Experimental study on the transport processes of different types of microplastics in rainfall runoff over urban road surface

Different types of microplastics (MPs) originating from urban areas have been recognized as major contributors to the deterioration of aquatic ecosystems. While rainfall runoff serves as the primary pathway for MPs to enter water bodies, the underlying transport processes remain poorly understood. In this study, a series of simulated rainfall experiments were conducted with varying rainfall intensities and slopes to investigate the transport processes of three types of MPs: polyvinyl chloride (PVC) fragments, polypropylene (PP) particles, and rubber (R) particles. The results revealed that various MPs exhibited similar transport processes, characterized by initial increases followed by decreases in both concentration and transport rate. Among the three types, rubber particles demonstrated the lowest transport potential, while PVC fragments showed higher transport potential under low rainfall intensities, and PP particles were more readily transported during heavy rainfall events. The shape, density and surface roughness of MPs, along with rainfall intensity, slope surface texture and slope, were identified as the dominant factors influencing MPs transport. Specifically, increased rainfall intensity enhanced MPs transport, whereas steeper slopes inhibited their movement. The exponential transport model proved effective in predicting MPs transport processes in rainfall runoff, achieving determination coefficients above 0.9. The wash-off coefficient of MPs showed positive correlation with rainfall intensity and negative correlation with slope. This study advanced the understanding of MPs transport processes in rainfall runoff over urban roads.