Heterogeneous electro-Fenton process using a novel catalytic electrode for the degradation of direct dye from aqueous solutions: modeling, optimization, degradation pathway and toxicity evaluation

In this study, Fe3O4 magnetite nanoparticles were coated on the granular activated carbon (GAC) surface and packed in stainless steel (SS) mesh, which was named MGACSS. It was developed as a novel three-dimensional cathode in the heterogeneous electro-Fenton (HEF) process for direct dye removal from colored wastewater. According to the Box–Behnken design, the Direct Blue 80 (DB80) dye removal efficiency and chemical oxygen demand (COD) degradation under the optimal conditions of initial dye concentration of 35 mg/L, current of 0.09 A and reaction time of 18 min reached 98.97% and 66.66%, respectively. Furthermore, scavenger studies confirmed that the surface-bounded ·OH was the major oxidant responsible for the degradation of DB80 dye. The MGACSS electrode exhibited a dye removal efficiency of 98.56% even after six consecutive operations, indicating excellent stability and reusability. The efficiency of the MGACSS cathode demonstrated an excellent treatment performance in removing other direct dyes of Direct Brown 103 (DB 103), Direct Red 23 (DR23) and a real sample with 97.34%, 98.15% and 93.1% efficiency, respectively. This study suggested a highly efficient and stable novel electrode for removing direct dye from wastewater through the HEF process, over a wide pH range.