Marine Conditions Enhance Acetochlor Adsorption on Biodegradable Microplastics: Mechanistic Insights and Management Implications

Biodegradable microplastics can adsorb organic pollutants in aquatic environments, worsening contamination. However, the molecular mechanisms behind this association remain poorly understood. This study employs molecular dynamics (MD) simulations and density functional theory (DFT) calculations to systematically explore the molecular interactions between polylactic acid (PLA) and the herbicide acetochlor (ACT) in freshwater and a seawater analog. Our simulations reveal that PLA demonstrates a notably higher adsorption capacity for organic pollutants in seawater than in pure water. This improvement stems from three main factors: (i) PLA forms a more compact microstructure under saline conditions, (ii) its specific surface area increases, offering more active adsorption sites, and (iii) surface adsorption between PLA and ACT molecules dominates. DFT calculations support the MD simulation findings, demonstrating stronger PLA–ACT interaction energies in seawater. The adsorption process is mainly driven by two fundamental mechanisms: van der Waals forces and hydrogen bonding. Importantly, dissolved salt ions in seawater act as molecular bridges, facilitating interactions between PLA and ACT. Based on these insights, the study proposes conservative, testable risk indicators and planning/management implications for coastal drainage infrastructure, contributing to broader sustainable development objectives.