Enhanced nitrogen and phosphorus removal and microbial community response in constructed wetlands using iron-manganese bimetallic modified biochar as substrate for rural wastewater treatment

Introduction Conventional constructed wetlands (CWs) often exhibit limited efficacy in removing total nitrogen (TN) and total phosphorus (TP) from rural wastewater with low and fluctuating carbon‐to‐nitrogen (C/N) ratios. This study developed an iron‐manganese (Fe‐Mn) bimetallic modified rice husk biochar (Fe‐Mn‐BC) as an enhanced CW substrate. Methods Fe‐Mn‐BC was synthesized via FeCl 3 /MnCl 2 co‐impregnation, alkali precipitation, and pyrolysis, and applied in four parallel vertical‐flow reactors (CW‐G, CW‐BC, CW‐Fe‐BC, CW‐FeMn‐BC) operated under three sequential C/N regimes (1, 3, 5). Material properties were characterized by XRD, SEM‐EDS, FTIR, and BET; microbial responses were assessed by 16S rRNA sequencing and qPCR. Results Characterization confirmed coexistence of α‐FeOOH, Fe 3 O 4 , and δ‐MnO 2 phases with Fe and Mn loading of 73.4 and 69.8 mg g ‐1 . Under the most stringent C/N = 1 condition, CW‐FeMn‐BC achieved TN and TP removal of 62.4% and 92.7%, with a low TN response slope of 4.55% per C/N unit (approximately 55% of CW‐BC). Sequencing and qPCR revealed selective enrichment of ammonia‐oxidizing, iron-oxidizing, and iron‐reducing bacteria, accompanied by marked upregulation of nirS (12.5‐fold), hzsA (21‐fold), cyc2 (15.8‐fold), and phoD (4.2‐fold) functional genes. Discussion These findings demonstrate that Fe‐Mn‐BC enables a material‐chemical‐biological tri‐synergy for enhanced N and P removal, offering a sustainable substrate strategy for decentralized rural wastewater treatment under fluctuating low C/N conditions.