Benzo[a]pyrene and heavy metal ion adsorption on nanoplastics regulated by humic acid: Cooperation/competition mechanisms revealed by molecular dynamics simulations

Nanoplastics adsorb pollutants and organic matter to aggravate or alleviate impact to the eco-environment and human health. However, the interaction mechanisms remain unclear and difficult to study using current experimental techniques. By means of molecular dynamics simulation, here we investigate adsorption of benzo[a]pyrene (BaP) and heavy metal ions (Cu2+) on nanoplastics of different materials and surface charges regulated by humic acid (HA). Among considered materials, polystyrene shows the highest capacity of adsorbing BaPs via forming sandwiched π-stacking structures with benzene rings. Driven by hydrophobic, electrostatic and hydrogen bonding interactions, HAs spontaneously aggregate into micelle-like structures with hydrophobic core and charged exterior accessible to BaPs and Cu2+, respectively. Cationic and neutral nanoplastics adsorb more HAs to form eco-coronas, which modulate BaP and Cu2+ adsorption via following cooperation/competition mechanisms. On one hand, the direct binding of BaPs to nanoplastics is hindered by HAs through BaP encapsulation plus competitive adsorption. On the other hand, adsorbed HAs expose carboxyl groups to offer rich binding sites to promote Cu2+ adsorption on neutral and cationic nanoplastics, while unbound HAs compete with anionic nanoplastics to inhibit Cu2+ adsorption. These results provide molecular level insights into transport, transformation and accessibility of nanoplastics with coexisting contaminants in the aqueous environment.