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 Fe3O4 nanoparticles to remove NPs from water. The magnetic biochars synthesized at various pyrolysis temperatures of 300 degrees C (MBC300), 500 degrees C (MBC500), and 700 degrees C (MBC700) were used to eliminate the unmodified (PS), aged under UV radiation (UVPS), amine-modified (PS-NH2) 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 effi-ciency 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-NH2 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). Mecha-nism analysis revealed that the hydrophobicity, electrostatic attraction, H-bonding formation and 7C-7C conjunc-tion 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.

Automated summary

This study successfully prepared magnetic biochars by ball-milling biochar with Fe3O4 nanoparticles, which showed high efficiency in removing polystyrene nanoparticles (NPs) from water. The magnetic biochars synthesized at different pyrolysis temperatures exhibited improved removal capacity compared to pristine biochars. The best removal efficiency was achieved by MBC500 due to its larger specific surface area and abundant oxygen-containing functional groups. Mechanism analysis indicated that hydrophobicity, electrostatic attraction, H-bonding formation, and 7C-7C conjunction contributed to the adsorption process.