Accelerated Fe2+ Regeneration in an Effective Electro-Fenton Process by Boosting Internal Electron Transfer to a Nitrogen-Conjugated Fe(III) Complex

The regeneration rate of Fe²⁺ from Fe³⁺ dictates the performance of the electro-Fenton (EF) process, represented by the amount of produced hydroxyl radicals (·OH). Current strategies for the acceleration of Fe²⁺ regeneration normally require additional chemical reagents, to vary the redox potential of Fe²⁺/Fe³⁺. Here, we report an attempt at using the intrinsic property of the electrode to our advantage, i.e., nitrogen-doped carbon aerogel (NDCA), as a reducing agent for the regeneration of Fe²⁺ without using foreign reagents. Moreover, the pyrrolic N in NDCA provides unpaired electrons through the carbon framework to reduce Fe³⁺, while the graphitic and pyridinic N coordinate with Fe³⁺ to form a C-O-Fe-N₂ bond, facilitating electron transfer from both the external circuit and pyrrolic N to Fe³⁺. Our Fe²⁺/NDCA-EF system exhibits a 5.8 ± 0.3 times higher performance, in terms of the amount of generated ·OH, than a traditional Fenton system using the same Fe²⁺ concentration. In the subsequent reaction, the Fe²⁺/NDCA-EF system demonstrates 100.0% removal of dimethyl phthalate, 3-chlorophenol, bisphenol A, and sulfamethoxazole with a low specific energy consumption of 0.17-0.36 kW·h·g^-1. Furthermore, 90.1 ± 0.6% removal of dissolved organic carbon and 83.3 ± 0.9% removal of NH₃-N are achieved in the treatment of domestic sewage. The purpose of this work is to present a novel strategy for the regeneration of Fe²⁺ in the EF process and also to elucidate the role of different N species of the carbonaceous electrode in contributing to the redox cycle of Fe²⁺/Fe³⁺.