K₂FeO₄-activated magnetic sludge biochar for the efficient remediation of polydisperse microplastics: synergistic adsorption mechanisms and practical application

The persistent accumulation of micro/nanoplastics (MNPs) in aquatic environments necessitates the development of efficient remediation technologies. Conventional biochar adsorbents face challenges including slow kinetics and poor adaptability in complex matrices. Herein, a magnetic sludge-derived biochar (MSDB) co-loaded with Fe₃O₄ and zero-valent iron was synthesized via a one-pot K₂FeO₄-assisted activation–magnetization pyrolysis of deashed sludge-derived biochar at 800 °C, where K₂FeO₄ served as the single additive to enable in-situ formation of Fe₃O₄/ZVI while promoting pore development, yielding a highly porous structure with a specific surface area of 123.58 m 2 ·g −1 . The MSDB exhibited exceptional performance for removing polystyrene microplastics (PSMPs, 0.1–1 μm), achieving a Langmuir model-fitted apparent maximum adsorption capacity (Q₀) of 440.24 mg·g −1 and rapid kinetics, thereby demonstrating its efficacy for polydisperse MP fractions commonly found in the environment. Multimodal characterization revealed synergistic adsorption mechanisms, including physical entrapment, π-π stacking, hydrophobic interactions, and weak interfacial electron redistribution. The MSDB maintained robust removal efficiency across a wide pH range (3−11) and in the presence of coexisting ions, organic matter, and antibiotics (sulfamethoxazole, SMX). Notably, the presence of SMX induced synergistic heteroaggregation, further enhancing PSMP capture. The material also showed outstanding performance in real water samples (tap water: 99.17%; lake water: 89.22%; seawater: 85.78%). Furthermore, the material exhibited excellent recyclability, retaining 75.5% of its initial capacity after five cycles, while enabling facile magnetic separation due to its sustained saturation magnetization (19.77 emu·g −1 ). This work provides a sustainable strategy for valorizing sewage sludge into a high-efficiency, magnetically recoverable adsorbent for microplastic remediation, aligning with circular economy principles. Future research should focus on large-scale continuous-flow applications and pilot-scale testing to validate its efficacy in end-of-pipe wastewater treatment.