Using flow-through reactor to enhance ferric ions electrochemical regeneration in electro-fenton for wastewater treatment

Abstract

Electrochemical regeneration of ferric ions catches more and more attention since it could decrease iron sludge to avoid the sludge problem while it could simultaneously obtain similar robust removal efficiency to refractory organic contaminants in wastewater treatment as the normal Fenton agent method. However, the electrochemical regeneration of ferric ions in aqueous solution was very slow. In this paper, the mass transfer limit of ferric ions in electro-Fenton reaction was evaluated. Koutecky-Levich equation reveals an extremely low diffusion coefficient (D₀) value of ferric ions since its complex coordination of [Fe(HO)_x(H₂O)_6-x]^3-x. The D₀ value was only 2.70 × 10⁻⁶ cm²·s⁻¹. Flow-through reactor was therefore introduced in which the ferric ions was designed to penetrate through the 73.1 μm porous holes in the graphite fiber electrode. The μm-scaled confinement of ferric ions diffusion inside the holes was proved to successfully enhance the reduction current of ferric ions by more than 200 % since the diffusion distance of the ferric ions was significantly decreased in the flow-through reactor. However, besides the benefit of the flow-through reactor, the ferric ions reduction electro-Fenton (FeRR electro-Fenton) in flow-through still faces both the pH limit and H₂O₂ decomposition problems. Hydrogen evolution reaction (HER) could also cause the decrease of pH which exceeded the optimal pH window for Fenton and therefore destroyed the electro-Fenton reaction consequently although the electrochemical reaction of FeRR was 770 mV prior to HER reaction. The regeneration of Fe (II) process simultaneous destruction of H₂O₂ since H₂O₂ e-reduction was 120 mV prior to FeRR reaction, which resulted in only very low H₂O₂ concentration suitable for FeRR electro-Fenton. Even using stepwise addition of H₂O₂ in electro-Fenton, the decomposition of H₂O₂ still could not be avoided. Although the decomposition of H₂O₂ quite limited the application of FeRR electro-Fenton in real application, FeRR electro-Fenton still supports the enhancement removal efficiency of the refractory organic contaminants under low organic contaminants application.