Electrostimulation enhances nitrate removal and mitigates nitrite accumulation from denitrification-induced...

Groundwater nitrate contamination has become a global environmental concern. The self-alkalization during denitrification often results in nitrite accumulation, limiting nitrogen removal rate. In this study, a cotton-based biofilm electrode reactor (CBER) was constructed to investigate the effects of electrical stimulation on denitrification performance, microbial enzyme activity, and microbial community. The system pH increased to 9.68, indicating severe self-alkalization with nitrite accumulation of 16.77 ± 1.71 mg-N/L at 0 mA/m². In contrast, nitrate removal rate (NRR) increased 21.4 % to 0.68 mg-N/(L·h) at 100 mA/m², while nitrite accumulation decreased to 0.14 ± 0.05 mg-N/L, representing a 99.2 % decrease compared with that at 0 mA/m². Electrical stimulation effectively suppressed nitrite accumulation by enhancing nitrite reductase activity from 0.20 to 0.34 U/mg prot (0-100 mA/m²). Enzyme activity assays demonstrated that moderate electrical stimulation significantly enhanced the activities of nitrate and nitrite reductases, whereas high current density 300 mA/m² induced oxidative stress, thereby suppressing microbial activity. Microbial community analysis revealed that electrical stimulation increased the relative abundance of Proteobacteria from 60.2 % to 82.6 %. Concurrently, the key functional genus Pseudomonas increased from 1.1 % to 13.1 %, indicating a functional shift from organic degradation to nitrogen removal. This study reveals a synergistic mechanism how electrical stimulation suppresses self-alkalization through enzyme regulation and community restructuring. These findings provide important insights for the design and optimization of bioelectrochemical nitrogen removal technologies.