Microplastics enhance soil residue of polycyclic aromatic hydrocarbons: Roles of pH and dissolved organic matter

Abstract

In agricultural ecosystems, the coexistence of microplastics (MPs) and polycyclic aromatic hydrocarbons (PAHs) poses a serious environmental challenge. However, a systematic understanding of their migration mechanisms and driving factors remains inadequate. The core innovation of this research was the integration of dynamic soil column leaching with multi-factor control. By employing a multi-factor controlled soil column leaching experiment, we aimed to elucidate the coupled mechanisms underlying the interactive effects of pH and dissolved organic matter (DOM) on the dynamic migration behavior of PAHs in MPs-contaminated soi, thereby establishing a more comprehensive and environmentally relevant mechanistic model. The results indicated that the migration dynamics of PAHs in the soil profile were significantly influenced by pH. Under acidic (pH 3 and 5) and alkaline (pH 9 and 11) leaching conditions, the residue of PAHs was significantly enhanced. When the pH was 11, the residual concentration of PAHs reached their peak at a depth of 30–40 cm (8264.22 ± 167 ng g⁻¹ in polyethylene (PE)-contaminated soil and 4026.17 ± 189 ng g⁻¹ in the control soil (CK)). At all pH levels, the PAHs residual amounts in PE-contaminated soil were consistently (21 ± 2.4)% - (73 ± 5.8)% higher than those in CK. Under acidic and alkaline leaching conditions, the residual concentrations of low-ring (2–3 rings) and medium-ring (4 rings) PAHs in both soils increased initially and then decreased with depth, reaching their peaks at 30–40 cm and 40–50 cm, respectively. pH variations altered the surface charges of PE-MPs and soil particles, establishing electrostatic interactions may be the predominant mechanism governing PAHs-MPs interactions. With the increase in DOM concentration (5–40 mg L⁻¹), the residual concentrations of PAHs in each ring were significantly reduced by (19.4 ± 1.5)% - (22.7 ± 2.6)%. Without DOM treatment, the residual amounts continuously decreased with depth, reaching the lowest at 40–50 cm. High-ring PAHs showed strong depth attenuation characteristics in the presence of DOM, with the residual percentages ((32 ± 3.6)% - (56 ± 5.2)%) significantly higher than those of low-ring PAHs ((14 ± 0.9)% - (26 ± 2.3)%). The results shown that the aromatic structures within DOM promoted the adsorption of PAHs throughπ-πinteractions. The residual amounts of each PAH component in PE-contaminated soil were (18.6 ± 1.3)% - (23.8 ± 0.8)% higher than those in CK. The PAHs adsorption phenomenon exhibited by MPs may be governed by the combined effects of electrostatic interactions, π-π interactions and hydrophobic partitioning effect. This study revealed that soil rich in MPs enhanced the environmental residue effect of PAHs through pH and DOM regulation mechanisms, thereby reducing the risk of groundwater pollution.