Effect of pH and salinity on the release of polystyrene microplastics derived dissolved organic matter as revealed by experimental studies and molecular dynamic simulations

Microplastics-derived dissolved organic matter (MPs-DOM) poses a significant risk to aquatic systems. This study characterized MPs-DOM from polystyrene microplastics (PSMPs) upon photoaging in freshwater and seawater. For pristine PSMPs, plastic additives are the predominant substances in MPs-DOM. As the degree of aging increases, intermediates emerge as the new predominant substances in MPs-DOM. Both higher pH and salinity accelerate the aging of PSMPs and MPs-DOM release. Molecular dynamics simulations align with experiments showing that increased pH and salinity levels enhance the release of MPs-DOM. Interaction energy calculations revealed a link between MPs-DOM release amount and the interaction intensity between PSMPs and MPs-DOM. Generally, MPs-DOM having lower interaction energy with PSMPs is more liable to release, and aging of PSMPs leads to a decrease in their interaction energy with MPs-DOM. For example, the interaction energies in the pH 10 seawater system were slightly lower than those in the pH 7 seawater system. In the pH 7 seawater system, the interaction energy between butyl acetate and PSMPs was -41.97 kJ/mol, while in the pH 10 seawater system, this value was -26.86 kJ/mol. These insights are crucial for assessing the environmental behavior of MPs and MPs-DOM in aqueous environments.