Removal of nitrobenzene from aqueous solution by graphene/biochar supported nanoscale zero-valent-iron: Reduction enhancement behavior and mechanism

To enhance the reduction capacity of nanoscale zero-valent-iron/biochar (nZVI/BC), novel composites modified by graphene (nZVI/GBC) were synthesized. The characterization demonstrated that nZVI/GBC-0.1 had larger specific surface area, higher graphitization level, more carbon defects and functional groups than nZVI/BC. The maximal nitrobenzene (NB) removal efficiency obtained by nZVI/GBC-0.1 was 71.7%, and the reduction removal increased by 2.64 times compared to nZVI/BC. XPS analysis revealed that carbon defect caused by graphene modification played a key role in the NB removal process. Furthermore, the electrochemical experiments demonstrated that graphene increased the current density in nZVI/GBC and the potential difference between nZVI and BC, and reduced the equivalent series resistance of nZVI/GBC. Accordingly, the electron releasing capacity could be enhanced. Then the electrons from nZVI/GBC composite could attack the NO2 on NB to form AN. Also, Fe2+ could reduce NB by donating electron. Moreover, among the functional groups on the surface of nZVI/GBC, the C-OH group was the main electron-donating group that could reduce Fe3+ to Fe2+. These findings provided insights for exploring the mechanism of improving the reduction capacity of nZVI/GBC composite.