Generation of hydroxyl radicals by metal-free bifunctional electrocatalysts for enhanced organics removal.

ABSTRACT

Low yields of H2O2 and a narrow range of appropriate pH values have been two major drawbacks for electro-Fenton (EF) process. Herein, metal-free electrochemical advanced oxidation processes (EAOPs) were developed with nitrogen and sulfur co-doped electrochemically exfoliated graphene (N, S-EEGr) electrocatalysts, which was confirmed as an outstanding bifunctional catalyst for synchronous generation and activation of H2O2 via (2 + 1) e- consecutive reduction reactions. Specifically, two elements (N, S) in metal-free N, S-EEGr-CF cathode synergize to promote the formation of H2O2 followed by its activation. With N, S-EEGr-CF cathode, phenol of initial 50 mg L-1 could be effectively removed within pH 3-11 and 6.25 mA cm-2, and 100% removal efficiency could be achieved within 15-min even at neutral pH. The pseudo-first-order rate constant for phenol removal in metal-free EAOPs with N,S-EEGr-CF at neutral pH was 10 times higher than that with EF process. Detection of active species, coupled with decay kinetics with specific trapping agents, confirmed that OH was the dominant oxidizing species promoting removal efficiencies of organics (phenol, antibiotics and dyes) at pH 3 and pH 7. In the actual wastewater treatment, the synergistic effect of bifunctional catalyst would also be used for improving the degradation efficiency of organics. Thus, the metal-free EAOPs with N,S-EEGr-CF cathode may serve as an alternative in wastewater treatment with a broadened range of solution pH values and avoiding Fe2+ (catalyst) addition.