Insight into the impact of Fe3O4 nanoparticles on anammox process of subsurface-flow constructed wetlands under long-term exposure

The increasing use of Fe3O4 nanoparticles (NPs) had posed an emerging challenge to wastewater treatment processes, and their potential impact on anaerobic ammonium oxidation (anammox) process of unplanted subsurface-flow constructed wetlands (USFCWs) was investigated firstly under the long-term exposure of different Fe3O4 NP concentrations. It was found that Fe3O4 NP exposure could improve total nitrogen (TN) removal. The abundance of Candidatus Anammoxoglobus increased significantly at 10 mg/L Fe3O4 NPs, while decreased under 1 mg/L Fe3O4 NP exposure. Desulfosporosinus and Exiguobacterium increased to some extent at 1 mg/L Fe3O4 NPs, suggesting that Fe-anammox played an important role in TN removal. The ROS production increased with the increase of Fe3O4 NP concentration, and the integrity of cell membrane was good under Fe3O4 NP exposure. The functional genes that related to inorganic ion transport and metabolism and lipid transport and metabolism were upregulated, and cell motility decreased after long-term exposure of 1 mg/L Fe3O4 NPs.