Unveiling microbial mechanisms for fluoride and nutrient removal in iron-carbon constructed wetlands under micro-oxygen regulation.

The co-occurrence of fluoride (F⁻) and nutrient pollutants in wastewater poses a significant challenge for treatment processes due to their distinct physicochemical behaviors. Constructed wetlands (CWs), as ecologically adaptive systems, offer nature-based solutions for the integrated attenuation of multifaceted contaminant mixtures. This study evaluated five types of CWs with varying substrates under micro-oxygen regulation, focusing on iron-carbon (Fe-C) micro-electrolysis for enhanced removal and the microbial response of F⁻, nitrogen (N), and phosphorus (P) under continuous flow conditions. The Fe-C CWs (CW-E) achieved the highest F⁻ removal efficiency (40.14 ± 15.81 %) and sustained total phosphorus (TP) removal (up to 99 %) under F^- stress, while maintaining moderate total nitrogen (TN) removal (72 %). Comparative analysis showed that CW-E outperformed other configurations in simultaneous multi-pollutant removal. However, CW with separately filled Fe-C substrates (CW-D) showed more stable nitrogen removal, indicating that substrate combination affects specific pollutant behavior. Fe-C micro-electrolysis CW under F^- and nutrient stress reduced microbial diversity but enriched key electroactive and functional bacteria (Bacillus, Desulfomicrobium) associated with extracellular electron transfer, N transformation, and P accumulation. Functional genes related to electron transfer (e.g., cyt c, pili, NADH dehydrogenase) and quorum sensing (QS) were upregulated, indicating that micro-electrolysis reshaped microbial community structure and function. Moreover, QS was significantly positive (P < 0.001) with the direct electron transfer (DET) process, indicating the role of DET in microbial cooperation. This work demonstrated that Fe-C micro-electrolysis CWs enhanced microbial function toward pollutant stress, providing a potential intensification strategy for multi-pollutant treatment in decentralized wastewater systems.