Accelerated Fe³⁺/Fe²⁺ cycle using atomic H* on Pd/Al₂O₃: A novel mechanism for an electrochemical system with particle electrode for iron sludge reduction in the Fe²⁺/peroxydisulfate oxidation process

The high cost associated with the disposal of iron sludge in Fe²⁺ activated oxidation systems significantly limits their widespread use. In this study, we constructed a trace iron-based peroxydisulfate (PDS) oxidation system (Pd-EFP) using Pd/Al₂O₃ as the particle electrode and externally added PDS as an oxidant. At an initial solution pH of 3.0 and a current density of 3.33 mA/cm², with the addition of 10 mM PDS, 50 mg Pd/Al₂O₃, and 2 mg/L Fe ions, 80.12% of 180 µM benzoic acid (BA) was degraded within 120 min. The Pd/Al₂O₃ catalyst provided sufficiently large surface area for atomic H* production from the adsorption of electrogenerated H₂ or H⁺ conversion via electro-induction on the Pd/Al₂O₃ surface, which subsequently accelerated the transformation from Fe³⁺ to Fe²⁺. Using this method, organics could be degraded by both SO₄ ^·- and ·OH via the Fe²⁺-activated PDS process. In the Pd-EFP process, the optimal dosage of Fe ions was determined to be 36 μM (2 mg/L). Correspondently, the optimal current density and PDS concentration in the Pd-EFP system were 3.33 mA/cm² and 20 mM, respectively. Furthermore, degradation of BA was efficiently promoted by the N₂ atmosphere, which could steer the reaction on the surface of Pd/Al₂O₃ in the right direction toward Fe³⁺ reduction by atomic H*, by dispelling accumulated H₂ above the reaction liquid and suppressing oxygen reduction. Finally, the Pd/Al₂O₃ catalyst was found to be durable in the Pd-EFP system according to reusability experiments and X-ray diffraction patterns of the fresh and used Pd/Al₂O₃ catalyst. This research provides an environmentally benign system for recycling Fe³⁺ in Fe²⁺/PDS processes and for suppressing iron sludge production.