Soil dissolved organic carbon governs the transport of polyethylene terephthalate microplastics in heterogeneous soil systems

Abstract

The fate and transport of microplastics (MPs) in heterogeneous soil systems remain poorly understood. This study systematically investigated the transport behaviors of three distinct MP polymers, polystyrene (PS), polypropylene (PP), and polyethylene terephthalate (PET), with twelve representative Chinese soil colloids using saturated column experiments packed with quartz sand. Without soil colloids, PS and PP exhibited high transport with breakthrough mass recoveries ranging from ∼70 to 90%. Notably, the introduction of various soil colloids exerted a negligible impact on the transport of PS and PP, suggesting their transport is largely independent of soil mineralogy. In contrast, individual PET displayed significantly lower transport (recovery ∼10%) in the absence of colloids. However, PET transport was markedly facilitated by the presence of soil colloids, with recoveries surging to ∼60% depending on the soil type. To decipher the underlying drivers, an XGBoost machine learning feature importance analysis was employed, identifying dissolved organic carbon (DOC) as the predominant soil physicochemical property governing PET transport. This finding was further corroborated by scanning electron microscopy (SEM), which revealed a stable PET-DOC interaction mechanism that enhances PET transport. Our findings underscore that MP transport in terrestrial environments is not solely determined by polymer identity but is governed by a critical interplay with soil DOC. These insights are essential for accurately assessing the environmental risks of MPs, particularly in agricultural soils characterized by fluctuating organic matter levels.