Hard versus soft EPS coronas govern divergent nanoplastic mobility in seawater-saturated porous media

In this study, we elucidate the chemical heterogeneity between tightly and loosely bound extracellular polymeric substances (TB-EPSs and LB-EPSs, respectively) secreted by marine Pseudomonas aeruginosa, emphasizing their critical role in modulating nanoplastic (NP) behavior. We demonstrated that hard and soft corona nanoplastics (HNPs and SNPs), formed by TB-EPSs and LB-EPSs, respectively, exhibit distinct transport mechanisms in seawater-saturated porous media. Compositional analyses revealed that TB-EPS exhibits significantly higher hydrophobicity than LB-EPS, evidenced by its 32% higher C/H ratio, 45% lower O/C ratio, and reduced carboxyl/ester group density (0.1472 vs. 0.2690). TB-EPS also contains 43% more C-O/N bonds (0.1701 vs. 0.1189), indicating enriched protein-polysaccharide linkages that strengthen NP binding. Following corona formation, HNPs and SNPs show increases of 9.51% and 6.64% in hydrodynamic diameter, and reductions of 28.5% and 26.7% in zeta potential magnitude, respectively. Paradoxically, mobility increased by 20.38% and 10.73% for HNPs and SNPs, respectively, demonstrating that electrostatic forces do not govern transport. Small-angle X-ray scattering analysis revealed that HNPs form compact coronas with a radius of gyration (R_g) of 32.35 nm and a slope of 3.52, driven by 76.78% of hydrophobic protein adsorption of TB-EPS. SNPs develop extended coronas with a R_g of 35.14 nm and slope of 3.68, enhancing sand retention by 16.05% through spatial confinement effects. In this study, we linked EPS subfraction chemistry to corona architecture, revealing that steric hindrance and hydrophobic interactions-rather than electrostatic repulsion-primarily regulate NP mobility. These findings provide a quantitative framework for refining oceanic plastic-fate models by addressing the critical knowledge gap regarding EPS heterogeneity-driven NP transport regulation.