-wrapped zero-valent copper nanoparticles (3D-GN@Cu⁰) were synthesized using a self-assembly process of liquid-phase reduction and characterized by field emission scanning electron microscopy, nitrogen adsorption/desorption isotherms, X-ray diffraction, Raman spectrum analysis, and X-ray photoelectron spectroscopy. The catalytic activity of 3D-GN@Cu⁰ was evaluated in view of the effects of various systems, the pH value, catalyst dosage, initial metronidazole concentration and temperature, and it showed a high efficiency for removing metronidazole with saturated dissolved oxygen (without adding extra H₂O₂) in a wide range of pH value from 3.2 to 9.8. Combined with the results of dissolved oxygen activation, determination of reactive oxidizing species, and X-ray photoelectron spectroscopy (XPS) analysis, the surface-bounded ·OH_ads formed by the reaction of the in situ generation H₂O₂ with 3D-GN@Cu⁰ was mainly responsible for the removal of metronidazole. The charge distribution and electrostatic potential (ESP) of 3D-GN@Cu⁰ further illustrated the distribution and transfer of electrons on the catalyst surface, which predicted a micro-electrolysis-promoted Fenton-like reaction mechanism.