PVP modified rGO/CoFe2O4 magnetic adsorbents with a unique sandwich structure and superior adsorption performance for anionic and cationic dyes

In this work, polyvinylpyrrolidone (PVP) modified magnetic rGO/CoFe2O4 (PVP/rGO/CFO) composites are successfully synthesized by a simple hydrothermal method. The addition of PVP effectively changes the morphology of the PVP/rGO/CFO composites. When the mass ratio of PVP to rGO is 1:1 during the preparation of the composites (named as sample S3), a unique sandwich structure is formed through the self-assembly behavior between rGO nanosheets and CoFe2O4 nanoparticles with the aid of PVP. The adsorption capacity of the as-synthesized samples for different dyes is investigated. Here, it is noted that, owing to the specific microstructure, sample S3 exhibits superior adsorption properties, where, in terms of Langmuir isotherm model, the maximum adsorption capacities for MB, RhB and CR are 333.3, 558.7 and 355.9 mg g(-1), respectively. Because of the typical magnetic characteristics, the adsorbents can be easily separated by a magnet. The adsorption kinetics accords well with the pseudo-second-order model. It is demonstrated that the excellent adsorption performance of cationic dyes MB and RhB is attributed to the synergistic effect of pi-pi conjugation interactions and electrostatic interaction, while the adsorption of anionic dyes CR is mainly due to the interaction between its functional groups and the lactam group in PVP. This study provides a new route to develop graphene-based magnetic adsorbents to simultaneously remove cationic and anionic dyes in the wastewater.