The occurrence characteristics and underlying removal mechanism of combined copper and sulfamethoxazole pollution in constructed wetlands

Constructed wetlands (CWs) can effectively remove heavy metals and antibiotics, providing a promising solution for rural wastewater treatment. However, mechanisms of the interaction and removal of coexisting contaminants in CWs remain unclear. The removal efficiency of coexisting copper (Cu) and sulfamethoxazole (SMZ), main removal pathways, and interactions between Cu and SMZ in CWs were investigated using both in small-scale CWs and laboratory batch-adsorption experiments. The removal efficiencies of SMZ and Cu in CWs were approximately 99 %, and the presence of Cu accelerated SMZ removal within 10 h. Cu removal was mainly dependent on ceramsite adsorption, and its contribution ratio decreased by 39 %–41 % with increasing concentrations. The coexistence of high SMZ concentrations enhanced the Cu bioaccumulation. Over 92 % of SMZ removal in CWs occurred via microorganisms; SMZ was more likely to accumulate in plant roots than in aboveground parts. Adding low levels of Cu increased bacterial diversity in CWs, and addition of 20 mg/L Cu under SMZ stress conditions significantly increased the relative abundance of anaerobic denitrifiers and SMZ-degrading Microscillaceae. Copper and SMZ interactions with ceramsite presented competitive and synergistic effects depending on SMZ concentration. Copper and SMZ adsorptions were stable under neutral and weakly alkaline conditions, respectively. Cation addition promoted SMZ adsorption via hydrophobic interactions but did not promote Cu adsorption. Sulfamethoxazole and Cu(II) competed for –NH sites, and the adsorbed Cu bridged additional anionic SMZ to form surface ternary complexes on ceramsite particles. SMZ − NH2+ bonding facilitated additional Cu–N coordination bonding via electrostatic interactions. These findings provide insights for optimising the purification efficiency of CWs and mitigating the environmental risks of compound pollution.