Experimental study on transport processes of fragment microplastics during urban road rainfall runoff

Urban runoff serves as a critical pathway for the transport of microplastics (MPs) from terrestrial to aquatic ecosystems. Although fragment-shaped MPs are consistently identified as the dominant morphological type in receiving waters, existing studies have focused on granular or spherical particles, leaving the wash-off dynamics of irregular fragments poorly understood. While the traditional exponential wash-off model has been applied to simulate MP transport, its key parameters remain empirical constants that necessitate case-specific calibration, thereby limiting predictive capability. To address these knowledge gaps, this study conducted systematically controlled laboratory experiments to investigate the wash-off processes of polyvinyl chloride fragments (1-5 mm) under simulated urban road rainfall runoff. The individual and coupled effects of key factors-rainfall intensity, road slope, fragment size, and initial load-were quantified. Results demonstrated that Increasing size from 1 mm to 5 mm not only suppressed the enhancing effect of rainfall intensity but also attenuated the influence of slope changes. Initial load exerts a nonlinear characteristics, first promoting then inhibiting transport The first flush effect, quantified by coefficient b (ranging from 0.055 to 0.778), is most suppressed under low rainfall intensity yet paradoxically strongest for larger fragments on steeper slope. Importantly, new empirical equations were developed to parameterize the exponential wash-off model coefficients (k and C_F), explicitly incorporating the effects of rainfall intensity, slope, initial load, and size, enabling reliable prediction of fragment MP wash-off. These equations enable reliable prediction of fragment MP wash-off without case-specific recalibration. Collectively, these findings advance the mechanistic understanding of urban fragment microplastics transport and provide a scientific basis for optimizing stormwater management strategies to mitigate pollutants export and protect aquatic ecosystem health.