Plant litter facilitates biotic and abiotic removal of benzo[a]pyrene in iron-based constructed wetlands

Plant litter (PL) inevitably enters iron-based constructed wetlands (ICW) during the operation period, particularly in the decay phase. However, the mechanisms by which PL influences the removal of polycyclic aromatic hydrocarbons (PAHs), for example, benzo[a]pyrene (B[a]P), in ICWs remain ambiguous. Herein, we systematically illustrated the biotic and abiotic removal mechanisms of B[a]P in ICW with PL (ICW-P). The results showed that ICW-P achieved higher B[a]P removal (92.25 ± 2.22%) and toxicity reduction (91.38 ± 0.82%) compared to ICW, thereby ensuring water security. Further analysis revealed that oxidizing surface-bound/low-crystalline Fe(II) modulated •OH production during the long-term operation of CW. PL addition elevated humic-like substances in extracellular polymeric substances, enhancing electron transport system activity (ETSA) and thereby facilitating surface-bound/low-crystalline Fe(II) oxidation and hydroxyl radical (•OH) generation. In the biotic pathway, elevated ETSA fostered higher microbial interactions, potentially facilitating interspecies electron transfer between electroactive bacteria (e.g., Geobacter) and PAHs-degrading bacteria (e.g., Acidovorax). The increased abundance of PAHs-degrading and ATP synthesis genes in ICW-P further reinforced the B[a]P biodegradation process. Furthermore, microorganisms involved contributed to •OH (27.19%) and Fe(II) (57.23%) production, which in turn sustained microbial metabolic activities. This mutual dependency indicated that synergistic abiotic-biotic interplay drove efficient PAHs removal in ICW-P. These findings highlight the role of PL in PAHs removal in ICW and provide insights for strengthening treatment performance in ICWs.