Enhanced removal of aged and differently functionalized polystyrene nanoplastics using ball-milled magnetic pinewood biochars

In this study, simple and environmentally friendly magnetic biochars were successfully prepared by ball-milling biochar with FeO nanoparticles to remove NPs from water. The magnetic biochars synthesized at various pyrolysis temperatures of 300 °C (MBC300), 500 °C (MBC500), and 700 °C (MBC700) were used to eliminate the unmodified (PS), aged under UV radiation (UVPS), amine-modified (PS-NH) and carboxylate-modified (PS-COOH) polystyrene NPs of 100 nm in size. Results showed that the removal efficiency of MBC300, MBC500, and MBC700 for PS were 43.67, 82.73 and 57.02%, which were 3.01, 5.76, and 3.10 times greater than that of corresponding pristine biochars at the same temperatures, respectively, and the strongest removal efficiency of MBC500 was 95.2% since it has the largest specific surface area and abundant oxygen-containing functional groups. The surface properties of the NPs affected their removal, and the PS-NH had the highest removal rate using magnetic biochars. Compared to pristine biochars, the magnetic biochars displayed faster adsorption kinetics. The Langmuir maximum adsorption capacity of magnetic biochars for NPs were 107.7181-229.5772 mg/g, much greater than those of the pristine biochars (55.4602-80.3096 mg/g). Mechanism analysis revealed that the hydrophobicity, electrostatic attraction, H-bonding formation and π-π conjunction between the NPs and MBCs contributed to the adsorption process. This work highlights the promising potential of ball milling to be used as a simple technique for the preparation of magnetic biochar to remove NPs, especially NPs with various surface groups.