Experimental and Computational Insights into Congo Red Adsorption by Polyethylene and Polyethylene Terephthalate Microplastics

<title>Abstract</title> Plastic pollution has emerged as a critical environmental issue, with microplastics (MPs) receiving increasing attention due to their persistence and ability to interact with chemical pollutants. While MPs are often considered environmental hazards, their potential valorization as low-cost adsorbents in contaminant removal is also being explored. In this study, we investigated the adsorption of Congo red (CR), a model azo dye commonly found in textile effluents, onto polyethylene (PE) and polyethylene terephthalate (PET) microplastics (0.05 mm). Comprehensive characterization combined with adsorption experiments revealed distinct behaviors: PE exhibited adsorption consistent with the Langmuir model, indicative of monolayer adsorption on homogeneous sites, whereas PET followed the Freundlich model, suggesting multilayer adsorption on heterogeneous surfaces. Kinetic studies showed rapid uptake in the first 60 minutes, reaching equilibrium with removal efficiencies of 4% (PE) and 10% (PET). To gain molecular-level insight into these differences, density functional theory (DFT), docking simulations, and non-covalent interaction (NCI) analyses were performed. Computational results confirmed that PET promotes π–π stacking and hydrogen bonding between its carbonyl/aromatic groups and CR, while PE interactions are limited to weak hydrophobic and van der Waals forces. The alignment between experimental and theoretical findings highlights the critical role of polymer structure in defining adsorption pathways. Overall, this integrative approach advances mechanistic understanding of microplastic–pollutant interactions and provides a framework for assessing the environmental risks and valorization potential of MPs.