Synergistic preparation of magnetic biochar from sewage sludge and iron tailings for dye adsorption: Contrasting effect of hydrothermal carbonization and pyrolysis

The utilization of solid waste to prepare biochar for pollutant adsorption has emerged as a promising strategy. This study innovatively engineered magnetic biochar (S 1 I 3 -HC 240 and S 3 I 1 -PC 800 ) via hydrothermal carbonization (HTC) and pyrolysis approaches using sewage sludge and iron tailings (IT) as dual precursors, and investigated their adsorption behaviors and underlying mechanisms for methylene blue (MB) removal. The incorporation of IT enhanced MB adsorption, achieving maximum adsorption capacities of 91.17 mg/g (S 1 I 3 -HC 240 ) and 82.30 mg/g (S 3 I 1 -PC 800 ), respectively. The adsorption processes followed the Elovich kinetic model and the Freundlich isotherm, indicating spontaneous and endothermic reactions dominated by chemosorption. The primary mechanisms involved in MB removal included electrostatic attraction, π-π interactions, hydrogen bonding, and surface complexation. Notably, HTC processing with IT promoted formation of oxygenic group and Fe 3 O 4 , strengthening hydrogen bonding and surface complexation. Conversely, in pyrolysis, IT enhanced electrostatic attraction and oxidative degradation by increasing nitrogen functional groups and generating Fe 0 . From both an energy consumption and adsorption performance perspective, the energy-efficient HTC technique offers a significant advantage for the preparation of magnetic biochar from solid waste, making it a promising approach for sludge resource utilization.