Functionality-dependent removal efficiency and mechanisms of polystyrene microplastics by a robust magnetic biochar

The widespread release and accumulation of microplastics (MPs) in aquatic environments highlight the necessity for effective remediation strategies. Biochar-based adsorption has received attention as an environment-friendly and cost-effective technology. However, the effect of the surface functionality of MPs on adsorption efficiency remains uncertain. This study synthesized iron-modified magnetic biochar (M-WB) from sawdust to evaluate its removal efficiency for polystyrene (PS) with varying surface functionalities. The adsorption process of PS was best described by the Elovich kinetic and Sips isotherm models, indicating a chemical and multilayer adsorption mechanism. The adsorption rates and capacities followed the order: CPS (carboxyl) > APS (amine) > UPS (plain), with maximum adsorption capacities up to 205 mg/g. The influence of environmental conditions, including pH, dissolved organic matter, ionic strength, as well as water matrices greatly depended on PS functionality. Meanwhile, M-WB showed remarkable reusability, maintaining over 72 % PS removal efficiency after six cycles of pyrolysis regeneration. The characterizations revealed that the O-containing functional groups (e.g., Fe-O, C-O, and CO) on M-WB played a crucial role in the removal of functionalized PS. The primary mechanisms involved included surface complexation, hydrogen bonding, and electrostatic interactions. Density functional theory (DFT) calculation further confirmed the selective adsorption of PS with varying functional groups by elucidating the specific interactions between M-WB and functionalized PS. The efficient remediation of different functionalized PS in complex aquatic environments and the excellent magnetic separation performance justify the practical applicability of magnetic biochar.