Turning Waste into Treasure: A Magnetic Sludge-Based Biochar for Efficient Co-Removal of Tetracycline and Cu ²⁺

The treatment and disposal of excess sludge, a major solid byproduct of wastewater treatment processes, pose significant environmental and public health challenges, making its rational utilization crucial for promoting resource recycling. This study presents a one-step synthesis of a magnetically responsive bifunctional biochar composite (Fe-SDB₆₀₀) derived from excess sludge for the simultaneous and enhanced removal of tetracycline (TC) and copper ions (Cu²⁺) from aqueous solutions. Characterization confirmed the successful loading of nanoscale iron oxide particles onto the biochar surface, resulting in a hierarchical porous structure. Under optimized conditions, Fe-SDB₆₀₀ exhibited significantly improved adsorption capacities for TC (269.39 mg/g) and Cu²⁺ (271.65 mg/g) compared to the pristine biochar. The composite demonstrated excellent resistance to interference from common coexisting ions and maintained a high TC removal efficiency of 83.28% after four adsorption-desorption cycles, with minimal iron leaching. Adsorption kinetics were best described by the pseudo-second-order model, and isotherm data fitted the Langmuir model, indicating a chemisorption-dominated process. Density functional theory (DFT) calculations provided atomic-level mechanistic validation, confirming the spontaneous coordination of Cu²⁺ to surface Fe-OH sites with a high binding energy of -2.05 eV and identifying the A-ring of TC as the dominant moiety for π-π electron donor-acceptor interactions with an adsorption energy of -1.85 eV. Furthermore, in the binary system, a metal-bridging effect was identified, where preadsorbed Cu²⁺ enhanced TC uptake by forming ternary complexes. This work provides a sustainable and efficient strategy for sludge valorization and the coremoval of organic and inorganic pollutants, highlighting the potential of sludge-derived adsorbents in practical wastewater treatment.