Transport of eco-corona coated nanoplastics in coastal sediments

Transport is a critical process for understanding the fate and risk of nanoplastics (NPs) in marine environments. In this work, vertical transport of polystyrene (PS) NPs with different surface properties in marine sediments were investigated. Two types of Pd-doped (PS-Pd, 100 nm) NPs with distinct hydrophobicity were successfully synthesized, and both types of NPs showed weak and insignificant transport, with ratio of maximum effluent to influent NPs concentrations (M_eff) at 10 %-11 %. This is mainly due to physical straining of both NPs as caused by strong aggregation as confirmed by advection-dispersion modelling. Two functionalized NPs (PS-COOH and PS-NH₂) showed strong transport in marine sediment, mainly because of weak physical straining. Negatively charged and less hydrophobic PS-COOH NPs exhibited stronger transport (M_eff, 43 %) than PS-NH₂ NPs (M_eff, 15 %). Heteroaggregation experiments and TEM-EDS mapping demonstrated that weak hydrophobic interaction with organic matter and electrostatic attraction with minerals were the main reasons for higher transport of PS-COOH than PS-NH₂. The formation of eco-corona on NPs inhibited the transport of PS-COOH NPs. However, eco-corona enhanced PS-NH₂ NPs transport (M_eff, 20 ± 5 %) because of decreased electrostatic attraction. Interestingly, for the two PS-Pd NPs, the transport performed differently after eco-corona coating, in which the transport of hydrophilic PS-Pd-1 decreased after eco-corona coating due to increased hydrophobic interaction, while the increased transport of hydrophobic PS-Pd-2 was caused by weakened physical straining in sediments. These results provide insight into transport mechanism of NPs, and highlight critical roles of eco-corona in the fate and processes of NPs in marine sediments.