Efficient removal of aged microplastics from seawater using hydrophilic Fe3O4 nanoparticles: A multi-scale study based on macroscopic experiments and microscopic calculations.

Effective microplastics (MPs) remediation technologies are required to combat marine MPs pollution. Magnetic adsorption is a promising approach for MPs remediation. However, current strategies typically employ hydrophobic magnetic adsorbents to remove pristine hydrophobic MPs, overlooking the fact that environmental aging can enhance the hydrophilicity of MPs. This study investigated the adsorption performance of unmodified hydrophilic Fe3O4 nanoparticles (NPs) on six types of physically aged MPs through a multi-scale approach, combining macroscopic experimental observations with microscopic explanatory mechanisms. Results demonstrated that the removal efficiencies of aged MPs at high concentrations (at 4 g each) exceeded 80% at a low dosage of 5 mg hydrophilic Fe3O4 NPs. The adsorption capacities of MPs reached up to (6.716-7.359) × 105 mg/g. The optimal adsorption performance was achieved under neutral conditions (pH = 7), due to favorable electrostatic attraction between positively charged hydrophilic Fe3O4 NPs and negatively charged MPs. Increasing salinity enhanced adsorption by compressing the electrical double layer and promoting aggregation. Multivalent cations (e.g., Al3+) facilitated adsorption through charge neutralization and coagulation induced by their hydrolysis products. The applicability of hydrophilic Fe3O4 NPs was assessed in a practical nearshore seawater sample. Results indicated higher removal efficiencies (87.1-93.6%) for aged MPs compared to deionized water. Characterization, desorption tests and density functional theory calculations revealed that the adsorption was controlled by electrostatic interactions, combined with van der Waals forces. These findings provide insights into the interaction mechanism between hydrophilic magnetite NPs and aged MPs, validating a robust and cost-effective strategy for MPs remediation in complex marine environments.