Mitigating chlorite stress to sustain nitrogen removal in reclaimed water using bioelectrochemical constructed wetlands.

Residual chlorite (ClO2-) in reclaimed water has become an easily overlooked emerging contaminant, which exhibits significant cytotoxicity, genotoxicity, and ecological hazards in aquatic environments. Considering the redox potential difference between chlorite and nitrate, this study proposes a novel enhancement strategy utilizing bio-electrochemical constructed wetlands (BE-CWs). The nitrogen removal performance and microbial responses of conventional constructed wetlands (CWs), microbial fuel cell-coupled CWs, and microbial electrolysis cell-coupled CWs under ClO2- stress were systematically evaluated. Results indicated that ClO2- significantly inhibited nitrogen transformation in conventional CWs, causing ammonia and nitrite accumulation. Conversely, BE-CWs (specifically the biochar-based electrolysis system) maintained a robust total nitrogen removal of 92.7 ± 0.3% under 1.0 mg L-1 ClO2-. While all systems efficiently removed ClO2- at 89.0% to 97.9%, BE-CWs exhibited rapid upstream chlorite attenuation in the bottom layer, effectively mitigating oxidant exposure to downstream functional zones. Mechanistically, electrochemical stimulation enhanced microbial antioxidant enzyme activities, bolstering cellular defense. Functional community and gene analyses confirmed that BE-CWs preserved key denitrifying activities and enriched functional taxa like Dechloromonas (up to 18.10%), while fostering localized auxiliary pathways such as anammox in specific micro-environments. Overall, this study demonstrates that bio-electrochemical coupling effectively mitigates disinfectant byproduct toxicity, providing a promising optimization strategy for the stable operation of reclaimed water ecological buffer zones.