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Further research on the impacts of humic acid in the aggregation of nanoplastics: The roles of molecular weight and surface functionalization
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Researchers studied how humic acid — a natural compound found in soil and water — coats nanoplastic particles and changes how they clump together, finding that larger humic acid molecules create thicker coatings that keep nanoplastics suspended and dispersed rather than settling. This matters because dispersed nanoplastics travel farther through water systems and are more likely to be ingested by organisms.
• The aggregation of polystyrene nanoplastics (PS NPs) depended on HA adsorption layer. • The molecular weight and mass of HA on PS NPs surfaces controlled layer thickness. • Thicker layers impeded PS-Bare/PS-COOH flocculation in NaCl. • Ca 2+ transforms PS-Bare/PS-COOH with thicker adsorption layer to larger flocs. • Small dosage of HA with low molecular weight formed larger PS-NH 2 flocs. Humic acid (HA) affects the aggregation behavior of nanoplastics. This study examines the adsorption of molecular weight (MW)-fractionated HA onto functionalized polystyrene nanoplastics (PS-Bare, PS-COOH, and PS-NH₂) in NaCl and CaCl₂, characterizes the properties of the adsorbed HA layer, and assesses their impact on nanoplastics stability. The results reveal that pristine and MW-fractionated HA form adsorbed layers with comparable functional groups and surface charge. However, the adsorbed layer thickness (ALT) exhibits a positive correlation with the adsorption mass and MW of HA. In NaCl, the maximum ALT values were 0.21∼5.93 nm for PS-Bare, 0.74∼9.43 nm for PS-COOH, and 2.23∼12.1 nm for PS-NH₂, whereas their ranges in CaCl₂ were 0.21∼7.1 nm, 1.05∼12.1 nm, and 2.61∼12.8 nm. The increased ALT elevated the critical coagulation concentration (CCC) values in NaCl from 343 to 426∼1266 mM for PS-Bare and 339 to 467∼1380 mM for PS-COOH, while the hydrodynamic diameter (D h ) of PS-NH₂ stabilized at approximately 143 nm, indicating enhanced steric repulsion and improved nanoplastics dispersion. Similar results were also observed in CaCl 2 containing low concentrations of HA. However, in CaCl₂, higher-MW HA at 10 mg C L⁻¹ can counteract steric repulsion through a bridging effect, leading to decreasing CCC to 19.2∼23.1 mM for PS-Bare and to 18.7∼21.6 mM for PS-COOH. Lower-MW HA at 5 mg C L⁻¹ promotes increasing D h of PS-NH₂ to 600∼900 nm via the synergistic effects of charge neutralization and weak steric hindrance. Additionally, HA <3kDa induces increasing D h of PS-NH₂ to 1300 nm through patch-charge attraction, irrespective of concentration.
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