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/cm(2), with the addition of 10mM PDS, 50 mg Pd/Al2O3, and 2 mg/L Fe ions, 80.12% of 180 mu M 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 H-2 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 center dot- and center dot OH via the Fe2+ -activated PDS process. In the Pd-EFP process, the optimal dosage of Fe ions was determined to be 36 mu M (2 mg/L). Correspondently, the optimal current density and PDS concentration in the Pd-EFP system were 3.33 mA/cm(2) and 20 mM, respectively. Furthermore, degradation of BA was efficiently promoted by the N-2 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 H-2 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.