Insights About Interactions between Microplastics and Humic Acids: An Approach Theoretical Modeling and Experimental

Microplastics (MPs) are ubiquitous and persistent pollutants that interact with various environmental components, especially with organic matter such as humic acid (HA) in sediments. This interaction is critical, for example, in estuarine sediments, which are key areas for MP deposition and where the presence of HA is frequently detected during extraction procedures. Considering the potential contact between HA and MPs, understanding their interaction mechanisms is essential. The present study employed both experimental and computational approaches to investigate the interaction between polyethylene (PE), polypropylene (PP), and polystyrene (PS) microplastics with HA. The experimental analyses, together with statistical evaluation, provided a more detailed view of interaction trends involving HA and MPs across the environmental parameters studied, particularly sediment granulometry (gravel, sand, and mud), organic carbon (OC), and organic matter (OM) revealing a clear correlation. To further investigate these interactions, computational analyses were employed. The computation also shows the clear tendency for HA-MP interactions, predominantly governed by van der Waals forces. Interaction potential energy (IPE) data suggest that PS exhibits a higher affinity with HA (-316 kJ mol^-1) than PP (-254 kJ mol^-1) or PE (-195 kJ mol^-1). Additionally, hydrogen bonding data indicate a greater reduction in HA-water hydrogen bonds for PS, favoring HA-PS interactions. Visualizations of these interactions confirmed the presence of van der Waals interactions, as well as stronger interactions involving between HA and PS, primarily involving oxygen atoms in HA. These findings highlight the affinity between HA and MPs, particularly PS, and emphasize the need for further studies on the environmental risks posed in environmental matrices, such as estuarine environments.