Materials
Secondary biological effluent and synthetic aqueous solution were used in experiments. More details are in the Supporting Information (SI). Ferric chloride (FeCl3·6H2O), ferrous chloride (FeCl2·4H2O), polyferric chloride (PFC), polyacrylamide (PAM), ferroferric oxide (Fe3O4), potassium nitrate (KNO3), and potassium sulfate (K2SO4) were obtained from Sigma-Aldrich (St. Louis, USA). 3-(trimethoxysilyl) propyl-n -octadecyldimethyl ammonium chloride (TPODAC) was purchased from Bailingwei (Beijing, China). Potassium dihydrogen phosphate (KH2PO4) was purchased from Macklin (Shanghai, China).