An iron-carbon bioretention system for enhancing nitrogen and phosphorus removal: Synergy of vadose and saturated zones

• An iron-carbon bioretention system was established. • Removal of nitrogen and phosphorus achieved 86.96-97.84% and 88.90-96.15%. • Efficient treatment of intermittent and high load stormwater runoff pollution. • The synergistic effect of vadose and submerged zones. • The GHG emissions of the CFe-B were significantly reduced by 198.80 mg/m²·d. To enhance the removal of nitrogen and phosphorus pollutants from urban stormwater runoff in bioretention systems, this study developed an iron-carbon bioretention system with a saturated zone. The system's performance in enhanced pollutant removal was systematically investigated, and the synergistic removal mechanisms between the vadose and saturated zones were elucidated. Experimental results demonstrated that the iron-carbon bioretention system achieved high and stable removal efficiencies for dissolved pollutants, with removal rates of 95.97±2.42% for nitrate-nitrogen, 84.63±3.75% for total nitrogen, 94.88±1.92% for total phosphorus, and 86.99±5.57% for COD. These values represent significant improvements of 69.05%, 44.73%, 49.11%, and 18.63%, respectively, compared to a conventional sand-based bioretention system. Mechanistic analysis of nitrogen and phosphorus removal reveals that the system establishes functional zones in the vertical direction. The aerobic environment in the vadose zone facilitated nitrification and the formation of iron oxides, enabling nitrogen transformation and phosphorus adsorption. Conversely, the anaerobic conditions in the saturated zone drove continuous iron-carbon micro-electrolysis, generating Fe 2+ and [H] as inorganic electron donors. This process promoted autotrophic denitrification and precipitated phosphorus as stable iron phosphate. The iron-carbon also enhanced microbial diversity and enriched functional genera involved in autotrophic denitrification (e.g., Hydrogenophaga, Geobacter ) and iron cycling (e.g., Shewanella, Geobacteraceae ). Furthermore, the presence of iron oxides suppressed CH 4 production by competing with methanogens for organic substrates. The higher abundances of Desulfobacterota and Bacteroidota contributed to reduced N 2 O emissions, thereby mitigating the greenhouse gas footprint of the bioretention system. This study provides a novel strategy for enhancing stormwater purification in bioretention systems.