Microplastics inhibit lead binding to sediment components: Influence of surface functional groups and charge environment.

The coexistence of heavy metals and microplastics in sediments is well recognized, yet the interactions within ternary systems remain underexplored, and comprehensive studies addressing the diverse sequences of sediment-microplastic-heavy metal coexistence are lacking. In this study, we systematically investigated the interactions among lead (Pb), polystyrene (PS) microplastics, and sediments (using goethite (Goe) and goethite-humic acid composite (GH) as examples) under different coexistence orders. The presence of PS significantly inhibited Pb adsorption by both Goe and GH. For Goe, adsorption kinetics and hydrochemical condition effects showed that PS reduced the electrostatic repulsion between Goe and Pb, leading to a fourfold increase in the mass transfer rate of Pb to the Goe surface. However, Pb 4f deconvolution indicated competition between PS and Pb for hydroxyl groups on Goe, resulting in a 7.4% reduction in Pb adsorption. In the GH system, hydrophobic interactions and coordination complexes between PS and humic acid on GH inhibited the electrostatic adsorption and mass transfer processes between Pb and GH. Pb adsorption behavior and changes in Pb-O content under different coexistence orders further verified that competition between PS and Pb for carboxyl and hydroxyl groups on GH led to a 28.0% reduction in Pb adsorption. This study highlights the inhibitory effect of PS on Pb adsorption by Goe and GH, providing a theoretical basis for understanding the migration and transformation patterns of microplastics and heavy metals in sediments.