Further research on the impacts of humic acid in the aggregation of nanoplastics: The roles of molecular weight and surface functionalization

• 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.