Novel preparation of sludge-based spontaneous magnetic biochar combination with red mud for the removal of Cu2+ from wastewater

[Display omitted] •Alternative methods for the resource utilization of municipal sludge and red mud.•Ammonium oxalate was applied to the activation of sludge and red mud for the first time, which provided amino functional groups and reduction.•Fe2O3 in the red mud is reduced to Fe3O4 and Fe0 during co-pyrolysis, biochar obtained in situ magnetization.•The actual adsorption capacity of adsorbent SRMBC-750 was 242.62 mg/g, which exhibited excellent adsorption capacity for Cu2+. In this study, municipal sludge and red mud were used as raw materials to prepare spontaneous magnetic biochar (SRMC-750) for the adsorption of Cu2+ from wastewater. The experimental findings demonstrated that the maximum actual adsorption capacity was 242.62 mg/g at 25 °C and pH 5. The adsorption process followed the pseudo-second-order kinetic model and the Freundlich isotherm model. The potential adsorption mechanisms were electrostatic attraction, complexation, cation-exchange, and “cation-π”. After five adsorption cycles, SRMBC-750 still had 75% removal efficiency for Cu2+. Overall, SRMBC-750 provides a potential pathway for the removal of Cu2+ from wastewater.