Core@Shell structured coal fly ash Magnetospheres@C/g-C3N4 for degradation of Rh B via photo-Fenton catalysis

The photo-Fenton technology has shown widespread application potential in the degradation of wastewater. Herein, solid waste coal fly ash magnetospheres (MS) composed of a large amount of Fe3O4 and Fe2O3 were used as raw materials to prepare a novel photo-Fenton catalyst MS@C/g-C3N4. The SEM characterization results showed MS@C had a special core@shell structure with many micro-cracks, and for MS@C/g-C3N4, the g-C3N4 nanosheets not only intersperse in the cracks of MS@C, but also adhere to the surface of MS@C. The FTIR characterization results showed the little changes of the peaks corresponding to -CN heterocycles in MS@C/g-C3N4 may be caused by the interaction between g-C3N4 and MS@C. The photo-Fenton performance of MS@C/g-C3N4 composites was evaluated by degrading of Rh B. MS@C/g-C3N4 composites displayed the highest rate constant (0.1532 min−1) than MS@C (0.0671 min−1) and pure g-C3N4 (0.0018 min−1)as well as samples of mixture of MS@C and g-C3N4 (0.0745 min−1) due to the presence of the synergistic effect between photocatalysis and Fenton reaction. The effects of carbon shells thickness, H2O2 concentration, Rh B concentration and pH value on the photo-Fenton degradation of Rh B by MS@C/g-C3N4 were studied. The ICP results showed that the carbon shells had an inhibitory effect on the leaching of iron ions, and the optimal photo-Fenton degradation conditions were H2O2 concentration of 50 mmol/L, Rh B concentration of 100 mg/L and pH = 3. The recycle experiment showed that the degradation rate of MS@C/g-C3N4 composites was only slightly decreased about 5% after four cycles, and the XRD results for the fresh MS@C/g-C3N4 and used MS@C/g-C3N4 showed that there were no obvious changes occurred in the crystal structure after fourth cycle, which indicated that MS@C/g-C3N4 possessed excellent stability. The main active species were •OH and h+ in this system, and a possible photo-Fenton mechanism was proposed. The carbon shells (coated on MS) could enhance the separation efficiency of photogenerated electron-hole pairs of g-C3N4. In addition, the leached iron ions in MS@C/g-C3N4 could be anchored to the surface by the pyridine nitrogen in g-C3N4 to form Fe-N bonds, which could accelerate the transfer of electrons and be regarded as the active site in the Fenton reaction.