Enhancing nitrogen and phosphorus removal for surface water remediation using ecological floating beds via FeC-plant coupling and root microenvironment reconfiguration.

Iron-carbon (FeC) materials can be incorporated into floating structures for in situ nitrogen and phosphorus removal during remediation of surface water with low C/N ratio. Plant incorporation may alleviate nitrification inhibition and limited reactive interfaces associated with FeC materials alone, yet the effects on nutrient removal performance and the underlying mechanisms remain poorly understood. Herein, an ecological floating bed coupling aquatic plants with FeC materials (EFB-FeC) was constructed, with a plant-gravel system (EFB-G) as control. At C/N ratio of 2, EFB-FeC achieved TN and TP removal efficiencies of 93.77 ± 3.19% and 91.22 ± 7.48%, representing increases of 51.11 and 42.43% over EFB-G. FeC promoted autotrophic denitrification and phosphorus immobilization through micro-electrolysis, and enhanced plant photosynthetic activity and growth. More importantly, FeC-plant coupling reconfigured the root microenvironment, which was the key to overcoming nitrification suppression associated with FeC application alone and expanding reactive interfaces for nutrient removal. Specifically, compared with EFB-G, EFB-FeC showed higher abundances of nitrifying bacteria and nitrification-related genes on root surface. Meanwhile, genes involved in iron acquisition and utilization were also enriched on root surface of EFB-FeC, accompanied by the enrichment of autotrophic denitrifying bacteria. Besides, root iron plaque expanded the reactive interface for phosphorus immobilization. Mass balance analysis further revealed that the root microenvironment contributed more to nitrogen and phosphorus removal than direct plant assimilation. These findings advance the understanding of FeC-based EFBs by identifying root microenvironment as a key functional interface for enhanced nutrient removal during surface water remediation.