Enhanced electro-Fenton degradation of sulfonamides using the N, S co-doped cathode: Mechanism for H2O2 formation and pollutants decay.

Facing low reactivity/selectivity of oxygen reduction reaction (ORR) in electro-Fenton (EF), N, S atoms were introduced into carbon-based cathode. "End-on" O2 adsorption was achieved by adjusting electronic nature via N doping, while *OOH binding capability was tuned by spin density variation via S doping. Results showed the optimized N, S co-doped cathode presented a 42.47% improvement of H2O2 accumulation (7.95 ± 0.02 mg L-1 cm-2). According to density functional theory (DFT), N, S co-doped structure favored the "end-on" O2 adsorption as adsorption energy dropped to - 2.24 eV. Moreover, O-O/C-O bond lengths variation proved a possibility for *OOH desorption. The elaborated cathode was used in EF for sulfonamides (SAs) decay. A 100% removal rate of sulfadiazine (SDZ), sulfathiazole (STZ) and sulfadimethoxine (SDM) was achieved within 60 min, among which SDZ tended to be degraded easily. Because the absolute hardness (η) of those pollutants is ranked as follows: ηSDM> ηSTZ> ηSDZ. Degradation pathways were proposed based on the detected byproducts, along with toxicity was evaluated by ecological structure-activity relationship (ECOSAR) program. Results showed that toxic intermediates generated were reduced or even disappeared. EF with N, S co-doped cathode provides a promising process for antibiotics wastewater treatment.