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

Abstract

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 (Dh) of PS-NH₂ stabilized at approximately 143 nm, indicating enhanced steric repulsion and improved nanoplastics dispersion. Similar results were also observed in CaCl2 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 Dh of PS-NH₂ to 600∼900 nm via the synergistic effects of charge neutralization and weak steric hindrance. Additionally, HA<3 kDa induces increasing Dh of PS-NH₂ to 1300 nm through patch-charge attraction, irrespective of concentration.

Keywords

• Nanoplastics
• Humic acid
• Molecular weight
• Adsorbed layer thickness
• Aggregation