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Hard versus soft EPS coronas govern divergent nanoplastic mobility in seawater-saturated porous media
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Researchers found that the type of extracellular polymeric substance corona — hard versus soft — formed by marine Pseudomonas aeruginosa governs divergent transport mechanisms of nanoplastics in seawater-saturated porous media. Tightly bound EPS produced hydrophobic 'hard coronas' that reduced nanoplastic mobility, while loosely bound EPS formed 'soft coronas' that enhanced transport, with distinct chemical compositions driving these differences.
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) 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 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.
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