Research On The Occurrence And Runoff Flushing Behaviors Of Urban Microplastics

In this study,we analyse the deposition of microplastics in urban surface sediments,the scouring effect of runoff and the migration behaviour of microplastics in urban pavements and stormwater pipes,reveal the pollution risk of microplastics,and elucidate the scouring mechanism and migration factors of microplastics.The mechanisms of microplastic scour and migration factors were also elucidated.The study focuses on the abundance,particle size,morphology,colour and polymer type of microplastics,as well as health risk and fragmentation stability analysis,by revealing the characteristics of microplastics in urban surface sediments.The results showed that the abundance of MPs in surface sediments of five functional districts in Daxing District ranged from 2060 items/kg to 8680 items/kg,with commercial district > transportation district >residential district > recreational district > cultural and educational district.Blue,green,black and transparent microplastics were more abundant and mainly in the form of fragments.The main polymeric materials collected in the sediment samples from the five functional districts were,from highest to lowest,polypropylene(PP),polyethylene(PE),polyethylene terephthalate(PET),polystyrene(PS),nylon(PA)and acrylonitrile-butadiene-styrene(ABS)plastics,with PP(43%–87%)and PE(5%–33%)currently being the main contaminants in the five functional districts.The average daily intake of microplastics is high in transport,residential and commercial districts,where infants and children are at greater risk of ingestion.In addition,the degree of fragmentation of microplastics is associated with the accumulation of microplastics in smaller particle sizes.The runoff scouring process of different particle sizes of microplastics on concrete pavements was investigated through artificially controlled experimental simulations.It was found that rainfall intensity,pavement slope and sediment properties all had an effect on the scouring rate of microplastics.As the rainfall intensity increased from 28.32 mm/h to 66.52mm/h,all five particle sizes of microplastics showed a large initial scouring phenomenon.As the slope increased,a light rainfall of 28.32 mm/h resulted in a significant increase in the scour rate of microplastics,and the co-migration of microplastics and sediment showed an opposite trend.This study can provide a simple and effective method to assess the migration behaviour of microplastics in surface runoff based on the mass per unit district of microplastics and sediment under different conditions.By studying the migration behaviour of microplastics in stormwater pipes,it was found that microplastic particle size,the amount of flow rainfall intensity,pipe slope and sediment properties are important factors influencing the migration of microplastics in stormwater pipes.At three flow rates(100 L/h,200 L/h and 400 L/h),pipe scour rates for five particle sizes of microplastics increased with increasing flow intensity,while the initial scour effect of sediment was not significant compared to microplastics.The accuracy of the scour equation fit was proportional to flow size and inversely proportional to microplastic particle size.As the particle size of the microplastic decreases,the magnitude of the change in scour rate difference at the three different flow rates tends to level off.This further suggests that the flow rate of water in the stormwater pipe is one of the key factors affecting the migration of microplastics and the accuracy of the fit.