Innovative resource utilization of sludge and red mud: development of in-situ magnetic and floatable biochar for the adsorption of basic fuchsin

The development of sustainable adsorbents that integrate low-cost separation and high contaminant removal efficiency remains a critical challenge in wastewater treatment. This study engineered a sludge-based magnetic floatable particle adsorbent (SMFA-800-0.5) through synergistic co-pyrolysis of municipal sludge, red mud, and hollow glass microspheres (HGM) for the removal of basic fuchsin (BF) from dye wastewater. The results indicated that the dual-function separation approach utilizing Fe 3 O 4 in situ formation from red mud (magnetization of 8.24 A·m −1 ) enables excellent recovery efficiency, eliminating the need for external Fe precursors, while the integration of HGM provides a self-floating capability with 76% surface retention over 3 h. The sludge-derived carbon matrix, enhanced by HGM’s structural support, provides hierarchical pores with a Bruner–Emmett–Teller (BET) surface area of 57.98 m 2 ·g −1 . The addition of bentonite clay effectively addresses powder dispersion issues in flow systems. This material demonstrates superior adsorption performance, achieving a capacity of 106.72 mg·g −1 for BF via pseudo-first-order kinetics. The enhanced adsorption performance is driven by multi-mechanism synergies, including electrostatic attraction and pore filling. This work pioneers a “zero-external-input, dual-recovery” paradigm for sustainable dye removal. Graphical abstract