Performance and mechanism of cathodic treatment of 2-chloro-4-nitrophenol contaminated wastewater in microbial electrolysis cells

2-Chloro-4-nitrophenol （2C4NP） is a typical chloronitrophenol （CNPs）， characterized by persistence and high environmental toxicity. Currently， research on the degradation of 2C4NP by microbial electrolysis cells （MECs） is still limited， and the degradation mechanism remains unclear. In this study， a two-chamber MECs was constructed to systematically investigate the effects of external voltage， initial concentration of 2C4NP， and co-substrate types on the cathodic degradation of 2C4NP. Scanning electron microscopy （SEM） was used to analyze the microbial morphology， and high-performance liquid chromatography （HPLC）， ion chromatography （IC）， and LC/MS/MS were employed for qualitative and quantitative analysis of 2C4NP and its degradation intermediates， thereby elucidating the mechanism of 2C4NP degradation at the MECs cathode. Results showed that the application of an appropriate external voltage significantly promoted the degradation of 2C4NP. The initial concentration of 2C4NP had a significant impact on the degradation rate， and the degradation process followed different reaction kinetics under different co-substrate conditions. Under the optimal conditions （external voltage of 0.5 V， co-substrate of glucose， and initial concentration of 2C4NP at 30 mg/L）， the 72 h removal rate and dechlorination rate of 2C4NP reached 96.77% and 67.74%， respectively. SEM analysis revealed that microorganisms on the cathode surface aggregated， enhancing electron transfer efficiency among microorganisms and providing a basis for efficient degradation. HPLC， IC， and LC/MS/MS analyses indicated that the degradation of 2C4NP produced intermediate products such as 4-nitrophenol， paracetamol， and 2-chlorophenol. The partially removed chlorine and nitrogen were detected as Cl⁻ and NO3-. These results suggest that the degradation of 2C4NP in the MEC cathode chamber mainly occurs through reductive dechlorination and denitration reactions.