A charcoal-shaped catalyst NiFe₂O₄/Fe₂O₃ in electro-Fenton: high activity, wide pH range and catalytic mechanism

A charcoal-shaped catalyst NiFe₂O₄/Fe₂O₃ in electro-Fenton (EF) was synthesized by a facile precipitation approach via sintering products of oxalate co-precipitation. This obtained NiFe₂O₄/Fe₂O₃ catalyst was easily separated via an external magnetic field and was used as a heterogeneous electro-Fenton catalyst for rhodamine B (RhB, a target pollutant) degradation. Characteristics of NiFe₂O₄/Fe₂O₃ catalyst were assessed using scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Barrett-Emmett-Teller (BET), respectively. SEM results revealed that the proposed NiFe₂O₄/Fe₂O₃ was charcoal-shaped with the size in the range of 0.5-5 μm. Experiment results show that the EF process with the proposed catalyst could work in a wide pH range from 3 to 9. Under optimized conditions, estimated 90% RhB degradation was achieved in 60 min under the following conditions: 0.6 g/L NiFe₂O₄/Fe₂O₃, pH 3. Radical scavengers and electron spin resonance (ESR) spectra results demonstrated that the main oxidant species involved was $\cdot \text{OH}$, accounting for RhB degradation in EF. Moreover, according to our research on interfacial reaction, $\cdot \text{OH}$ was mainly generated from the homogenous Fenton reaction rather than the surface Fenton reaction, stimulating by the dissolved Fe²⁺, Fe³⁺ and Ni²⁺ from catalyst. The reusability of NiFe₂O₄/Fe₂O₃ catalyst was evaluated for recycling the same catalyst for 5 runs. In conclusion, the facile fabrication NiFe₂O₄/Fe₂O₃ catalyst shows great potential in wastewater treatment with promising activity.