The transport of polystyrene microplastics in saturated porous media: Impacts of functional groups and solution chemistry

Global attention to microplastics (MPs) pollution has been increasing as it has become a novel environmental issue. Natural aging processes alter MPs surface properties, introducing charged functional groups that affect their transport in porous media. This study investigated the transport of polystyrene microplastics (PSMPs) in saturated porous media through column experiments, including non-functionalized PSMPs (PS-Bare), carboxyl-modified PSMPs (PS-COOH), and amino-modified PSMPs (PS-NH). Unlike previous studies focusing on pristine microplastics, our research integrated the effects of surface functionalization with complex solution chemistry, including ionic strength, cation valence, and pH. Results indicated that surface functional groups and solution chemistry combined to impact PSMPs migration through zeta potential and hydrodynamic size. Increasing ionic strength decreased migration rates due to double-layer compression and charge screening. Higher cation valence and lower pH decreased PS-Bare and PS-COOH migration rates, while PS-NH showed the opposite trend due to differences in surface charges. As pH increased, carboxyl groups dissociated, enhancing the negative charge on PS-COOH and promoting its migration, while amino groups deprotonated, reducing the positive charge on PS-NH and inhibiting its migration. PS-NH exhibited higher mobility than expected. Despite its positive charge, PS-NH preferentially occupied active sites on sand surfaces, reducing aggregation and enhancing transport. In the presence of Al, PSMPs recovery rates were PS-NH (94.60%) > PS-COOH (41.48%) > PS-Bare (41.12%). This study enhances understanding of functionalized microplastics transport and its potential impact on groundwater contamination.