Objective Biochar has been widely applied in the removal of microplastics (mPS) and nanoplastics (nmPS) from aquatic environments. However, pristine biochar suffers from challenges such as difficult separation, limited adsorption capacity, and poor reusability, which hinder its practical application in efficient and repeated adsorption processes. To address these limitations, this study proposes a novel approach involving co-pyrolysis of cork and iron (Fe), followed by surfactant modification, to synthesize modified magnetic biochar with enhanced adsorption performance and recyclability, while systematically elucidating the underlying adsorption mechanisms.

Methods Magnetic biochar (MBC) was prepared by impregnating cork powder with FeCl3·6H2O solution followed by co-pyrolysis, enabling magnetic separation. Subsequently, cetyltrimethylammonium bromide (CTAB) was used to modify MBC, yielding CTAB-modified magnetic biochar (C-MBC). The microstructure and physicochemical properties of the materials were characterized using SEM-EDS, BET, FTIR, XPS, XRD, VSM, and zeta potential analysis. Batch adsorption experiments were conducted to evaluate nmPS removal efficiency, and adsorption mechanisms were interpreted through kinetic, isotherm, and thermodynamic modeling combined with material characterization.

Results The resulting C-MBC exhibited a rougher surface and larger average pore size (increased to 13.931 4 nm), with a specific surface area of 21.94 m²/g and total pore volume of 0.076 4 cm³/g. Fe incorporation endowed the biochar with magnetism and a positively charged surface, facilitating both separation and electrostatic attraction toward nmPS. CTAB modification further enriched the surface chemistry and significantly enhanced the positive surface charge, thereby improving nmPS adsorption capacity. The maximum adsorption capacity of C-MBC reached 277.44 mg/g, which was 3.43 times higher than that of pristine biochar (BC: 54.33 mg/g) under identical conditions (C-MBC: 186.33 mg/g). Moreover, C-MBC demonstrated excellent recyclability, maintaining an nmPS removal efficiency of 81% after four consecutive adsorption-desorption cycles. The adsorption process followed the pseudo-second-order kinetic model and Langmuir isotherm model, indicating a combination of physisorption and chemisorption with monolayer coverage on the biochar surface. Thermodynamic analysis confirmed that the adsorption was spontaneous and endothermic, reflecting strong interaction and high affinity between C-MBC and nmPS. Notably, C-MBC maintained high removal efficiency across a broad pH range (3–11), various ionic strengths, and in the presence of coexisting ions, demonstrating wide environmental applicability. Comprehensive mechanistic analysis revealed that nmPS adsorption onto C-MBC was governed by multiple interactions, including electrostatic attraction, complexation, hydrogen bonding, and pore-filling effects.

Conclusion This study demonstrates that co-pyrolysis with Fe and CTAB surface modification significantly enhances the nmPS adsorption capacity of cork-derived biochar. Furthermore, the synthesized C-MBC exhibits excellent environmental adaptability and reusability, showing great promise for practical applications in the removal of nanoplastics from water systems.