Current-dependent Nutrient Removal and Microbial- Electrochemical Synergy in an Iron-carbon Enhanced Vertical Flow Constructed Wetland

Constructed wetlands (CWs) frequently exhibit suboptimal nitrogen and phosphorus removal under low carbon-to-nitrogen conditions due to electron donor limitations and insufficient phosphorus retention. This study investigates a vertical flow constructed wetland (VFCW) hybridized with iron-carbon micro-electrolysis (ICME) and external electrochemical intensification (EI) to overcome these constraints. The system was operated under simulated nutrient-rich influent (COD: 250 mg/L, NH₃-N: 30 mg/L, TN: 130 mg/L, TP: 5 mg/L) across four direct current intensities (0, 10, 20, and 40 mA). Results revealed a distinct current-dependent performance threshold: moderate electrochemical stimulation (20 mA; 0.016 mA/cm²) maximized nutrient attenuation, achieving 90% COD, 92% NH₃-N, 85% TN, and 37% TP removal. Oxidation-reduction potential (ORP) decreased linearly with current, stabilizing within the optimal denitrification window (− 110 to − 150 mV) at 20 mA. However, excessive current (40 mA) depressed ORP below − 250 mV, triggering microbial stress and collapsing TN removal to 62%. High-throughput 16S rRNA sequencing demonstrated that moderate EI enriched key denitrifying phyla (Proteobacteria, Firmicutes, and Bacteroidota) and increased alpha diversity, whereas over-polarization induced community simplification and functional loss. Phosphorus removal was primarily driven by Fe²⁺/Fe³⁺ precipitation and hydrolytic adsorption. This work identifies a critical electrochemical operating window that balances redox manipulation, microbial ecology, and energy efficiency, offering a scalable pathway for advanced nutrient polishing in decentralized and tertiary treatment systems.