Dual inhibitory effects of high ammonia and nitrite accumulation on biocathode in three-electrode bio-electrochemical system: Electrochemical activity and microbial succession

Applying a constant potential (20 mV vs. saturated calomel electrode) to a three-electrode bio-electrochemical system is considered an effective method to investigate the biocathode's response to high ammonia load without other interferences. In this study, the ammonia nitrogen concentration was gradually increased in steps of 100, 300, 450, and 600 mg L⁻¹. The sharp decrease in the maximum output current, from 26.12 ± 1.51 mA to 11.64 ± 1.35 mA, was partly due to the inhibition caused by free ammonia. Meanwhile, the prolonged duration of nitrite accumulation with increasing ammonia nitrogen correlated with the lag phase of output current, suggesting that nitrite toxicity was another factor limiting the biocathode's output performance. Moreover, the cathodic reduction current was significantly inhibited by higher ammonia concentrations, with a marked decline when ammonia nitrogen levels exceeded 450 mg L⁻¹. However, the increase of conductivity based on high ammonium concentration only slightly reduced the charge transfer resistances (Rct) but could not fully counteract the free ammonia inhibition. Notably, the dual inhibitory effects of high ammonia and nitrite accumulation was also reflected in the reduced richness of microbial community, along with a decrease in the abundance of denitrifying functional bacteria (e.g., Ferruginibacter) and nitrifying bacteria (e.g., Nitrospira). Thus, a comprehensive investigation into the inhibitory effects of different ammonia loads on the biocathode provides theoretical support for understanding the impact of high ammonia shocks on biocathode in microbial electrochemical system (MES) during practical wastewater treatment.