Immobile iron-rich particles enhance simultaneous nitrogen removal and phosphorus retention in treatment wetlands

Eutrophication control requires cost-effective and sustainable technologies capable of simultaneously removing nitrogen and phosphorus from wastewater treatment plant (WWTP) effluents. Widely used post-treatment systems, treatment wetlands (TWs) typically exhibit limited nutrient removal because of imbalances in electron supply and demand, and rapid saturation of substrate adsorption capacity. In this study, immobile iron-rich particles (IIRPs) were introduced into TWs via a drainage-injection strategy to enhance nutrient removal from municipal WWTP effluent. Following the start-up phase, the IIRP-amended TWs consistently achieved effluent concentrations that met the target quasi-Class IV surface water quality standards (TN ≤ 10 mg L^-1; TP ≤ 0.3 mg L^-1) for 300 days of continuous operation. The enhanced NH₄⁺-N and TN removal with iron-rich particles amendment could not be attributed to nitrification or anammox, as evidenced using qPCR, metagenomic binning, and removal profiles. Instead, Fe-N redox-coupling processes, including Fe(III) reduction-driven and Fe(II) oxidation-driven nitrogen-removal pathways, contributed to enhanced nitrogen removal. The IIRPs amendment increased the equilibrium phosphorus adsorption capacity of wetland substrate by threefold, and the improved phosphorus retention was attributed to Fe-P interactions. These findings reveal a coupled Fe-N-P mechanism that enables efficient and stable nutrient removal and provide a mechanistic foundation for developing low-carbon, sustainable strategies to upgrade existing TWs for advanced wastewater polishing.