Effects of micro/nanoplastics on physicochemical properties and cadmium(II)-sorption capacity of pig-bone biochar

Biochar, when introduced into the environment, has a high potential to coexist with micro/nanoplastics (MPs/NPs), yet the effects of these plastics on biochar’s properties and metal removal capacity remain unclear. This study investigated how pig-bone biochar of sizes 25–75 μm (B75) and 250–425 μm (B425) interacts with polystyrene particles of 25, 200, and 5000 nm (PS25, PS200, and PS5000, respectively). Results indicated that the surface area of B75 was significantly reduced after interacting with MPs/NPs, while the surface area of B425 tended to increase. Furthermore, the Cd(II)-sorption performance of B75 was mostly unaffected by MPs/NPs. In contrast, the Cd(II)-sorption capacity of B425 increased significantly by 18.6%, 24.2%, and 7.0% following interactions with PS25, PS200, and PS5000, respectively. Kinetic and isothermal analyses revealed that, irrespective of interaction with MPs/NPs, the active sites on B75 for Cd(II)-sorption remained heterogeneous and the Cd(II)-sorption process was primarily driven by chemical forces. Conversely, after interacting with MPs/NPs, physical adsorption played a major role in the Cd(II)-sorption on B425. Using 2D-FTIR-COS analysis, we determined that COO⁻ groups on the pig-bone biochar were the most sensitive for Cd(II)-sorption. However, following interaction with MPs/NPs, the contribution of COO - groups to the Cd(II)-sorption diminished. Additionally, after interacting with MPs/NPs, the polystyrene benzene ring skeleton and C-H bending outside the benzene ring contributed to the Cd(II)-sorption process. This study emphasizes that the effects of MPs/NPs should be considered when using biochar to remediate metal pollution in the environment. • B75’s surface area reduced after MPs/NPs interaction, while B425's increased. • MPs/NPs did not affect B75’s Cd(II)-sorption, but enhanced B425’s Cd(II)-sorption. • COO - groups on biochar are crucial for Cd(II)-sorption but diminished by MPs/NPs. • MPs/NPs introduce polystyrene-specific groups for Cd(II)-sorption on biochar.