Molecular-Level Insights into the Influence of Natural Organic Matter on Nanoplastic-Small Molecule Emerging Contaminant Interactions

A diverse range of emerging contaminants (ECs) coexist in aquatic environments, and a deep understanding of the interactions among these ECs is crucial for evaluating their behavior. This study investigates the interactions between polystyrene nanoplastic (PSNP), 2,2',5,5'-tetrachlorobiphenyl (PCB), 4-hydroxy-2,2',5,5'-tetrachlorobiphenyl (OHPCB), bisphenol S (BPS), phenanthrene (PHE), sulfamethazine (SM2), dichlorodiphenyltrichloroethane (DDT), and perfluorooctanesulfonate (PFOS) in water, as well as the impact of natural organic matter (NOM) on their interactions. The results indicate that pristine PSNPs are more likely to adsorb nonpolar ECs, whereas aged PSNPs preferentially adsorb polar ECs with oxygenated functional groups and that the adsorption of hydrophilic ECs on the PSNP promotes the adsorption of hydrophobic ECs. The presence of NOM significantly altered the adsorption mechanism of PSNP toward ECs. For the adsorption of ECs on pristine PSNP, the interaction between ECs and NOM facilitated their migration to pristine PSNP, and the ECs initially bound to NOM transferred to the surface of pristine PSNP. Further analysis indicates that the conformational changes of NOM upon adsorption, along with the resultant alterations in the interaction intensities of PSNP-ECs and NOM-ECs, are the key mechanisms driving the transfer of ECs. Through a "bridge" effect and a "protective layer" effect that impede the diffusion of ECs, NOM significantly enhances the adsorption stability of ECs on pristine PSNP. For aged PSNP, however, NOM reduces the adsorption efficiency of PSNP toward certain ECs, such as PCB and DDT, through a competitive adsorption mechanism. These results provide new insights for evaluating the environmental behavior of ECs.