The Release And Migration Behavior Of Microplastics In Urban Rainwater Facilities

As the demand for urban flood prevention and drainage increases,a large number of plastic rainwater facilities are in use.Plastic rainwater facilities will be affected by various environmental factors,such as temperature,light,mechanical force,hydraulic scouring,etc.As the aging time increased,cracking and peeling,microplastics will inevitably be released into the rainwater system and migrate with it.Microplastics not only adsorb various pollutants in the environment,but also are easy to be enriched by biological inhalations such as large algae,animals and plants,which has great environmental risks.Information on the release and transport behavior of microplastics in urban rainwater systems is still lacking,and it is important to assess the risk of microplastic pollution in urban rainwater systems by understanding the release behavior of microplastics in plastic rainwater facilities and their migration mechanisms in environmental media.In this study,three typical plastic rainwater facilities,namely rainwater inspection wells,rainwater storage modules,and rainwater pipes（with HDPE,PP,and PVC as the main components,respectively）,were selected as research objects to study the microplastic release behavior of rainwater facilities during the aging process by simulating the environmental conditions in which the rainwater facilities are located.Then,based on this,we simulated the scouring process in the rainwater pipe environment to study the migration behavior of microplastics in rainwater pipe under different hydraulic and sediment conditions,and combined with model fitting to further reveal the migration behavior of microplastics in rainwater pipe sediments.The results of the study are as follows.（1）After 15–45 days of UV aging and 72 h of hydraulic scouring,the surfaces of the three facilities were found to exhibit increases in roughness,cracks,folds,and cavities,with the most pronounced changes occurring in the rainwater storage module.As the aging time increased,oxygen-containing functional groups formed and led to carbon chain scission.Fourier transform infrared spectroscopy（FTIR）,two-dimensional correlation spectroscopy（2D-COS）,and X-ray photoelectron spectroscopy（XPS）of facility surfaces showed that the formation of oxygen-containing functional groups was an important factor affecting the release of microplastics.The amount of microplastics released from the three facilities ranged from 160to 1905 items/g（microplastics/facilities）,following in the order of rainwater inspection well>rainwater storage module>rainwater pipe.The particle size of the released microplastics ranged from 3 to 1363μm,with 10–30μm accounting for the greatest proportion of particles,50.10%.The size of microplastics released from the rainwater inspection well and rainwater storage module increased with the aging degree,while the release from the rainwater pipe decreased.The release behavior depends mainly on the composition of the materials and the aging time.Thus,microplastics can be released from plastic rainwater facilities under suitable conditions.The results can be used to further evaluate microplastic pollution caused by urban rainwater facilities.（2）Plastic rainwater facilities have become an irreplaceable and important component of rainwater systems.These facilities could be a significant source of microplastics during the aging process.However,the information on microplastic release behavior during aging process is still lacking.To complement this research gap,Fenton reagents were used to simulate the reactive oxygen species（ROS）induced aging process of three typical rainwater facilities（i.e.,rainwater inspection well,rainwater storage module,and rainwater pipe;mainly composed of high-density polyethylene,polypropylene and polyvinyl chloride,respectively）and the subsequent microplastic release behavior.After 6 days of Fenton aging,an increase in sharpness,wrinkles,and voids was observed on the surface of the three facilities by scanning electron microscope（SEM）.The functional groups on the facility surface were analyzed by FTIR,XPS,and 2D-COS.The quantities of microplastics released from the three facilities ranged from 158to 6617 items/g（microplastic/facilities）,following in the order of rainwater storage module>rainwater inspection well>rainwater pipe.The release amount mainly depends on the material and aging time.The particle size of the released microplastic ranged from 2 to 1362μm,with10–30μm accounting for the greatest proportion（62.7%）of the particles.The size of microplastics released from the rainwater facilities decreased with the aging degree.This study indicates that the aging process caused by ROS in the environment may lead to the release of large amounts of microplastics from plastic rainwater facilities.（3）Rainwater pipes（PVC）are more stable under UV and Fenton aging conditions compared to the other two rainwater facilities.The microplastic release behavior of rainwater pipes was further analyzed by simulating acidic,alkaline and oxidizing environments.It was found that the surface morphology of the rainwater pipe in the acid,alkali,and oxidizing environments all changed with aging time,with a large number of holes and folds,and the interior of the holes gradually became smooth,with alkali aging having the most significant effect on the plastic rainwater pipe.Through FTIR and XPS characterization of the rainwater pipe,it was found that the carbonyl group was most affected during the aging process of Na OH and HCl,and the carbonyl absorption peak gradually increased and broadened with the increase of aging time.For HNO₃ and H₂O₂ aging conditions,the strong peaks at 1400–1500 cm^-1 increased significantly with aging time.The release of microplastics in rainwater pipes increases significantly after aging,with the largest amount of Na OH being released,followed by H₂O₂,HNO₃,and HCl,so the alkaline environment is more likely to lead to the release of microplastics from rainwater pipes.With the increase of aging time,the number of microplastics was observed to decrease gradually,which may be the further decomposition of primary microplastics,resulting in difficulty in observing the statistics.（4）The migration behavior of microplastics in rainwater pipes under different hydraulic,pipe,and sediment conditions was revealed by simulating the scouring process of microplastics in rainwater pipe sediments combined with mathematical model fitting.It was found that the larger the slope and sediment thickness,the higher the concentration of microplastics in the discharge water at the end of the pipe in the initial stage of scouring（within 10 s）,and the more drastic the change of microplastic concentration in the first 30 s.The higher the slope and sediment thickness,the higher the concentration of microplastics in the discharge water at the end of the pipe.When the sediment particle size was small,the SS and microplastic concentration in the effluent decreased due to the increase of adhesion;while at a larger flow rate,the microplastic concentration in the initial scouring decreased the fastest and the scouring intensity was the highest,but the volume of the solution increased per unit time,resulting in the decrease of the obtained microplastic concentration.When the slope,flow rate,and sediment thickness increase will bring a larger cumulative scouring rate,which mainly shows a large increase in the early stage and a slower increase in the later stage.If the scouring process is long enough,the scouring rate can keep increasing and approach 100%.The small size sediment is less affected by water scouring due to its strong adhesion ability,thus the sediment retains more microplastics.The scouring flux decreased exponentially with time under different conditions,and eventually the flux tended to be close to 0,with the most significant decrease in flux during the first 30 s.The increase in slope led to a greater scouring flux;while the increase in flow rate led to a very high flux in the first 15 s,followed by a rapid decrease in flux.The decrease in sediment particle size and the increase in adhesion to the pipe wall resulted in a decrease in microplastic scouring flux.