Magnetic Fe3O4@C nanoparticles separated from cold rolling mill sludge for 4-nitrophenol reduction

Magnetic iron oxides are usually employed as main components of 4-nitrophenol (4-NP) reduction catalysts. Here, magnetic Fe3O4 nanoparticles (Rec-Fe3O4) recycled from rolling oil sludge (ROS) were directly applied in 4-NP reduction reaction. The Rec-Fe3O4 nanoparticles were high purity of Fe3O4 with 0.9 % C content, which were further probed as carboxylates and hydrocarbon alkyl species. Compared with commercial nano-Fe3O4 (Com-Fe3O4), the Rec-Fe3O4 exhibited excellent 4-NP reduction performance with easy magnetic separation showing complete 4-NP conversion to 4-AP within 270 s, and its activity remained stable even after 5 consecutive cycles. Its activation energy was also estimated to be far much less than that of Com-Fe3O4. Benefiting from its special spontaneous formed Fe3O4@C structure during ROS generation, more active low valence Fe species remained and C shell accelerated electronic transfer on the Rec-Fe3O4 surface, both of which led to superior activity during 4-NP reduction reaction. This finding opens up a new insight to both of utilizing ROS waste resource and green synthesis of highly efficient 4-NP reduction catalysts.