A Fixed-Bed Reaction System Packed with a Graphene/Multiwalled Carbon Nanotube Scaffold for the Oxidative Removal of Naproxen

Most common adsorbents in research reports are powdered two-dimensional carbon materials, including powdered activated carbon and graphene, which tend to be lost during water treatment and cause secondary contamination. In this work, a hydrogel composed of three-dimensional graphene-doped acidified multiwalled carbon nanotubes (3DG/A-MWCNTs) was synthesized. The hydrogels with large specific surface areas and defect structures exhibited outstanding integration, compressive capacity, and negligible loss. The as-obtained material was added to a column as a continuous fixed-bed reactor (FBR). The 3DG/A-MWCNTs packed in the FBR exhibited superior adsorption and peroxymonosulfate (PMS) activation to achieve 56.2% naproxen (NPX) removal efficiency after 1150 min. The operation parameters were systematically optimized including the PMS concentration, initial pH, coexisting anions, bed height, and hydraulic retention time. Significantly, quenching, electron paramagnetic resonance, and electrochemical tests were used to demonstrate that reactive oxygen species (SO4•–, •OH, O2•–, and 1O2) and electron transfer were involved in NPX degradation. The presence of active sites, oxygen vacancies, electron-rich, oxygen-containing functional groups, and defect structures within 3DG/A-MWCNTs promoted their preeminent catalytic activity. The FBR maintains a removal efficiency of 66.2% for NPX after six repetitions and exhibits excellent purification ability for various pollutants and actual surface water. These results suggest that FBR performs well in practical applications. Overall, this study expands the environmental application of three-dimensional graphene and provides promising metal-free carbon materials for eliminating refractory contaminants in continuous flow mode by using adsorption and PMS-based advanced oxidation processes.