Aging-dependent cotransport of microplastics and plastic additives in soil: Contrasting vector roles, interfacial...

The coexistence of microplastics (MPs) and plastic additives in subsurface environments threatens groundwater quality, yet how aging influences their interactive transport remains poorly understood. This study systematically investigated the cotransport of two prevalent polymer types (polystyrene, PS; polyvinyl chloride, PVC) and typical additives (bisphenol A, BPA; diethyl phthalate, DEP). Mechanical-thermal coupled aging was applied to simulate landfill conditions. Column experiments revealed that transport behavior is critically governed by polymer identity and aging state. In contrast to pristine MPs, aged PS exhibited enhanced mobility (mass recovery of 90.3%) and functioned as a potent contaminant carrier, whereas aged PVC acted primarily as a retention sink, with its transport impeded by aging-induced surface roughness and physical straining. Sequential pulse experiments further demonstrated that, while stationary microplastics enhance additive retention, mobile microplastic colloids can trigger a distinct secondary elution of sequestered pollutants through competitive partitioning. Mechanistic probing via atomic force microscopy (AFM) coupled with DLVO theory showed that hydrophobic interactions effectively counteract long-range electrostatic repulsion, thereby promoting contaminant capture. Density functional theory (DFT) calculations elucidated the molecular-level driving forces, identifying π-π stacking and hydrogen bonding as key binding interactions. Collectively, these findings underscore that aging fundamentally redefines microplastics as dynamic vectors, substantially amplifying the vertical dispersion risk of co-existing organic contaminants in groundwater systems.