The influence of oxidation and hydrophobic coupling on the transport behavior of polyethylene microplastics: The synergistic effect of ultraviolet aging and surfactants

Microplastics (MPs) are inherently hydrophobic, limiting their environmental mobility. Environmental active substances adsorb onto MPs surfaces through hydrophobic interactions, influencing their transport. However, prolonged environmental aging increases oxygen-containing functional groups on MPs surfaces. This oxidation reduces hydrophobicity, consequently decreasing substance adsorption while potentially enhancing mobility. However, the coupled effects of oxidative aging and surface attachment on MPs transport remain largely unexplored. Current research focuses on isolated factors, with transport under combined effects so far significantly understudied. In this study, the transport behavior of polyethylene (PE) MPs under the combined effect of oxidative aging and surface attachment was investigated by column experiments. Surface oxidation was induced by ultraviolet irradiation, and three different types of surfactants were used for external substances. The results demonstrate a non-monotonic U-shaped trend in PE mobility with aging time (transport capacity: 1-day aged >30-day aged >15-day aged > 3-day aged > 7-day aged). The strong hydrophobicity of pristine PE MPs facilitates substantial adsorption of surfactant, significantly enhancing their hydrophilicity and colloidal stability, thereby maximizing initial mobility. Short-term aging introduces oxygen-containing functional groups that impede adsorption coverage of reactive species via electrostatic repulsion. Crucially, these nascent functional groups cannot fully substitute the stabilizing role of adsorbed species, resulting in reduced mobility. Prolonged aging accumulates sufficient surface oxygen-containing functional groups to independently stabilize MPs suspension, leading to mobility recovery. Notably, at high concentration of positively charged surfactant, the transport ability increases with the aging time, while at low concentration, the mobility gradually decreases. This is attributed to the positively charged surfactants forming a bilayer on the MPs surface, which weakens the influence of surface functional groups. These findings provide novel insights into understanding MPs environmental behavior under multiple concurrent aging effects and provide a more robust basis for objectively evaluating MPs transport and fate in soil-groundwater systems.