Horizontal transport characteristics of microplastics under simulated hydrodynamic conditions

The horizontal transport of microplastics on soil surfaces represents a crucial pathway for their distribution in the environment. This study systematically investigated the horizontal transport characteristics of microplastics under simulated hydrodynamic conditions to understand their behavior during surface runoff scouring. Experiments were conducted using 1 μm polystyrene particles and quartz sand (250-425 μm) to examine the effects of runoff velocity (10, 16, 23 cm/s) and slope gradient (0 %, 3 %, 5 %) on microplastic transport. Results showed that the total microplastic loss ranging from several hundred to a few thousand items in effluent exhibited a positive correlation with flow velocity after 30 min flow erosion, with no significant relationship with slope gradient-possibly due to interference from flow patterns. Under conditions of minimal or no quartz sand loss, high-velocity flow initially caused more pronounced erosion of microplastics, followed by a sharp decline, ultimately resulting in lower loss rates compared to low-velocity flow in the later stages. During extensive quartz sand erosion, microplastic loss remained consistently high throughout the experiment period, with a maximum loss of up to 8500 items per minute. The study revealed that flow primarily eroded microplastics dispersed in the pore water surrounding quartz sand, while also increasing microplastic adherence to sand surfaces. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was applied to analyze the retention and release mechanisms of small colloidal microplastics on quartz sand surfaces. This study elucidates the microscale mechanisms underlying small colloidal microplastic release during surface runoff by integrating XDLVO theory and torque analysis with hydrodynamic experiments. The results provide a scientific foundation for predicting small colloidal microplastic mobility and support the development of practical strategies to mitigate soil microplastic pollution in agricultural and urban environments.