Nitrogen and phosphorus removal efficacy and mechanism of by iron-carbon enhanced bioretention facility under different operating conditions

To address the issues of low and unstable removal efficiencies of nitrogen and phosphorus pollutants from traditional bioretention facilities, an iron-carbon based bioretention facility (IC-B) was constructed, and a biochar group (BC-B) and a traditional gravel group (TR-B) were set up as controls. This study investigated the efficacy of removing nitrogen and phosphorus from stormwater runoff, by simulating the conditions of different rainfall intensities (7, 17, and 21 mm/h), pre-rain drought periods (1, 3, and 10 d), and the presence or absence of satuated zones. The associated microbial community and removal mechanism were evaluated in combination with high-throughput sequencing and potential nitrification/denitrification capacity of the substrate. The results showed that the average removal rates of nitrate nitrogen and total phosphorus in the IC-B group were 94.17% and 97.57%, respectively. Furthermore, the removal rates of nitrogen and phosphorus pollutants in the IC-B group were the least affected by the changes in rainfall intensity. The stability of the removal of nitrogen and phosphorus in the BC-B group and the IC-B group was significantly better than that of the TR-B group under different pre-rainfall drought periods. The presence of a saturated zone enhanced \mathrmNH_4^+ -N removal in the TR-B group, while significantly improving denitrification in the IC-B group, which consistently achieved optimal TP removal. Mechanism analysis showed that the microelectrolysis of iron carbon could promote the electron transfer efficiency and significantly enhance the nitrogen and phosphorus removal capacity of the facility. Biochar formed an adsorption barrier in the vadose zone, iron filings accelerated the dissolved oxygen depletion, and the two synergistically created an anoxic environment to promote the enrichment of denitrifying bacteria (Proteobacteria relative abundance of 38.57%). At the same time, the iron-reducing bacteria (Acidobacteriota) proliferation drove Fe³⁺/Fe²⁺ cycling, enhanced system stability through coupled microbial metabolism, and synchronized the synergistic enhancement of chemical phosphorus removal and biological nitrogen removal. This study reveals the synergistic pollutant removal efficacy and mechanism of the iron and carbon amendment retention facility, providing technical support and practical reference for urban surface pollution control.