Insights into the enhanced effect of biochar on cadmium removal in vertical flow constructed wetlands

Biochar has been increasingly applied in constructed wetlands (CWs) to remediate heavy metal (HM)-polluted water. Nevertheless, only few studies have elucidated the enhanced mechanism and potential synergies related to the HM removal from biochar-based CWs (BC-CWs) for HMs removal. This study used cadmium (Cd) as the target HM and added biochar into CWs to monitor physicochemical parameters, plant' physiological responses, sub-strate accumulation, and microbial metabolites and taxa. In comparison with the biochar-free CW (as CWC), a maximum Cd2+ removal of 99.7% was achieved in the BC-CWs, associated with stable physicochemical pa-rameters. Biochar preferentially adsorbed the available Cd2+ and significantly accumulated Fe/Mn oxides-bond and the exchangeable Cd fraction. Moreover, biochar alleviated the lipid peroxidation (decreased by 36.4%) of plants, resulting in improved growth. In addition, extracellular polymeric substances were increased by 376.9-396.8 mg/L in BC-CWs than compared to CWC, and N and C cycling was enhanced through interspecific positive connectivity. In summary, this study explored comprehensively the performance and mechanism of BC- CWs in the treatment of Cd2+-polluted water, suggesting a promising approach to promote the plant-microbe -substrate synergies under HM toxicity.