Study On Characteristics Of Microplastic Pollution In Main Types Of Soil In Suburb Of Shanghai

In the environment,plastic debris can gradually break down into small plastic fragments through physical,chemical,and biological drivers.Plastic fragments with a size of less than 5 mm are called microplastics?MPs?.Many studies have revealed serious MP pollution in the ocean,which attracts great attention to this emerging pollutant.In fact,Soil is also a large reservoir of MPs,and MP pollution has become a new threat to terrestrial ecosystems.However,there is still a lack of effective analytical methods for soil MPs,which makes it difficult to study MP pollution in soil.Therefore,it is very important to develop an analytic method for separating MPs from soil,and then reveal the characteristics of MP pollution in soil.In this study,a method for separating soil microplastics was established,and then were used to analyse several types of soil in the suburbs of Shanghai,in order to systematically explore the pollution characteristics of soil MPs.First,we established a device and method for extracting soil MPs through sodium bromide?NaBr?solution circulation.The device was based on the principle of density flotation by semi-automatic separation,included a separation,vacuum filtration and solution recovery system.Saturated sodium chloride?NaCl?,calcium chloride?CaCl₂?and NaBr solutions were selected as candidate separation solutions based on the environment-friendly type,then hydrogen peroxide?H₂O₂?solutions were used for digestion.The separation efficiency of candidate separation solutions was tested with ten different types of polymers,three different size classes and three different shapes of MPs,respectively.The results showed that the recovery rate of NaCl solution is 0%when floating three types of higher density MPs,such as PET,POM and PVC,while CaCl₂ solution and NaBr solution can effectively float all ten types of MPs with average recovery more than 90%.The average recovery rate of saturated NaBr solution for small-sized fragment polyethylene MPs of 100-500?m and 500-1000?m was 96.67±3.33%and 98.33±1.67%,respectively,and the average recovery rates of saturated NaBr solution for fragment,fiber and film MPs was in the range of 85.00%-98.33%,which were higher than that of NaCl or CaCl₂solution.Moreover,the use of the circulating separation device can significantly improve the recovery rate of MPs of 100-500?m?p<0.01?,shorten the separation time from 72 h to 3 h,and save the separation reagents.In addition,after NaBr and H₂O₂ solution treatment,and the size or mass change rates of ten types of MPs were in the range from-14.55 to 5.08%,or from-99.95 to 4.89%,respectively,but there was no significant difference from the control groups with the treatment of distilled water.The results indicate that NaBr solution is a preferred reagent for floating soil MPs,and the device and method of NaBr solution circulation for extracting soil MPs are suitable for separating different types of MPs,which have characteristics of high efficiency,low cost and environment-friendly.Second,we comparatively studied the MP pollution characteristics in four different types of soil using NaBr solution circulating separation method.Fluvo-aquic soil,yellow-brown loam soil,paddy soil,and coastal saline soil were collected from12 plots in the southern suburbs of Shanghai.Based on the established devices and methods,the abundance,size,shape,and polymer type of MPs in different soil were analyzed.The results showed that different abundances of MPs were detected in the four types of soil,with an average abundance of 136.67 to 256.67 items/kg,and with the highest abundance in coastal saline soil.The size of MPs ranged from 32.71?m to4765.24?m.The average size of MPs in fluvo-aquic and yellow-brown soil was significantly lower than that in paddy soil and coastal saline soil?p<0.05?.Paddy and yellow-brown soil were mainly composed of fiber MPs,accounting for 90.19%and47.22%,respectively,while fragment MPs were dominant in fluvo-aquic soil and coastal saline soil,with the proportions of 59.64%and 56.85%.Various MPs including polypropylene?PP?,polyethylene?PE?,acrylic,polyethylene terephthalate?PET?,and polyamide?PA?were found.In addition,the main physical and chemical properties of the soil were detected,the abundance of MPs had a positive correlation with organic matter content and water content in the soil,R² was 0.993 and 0.828,respectively.These results indicate that the main types of soil in the suburbs of Shanghai are contaminated by MPs with various MPs distribution characteristics,and MP pollution is related to the physical and chemical properties of the soil.Finally,MPs in dry farmland soil were emphatically analyzed in the suburbs of Shanghai.Shallow?0-3 cm?and deep?3-6 cm?soil samples were randomly collected from twenty farmland fields.After the samples were shipped back to the laboratory,the abundance,size,shape,color,and polymer type of MPs were systematically assayed.The results showed that the average abundance of MPs was 78.00±12.91and 62.50±12.97 items/kg in shallow and deep soil,respectively,and with significant differences?p<0.05?.MPs in the size of less than 1 mm accounted for 48.79%and59.81%in shallow and deep soils,respectively;and the average size of MPs?1.32±0.07 mm?in shallow soils was significantly higher than that?1.12±0.07 mm?in deep soil?p<0.05?.The main morphology of MPs was fiber,and the proportion of fiber in shallow and deep soil was 53.33%and 37.62%,respectively.Microplastics were mostly in color of black or transparent,accounting for 46.30%and 39.39%.The vast majority of MPs were PP?50.51%?and PE?43.43%?,which may be related to agricultural film residues.These results indicate the widespread MP pollution in the farmland soil in the suburbs,and the residues of plastic products such as plastic film during agricultural production are important sources.The size,shape and distribution of MPs in different depths of soil are different,suggesting that MPs will migrate within different soil depths.In summary,an efficient circulating flotation device was specifically designed for the detection of soil MPs in this study.A separation method based on saturated NaBr solution was,for the first time,developed to assay MPs in soil,of a certain application prospect.Basing on this method,this study revealed the prevalence of MP pollution in the soil and the characteristics of MP pollution in dry farmland soil from the suburbs of Shanghai.The residuals of plastic films are suggested as the main source of MPs in farmland soil.Microplastic pollution may influence the soil environment and crop growth for a long time,posing ecological risks.The issue of the pollution of soil MP should be largely concerned,especially about the supervision of soil quality.The residual of plastic films and the input of MPs should be controlled in order to protect the ecological environment of farmlands.