Adsorption and mechanism of magnetically modified industrial hemp straw biochar on microplastics in aqueous solution

Microplastics (MPs) originating from plastic waste in groundwater have attracted much attention worldwide due to their wide distribution, small particle size and high mobility, as well as their potential threat to organisms. In this study, a magnetically modified biochar adsorbent for the removal of microplastics was prepared using KOH-activated industrial hemp straw biochar as a precursor. For polystyrene (PS) microspheres (1 μm, 60 mg/L) in aqueous solution, magnetic biochar (MBC) removed 89.97 %, which was 9.30 times higher removal rate compared to pristine biochar (BC). It is hypothesized that the adsorption process was a result of electrostatic interactions and chemical bonding interactions between microplastics and biochar. The adsorption process was affected by solution pH and interfering ions, and the MBC had good stability, and its removal efficiency of MPs remained above 80 % after 5 cycles. Kinetic, isothermal and thermodynamic modeling analyses showed that the adsorption reaction was spontaneous, higher temperature contributed to the enhancement of adsorption, and the adsorption mechanism involved electrostatic interactions, surface complexation, metal-O-MPs complexation and π-π interactions. These results would provide an idea for obtaining MBC to remove the MPs from aqueous systems.