Fate analysis of heavy metals and hydrophobic organic pollutants in tidal flow wetlands: Insights for pollution-specific ecological remediation strategies

Coastal wetlands play a critical role in modulating biogeochemical cycles and attenuating anthropogenic pollutants. However, limited attention has been paid to quantifying the efficacy of wetlands in intercepting upstream pollutants. In this study, lab-scale tidal flow wetlands (TFWs) simulating the hydrodynamic conditions of the Shanghai Nanhui Dongtan Wetland (NDW) were established to evaluate the synergistic removal efficiency of diverse contaminants. Results indicated exceptional composite pollutant removal capacities: chemical oxygen demand (COD_Mn: 0.45 g/m²/d), total nitrogen (TN: 0.38 g/m²/d), total phosphorus (TP: 0.051 g/m²/d), lead (Pb: 3.50 mg/m²/d), cadmium (Cd: 0.71 mg/m²/d), Σpolycyclic aromatic hydrocarbons (ΣPAHs: 1023.64 μg/m²/d), and Σbisphenol analogs (ΣBPs: 404.60 μg/m²/d). Behavior analysis demonstrated that substrate adsorption (63.18 %) dominated the removal of heavy metals (HMs), while biodegradation (62.15–70.86 %) was identified as the primary pathway for the removal of hydrophobic organic compound (HOCs). The introduction of iron-modified biochar significantly increased the abundance of Altericroceibacterium, Arcobacter, and Methylophaga, with these genera exhibiting specific degradation capabilities for HOCs in TFWs. Meanwhile, plant uptake (7.58–11.94 %) and benthic bioaccumulation (4.07–5.21 %) emerged as non-negligible pathways for HMs and HOCs. Therefore, substrate modification should be prioritized for HM-dominated scenarios in NDW remediation, while substrate modification could be coupled with Scirpus mariqueter planting in HOC-dominated scenarios. The Level IV fugacity model can accurately predict the partitioning of different pollutants across sediment-water-plant-biota phases. However, the amendment of iron-modified biochar elevated sediment-water partition coefficients (Ks), thereby underestimating the substrate-associated HMs. Similarly, root-induced “iron plaques” increased aquatic plant bioaccumulation factors (K_G), resulting in underestimated concentrations of HOCs in aquatic plants. These discrepancies emphasized the need for parameter optimization in hydrodynamic-ecological coupled models.