Overlooked yet critical pathways for microplastics input to soil and groundwater system: Transport mechanisms and...

Microplastics (MPs) have emerged as critical environmental pollutants in landfill ecosystems, where their transport into soil and groundwater present considerable ecological risks. However, the influence of landfill environments on the fate of MPs remains poorly understood. This study systematically investigated the transport mechanisms of MPs in two landfill soils, focusing on the interplay between microplastic properties (density, size, and polymer type) and landfill environmental conditions (temperature, salinity). Using soil column transport experiments, X-ray tomography (Micro-CT), and pore network modeling (PNM), we evaluated the transport behavior of four common MPs in landfills: polypropylene (PP), polyethylene (PE), polystyrene (PS), and polyethylene terephthalate (PET). Results indicate that MPs transport is primarily influenced by their density and chemical properties, with 0.5 and 1.0 μm MPs exhibiting the highest mobility. Smaller MPs (0.1 μm) exhibited a blocking effect, occupying soil adsorption sites and reducing transport efficiency. High temperature (up to 75 °C) and high salinity significantly hindered MPs transport. Elevated temperatures increased soil particle polarity and dissolved organic matter release, altering pore structure. Micro-CT analysis revealed that rising temperatures reduced soil porosity from 0.379 to 0.289, leading to a 31 % to 36 % reduction in migration efficiency due to physical strain. Increased salinity compressed the electric double layer, diminishing electrostatic repulsion between MPs and soil particles. Simulation predictions showed that heavy rainfall accelerated MPs transport, with maximum migration depths reaching 58 cm and 64 cm in two landfill soils, respectively. These findings provide critical insights into the environmental behavior of MPs in landfills and inform strategies for mitigating soil and groundwater contamination.