| Plastic pollution is one of the most significant global environmental problem.The soil is a main sink of plastic waste in terrestrial environments.The plastic on the soil surface would be weathered and broken into plastic fragments less than 5 mm in size,which were called microplastics(MPs).The microplastics in the surface soil may be transported to the deep layers,which would further pose threats to the soil ecosystem and groundwater system.The migration behavior of MPs in the soil affects its environmental fate and possible eco-environmental risks.However,there are no comprehensive and in-depth research reports on the migration of MPs in actual farmland soil,and the migration of MPs in surface soil and related factors are not yet clear.Regarding this issue,we have conducted indoor simulative leaching experiments,outdoor soil box experiments,and animal incubation experiments in this study,in order to systematically study the migration behavior of MPs in topsoil.In this study,sizes,shapes and polymer types of microplastics were firstly characterized and analyzed by laser particle size analyzer,stereo microscope and Fourier Transform Infrared Spectrometer.Then,a set of simulative rainfall device was designed.Based on a 21-day soil box leaching experiment and subsequent layered sampling analysis,the migration behavior of different sizes,shapes and types of MPs under different rainfall and soil slope conditions was explored.Subsequently,a soil box experiment with the same experimental duration was carried out in summer in Shanghai to observe the migration of MPs from the soil surface to the deep soil layer,and related runoff behaviour.The migration of MPs under actual natural conditions and its related factors were further investigated through analyzing meteorological factors.Finally,by soil incubation experiments on the common soil animal Lumbricus terrestris,we have investigated impacts of feeding and excretion,body surface adhesion and other activities on the migration of additive MPs in soil.The main results are as follows:(1)Under simulative leaching action of 5,15 and 25 mm daily rainfall,the total mobility of MPs on soil surface were 0.74±0.03%,3.79±0.17%and 4.85±0.13%,respectively,which were significant positive correlation with rainfall(p<0.05).The maximum depth of vertical migration of MPs was 4-7 cm.Simulated rainfall caused the MPs to migrate horizontally into the runoff water body outside the box.The MPs collected from the runoff in three rainfall groups were 0%,0.30±0.01%and 2.41±0.07%,with an increasing trend.(2)Under condition of simulative rainfall of 15mm/d,the mobility of microplastic particles in the three slope soils of 0%,8%and 15%were 2.72±0.03%,3.79±0.17%and 3.83±0.33%,respectively.The total mobility was significant positive correlation with soil slopes(p<0.05).Among them,the maximum migration depth of soil MPs with a soil slope of 8%was 7 cm,and the maximum migration depth of soil MPs with a slope of 15%and 0%drops to 6 cm and 5 cm,and the distribution of MPs numbers were significantly different among three groups.(3)After 21 days of simulated rainfall(15 mm/d),the total mobility of microplastic with three sizes of<0.3 mm,0.3-1 mm and 1-3 mm were 5.00±0.21%,4.07±0.29%and 2.88±0.21%,respectively.Among them,the migration distance of microplastic particles>1 mm in the horizontal direction of the topsoil was the shortest,with a migration distance of 12 cm,and MPs<0.3 mm migrate the deepest in the vertical direction of the soil,at 7 cm.The foam has the highest mobility in the horizontal direction of the soil surface,which is 2.08±0.42%,followed by particles.Fibers and films are more likely to flow into the water body,and the total amount was1.93±0.65%and 2.07±0.69%,respectively.The mobility of MPs particles and fibers in the soil profile were relatively higher,0.60±0.12%and 0.52±0.31%,respectively.(4)Under the action of an outdoor rainfall of 3.1mm/d,the total mobility of PET MPs particles was 2.37±0.07%.Among them,the relative mobility of MPs in the horizontal direction of the soil surface,in-situ soil profile,and runoff water were 1.28±0.04%,1.13±0.02%,and 0.18±0.03%,respectively.The maximum migration depth in the soil was 5 cm.Among the three kinds of MPs with particle sizes,0.3-1mm MPs have the highest total mobility at 3.09±0.28%,and MPs with particle diameters of 1-5 mm have the lowest total mobility at 1.37±0.07%.Among the four types of MPs,the fiber has the highest total mobility,which was 2.54±0.14%;following the film,which was 2.20±0.34%;the foam has the lowest total mobility,which was only 0.56±0.07%.(5)In the earthworm box incubation experiment with the average densities set as8.55,28.49 and 45.58 individual/m~2,respectively,the total mobility of microplastics in the soil profile was 8.42±3.81%,61.80±12.43%and 72.43±4.19%,respectively.In addition,the total mobility of small-size microplastic particles is higher than that of large-size microplastic particles,which were 92.01±2.82%and 61.80±12.43%,respectively;the analysis of microplastics in the excrement of earthworms after incubation shows that small particle size and fibrous microplastics are the majority,but film and spherical MPs are less.By simulating the adhesion experiment on the surface of the earthworm body,the results show that the adhesion rate of PVC particles on the surface of the earthworm body is the highest,PP is the second,and PE is the least;in terms of different microplastic shapes,the adhesion rate to the film is higher than that of the particles,followed by fibers.The above results indicate:(1)Rainfall is a key driving factor for the migration of MPs in soil.With the increase of rainfall,the amount of microplastic particles on the soil surface in the horizontal direction and in runoff water increased significantly,but the migration into the deep layer of the soil profile was relatively reduced.A certain slope of the soil slope will facilitate transport of MPs.(2)The density,size and shape of MPs will affect their migration in the soil.The smaller the size,the more easily MPs migrate.The denser MPs are relatively easy to migrate to the deeper layers of the soil,and fiber and film MPs are more likely to flow to runoff water.(3)Compared with indoor simulated rainfall,MPs migrate deeper in the vertical direction of the soil and farther in the horizontal direction under similar outdoor natural rainfall conditions.This reflects that many factors such as temperature,rainfall intensity and frequence in the natural climate environment will affect the environmental behavior of MPs in the soil.(4)The disturbance,feeding and excretion of soil animals,body surface adhesion and other behaviors will accelerate the transportation of MPs in the soil,and promote the distribution of MPs into deep soil layers.Realistic farmland soil was utilized in this study,through simulatively leaching experiments,outdoor soil box experiments and soil animal incubation experiments,the migration behavior of various types of MPs in the soil have been revealed,and the driving factors and influencing factors of the migration of MPs in the soil have been analyzed.The research results provide an important basis for the analysis of environmental fate of MPs in the farmland environment,and provide a reference for soil MPs pollution control and treatment of MPs in soil. |
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