Effects of Polymer Morphology on Humic Acid Adsorption onto Model Microplastics and Nanoplastics

Eco-coronae, formed by natural organic matter, such as humic and fulvic acids, on micro- and nanoplastics (MNPs), can enhance surface oxidation, increase human exposure to contaminants, and affect interactions between MNPs and plasma membranes. To better understand the formation of eco-coronae around MNPs, we investigate the influence of crystallinity, an intrinsic property of most commercial semicrystalline plastics, including polyolefins and polyamides, on the adsorption of a model humic acid (HA) in aqueous conditions using all-atom molecular dynamics. Adsorption was found to be thermodynamically favorable across all polymers and morphologies. Crystalline surfaces promote binding by providing planar hydrophobic interfaces that maximize contact with the aromatic groups of HA, and the presence of polar groups on crystalline MNP surfaces enables additional dipole-dipole interactions. Diffusive interfaces of amorphous polyolefins extend the polymer-HA interaction range and, in turn, yield stronger net binding affinities than those of crystalline surfaces. Reduced segmental dynamics in glassy amorphous polyamides hinder structural reorganization upon HA association, mitigating the hydrophobic contacts that drive adsorption. Overall, our simulations demonstrate the effects of MNP surface morphology on HA adsorption and provide molecular-level insights into the factors controlling eco-corona formation on MNPs.