Vertical transport behavior of soil polycyclic aromatic hydrocarbons (PAHs) microplastic-mediated based on column leaching experiment

The transport behavior and key drivers governing the fate of microplastics (MPs) and polycyclic aromatic hydrocarbons(PAHs) in agricultural ecosystems are not yet fully understood, despite their widespread co-contamination and recognized environmental risks. This study investigated the longitudinal leaching behavior of PAHs in MP-contaminated soil by combining soil column leaching experiments with controlled factors such as pH and dissolved organic matter (DOM). Results indicated that the PAHs leaching concentration exhibited a fluctuating decline followed by a marked increase with rising pH, reaching its minimum at pH 7 and peaking at 1.6 times the concentration under neutral conditions within the alkaline range (pH 9-11). The presence of MPs reduced the leached PAHs concentrations by 8.2%-20.2%, significantly enhancing soil retention capacity. In surface soil, the PAHs residual content was lowest (993.3 ng·g⁻¹) under acidic conditions and highest (3091.97 ng·g⁻¹) under neutral conditions. Increased DOM concentration elevated PAHs concentration in leachate by 44%-75%, with the transmission and migration capacity of low- and medium-ring PAHs being significantly higher than that of high-ring PAHs. The PAHs residual rate in surface soil decreased with increasing DOM concentration, and DOM-promoted PAHs migration showed distinct ring-number dependence: the residual rate of 5-6ringPAHs (32%-56%) was significantly higher than that of 2-3 ring PAHs (14%-26%). This study confirms that MP-contaminated soil enhances the environmental retention effect of PAHs through pH and DOM regulation mechanisms, inhibiting PAHs migration.