Complementary removal of municipal pollutants via biochar-amended wetlands planted with <i>Lemna minor</i> and <i>Arundo donax</i>

This study evaluates the treatment performance of pilot-scale vertical subsurface flow constructed wetlands (VSSF CWs) for decentralized municipal wastewater management in Iraq's arid and semi-arid regions. Two vegetated systems-VFpL (<i>Lemna minor</i>) and VFpA (<i>Arundo donax</i>)-were assessed against an unplanted control (VF) in Al-Midhatiya, Babylon, Iraq. Each unit (0.82 m diameter × 0.80 m depth) operated in batch mode between November 2024 and March 2025, with a hydraulic retention time (HRT) of 5 days, extended to 8 days for nutrient removal. The influent rate averaged 0.0527 ± 0.012 m³/day, simulating real-world decentralized flow conditions. All wetland beds were filled with gravel amended by locally sourced date palm frond biochar, selected for its high porosity, mineral content, and functional group diversity. FTIR, SEM, and XRD analyses confirmed the presence of surface-active groups (<i>e.g.,</i> carboxyl, hydroxyl), crystalline phases such as quartz and calcite, and microstructural changes associated with microbial colonization. Spectral shifts and morphological smoothing in the planted units reflected strong microbial-root interactions and biofilm formation. Treatment performance was superior in vegetated systems: VFpL achieved 86.2% (COD), 93.3% (BOD), 71.8% (TN), 71% (TP), and 86.2% (TSS) removal, while VFpA recorded 84.3%, 91%, 68.4%, 73.4%, and 89.2%, respectively. TDS removal was moderate (26.7% VFpL, 29.3% VFpA). The unplanted VF exhibited significantly lower efficiency. Statistical analysis <i>via</i> one-way ANOVA (<i>p</i> < 0.001) confirmed the enhanced performance of biochar-plant-integrated VSSF CWs as a sustainable approach for wastewater treatment in water-scarce environments.