A Study On Pollution Characteristics Of Microplastics In Agricultural Soils And Environmental Behaviors Of Typical Mulching Films

The extensive use of mulching film and other plastics products has caused plastic pollution in farmland soil.The plastic wastes can be further broken into microplastics(< 5 mm,MPs)in the environment.As emerging pollutants,MPs have the characteristics of large quantity,small size and large specific surface area,which have potential environment risks to the farmland ecosystem.To reveal the environmental risks of MPs,it is important to investigate the characteristics of microplastics in agricultural soil and environmental behaviors of secondary MPs from mulching films.Although there have been some regional reports on MPs in soil,there is a lack of research on MPs in farmland soils on a large scale nationwide.In addition,there are few reports on the source of MPs produced by different types of mulching films in actual farmland soils.In this paper,a large-scale survey of soil MPs was carried out in farmland across China.Moreover,the aging characteristics of three types of typical plastic mulching films and the environmental behavior of secondary MPs were systematically studied through field experiments.In this study,soil samples were collected from 30 farmlands in 21 provinces.Micro(meso)plastics(MMPs)in soil were separated by the method of density separation based on saturated sodium bromide,and identified by microscope and μ-FTIR.The distribution and characteristics of MPs in farmland soil were analyzed systematically.Based on the results,we selected traditional polyethylene(PE)film,biodegradable film PLA/PBAT and calcium carbonate modified degradable mulch film(calcium carbonate-filled,CCF)as test materials,and conducted field experiments for 12 months in Shanghai Fengxian Zhuanghang farmland base.We collected periodically film and soil samples from the field during 12 months.We analyzed the morphology changes of residual film and measured carbonyl index,hydrophobicity of films.Secondary MPs from mulching films were also separated and measured.The main results are as follows:(1)In agricultural soils in our country,the MMP abundances are in the range of25.56-2067.78 items/kg,with the mean abundance of 358.37± 89.33 items/kg.The abundance of MMPs is high in the North,Central,and some East provinces of China,while the overall abundance of MMPs is relatively low in Southwest China.The abundance of MMPs in the shallow layer(0-5 cm),the middle layer(5-10 cm)and the deep layer(10-15 cm)were 435.67 ± 108.95 items/kg,367.44 ± 103.93 items/kg,and272.00 ± 64.90 items/kg,resepectively The abundance in shallow layer was significantly higher than that in the deep layer(p < 0.05).(2)The shapes of MMPs contained fragments,fibers and films,with the mean proportions of 42.50%,37.08% and 20.42%.The mean proprotions of MPs in the size of < 1 mm,1-3 mm,3-5 mm,and > 5mm were 38.74%,39.93%,12.34%,and 8.98%.The mean size of MMPs in deep layerer was significantly larger than that in shallow and middle layers(p < 0.001).The mean size of fibers was the largest,followed by films,and fragments.The polymer types of MMPs included PP,PE,PES,acrylic,PA,PVC and PS.PP accounted for the largest proportion,and with an average of 58.10%.The fragments are mainly PP.The fibers are mainly PP,PES and acrylic.Films are mainly PE.The mean abundance of films was 56.31 ± 16.29 items/kg,in which PE accounted for 96.55%.(3)After treatment in farmland field for 2 months,PLA/PBAT and CCF films were seriously damaged.PLA/PBAT could not be found any more after 4 month treatment.The degradation of CCF films lasted for a longer time,but with very few residual films after 12 months.The degradation of PE was very slow,and only some holes could be observed after 12 months.By using scanning electron microscope(SEM),we observed cracks and gullies on PE,and deep hole on CCF film.The degradation of films buried in the soil were relatively slow.But all films appeared many holes of different sizes after 2 months,which were mainly caused by plant roots or soil animals such as earthworms.Suspected microbial corrosion could be observed on the surface of PE and PLA/PBAT under SEM.(4)The contact angle of PLA/PBAT was decreased from 104.6° to 62.73° after mulching treatment for 2 months,but that of PLA/PBAT buried in soil didn’t change significantly.There was no significant change in the contact angle of mulching CCF,but that of CCF buried after 12 months was obviously decresed(p < 0.05).There was no significant change in the water contact angle of mulching PE fim and buried PE.The carbonyl index of PE,PLA/PBAT and CCF increased by 1.68 time,0.15 times and 2.61 times,respectively.The change of carbonyl index reflected the aging of films and was correlated with the accumulated change of aire temperature in the experiment field.(5)There were very few PE MPs in soil after mulching treatment.The abundance of PE in 0-5 cm soil layer was only 23.33 ± 8.82 items/kg after 12 months.The abundance of PLA/PBAT MPs reached 60.00 ± 35.12 items/kg after mulching treatment of 2 months,and all PLA/PBAT MPs disappeared later.CCF MPs could be detected after 4 months of mulching,and the abundance of CCFs increased during the treatment.After mulching treatment for 12 months,the abundance of CCF reached 273.33 ± 53.65items/kg and the majority of CCF MPs were smaller than 3 mm.MPs could also be detected in 5-10 cm layers,but the abundances were obviously lower.All three types of films buried in the soil produced secondary MPs,and the abundance ranged from 0to 0-46.67 items/kg,which was relatively low than that under mulching treatment,of no obvious change rules.These results indicated:(1)There is widespread pollution microplastics in agricultural soils across China.The abundance of MPs varies greatly among different regions.MP pollution is relatively serious in arid and cold northern regions,and central and eastern China with developed agriculture.Less MPs were detected in agricultural soils in southwest China.The regional difference might closly related to the use of agricultural plastic products.(2)The main type of MPs in agricultural soils were PP fragments,PE films and PES fibers,which may mainly come from plastic wastes,mulching films,sewage and sludge.The size and shape of MPs showed characteristic distribution in different depth layers of soils,which may be related to environmental factors and agricultural activites.(3)Under the actural conditions of farmlands,both mulching and burial treatment lead to degradation and fragmentation of films,of high aging rates in mulching treatment.It indicates light,oxygen,heat and other environmental factors may accelerate the aging of films.Of three types of mulchings,the degradation rate of PLA/PBAT is fastest,followed by CCF,and the degradation rate of PE is very slow.During the degradation,the hydrophobicity of PLA/PBAT decreased significantly,formed some soluble degradation products due to hydrolysis.After treatment,holes appearred on the surface of CCF,indicating the structural integrity of CCF may be damaged.(4)In the process of plastic film degradation,secondary MPs will be produced and migrated into the soil.The abundance of CCF MPs is the highest,and CCF may accumulate in soil for a long time.PLA/PBAT produced a large number of MPs in a short period of time and then completely degraded in soil.PE degrades slowly and only produced few secondary MPs.This study first conducted a systematic survey on MP pollution in agricultural soils on national scale,and get the basic level of microplastic pollution in farmland soils in China and provided first-hand information and important data for the eco-environment risk assessment of MPs.Field experiments based on mulching films revealed the characteristics of aging and secondary MPs of different types of films.These results indicated that the secondary microplastics should be taken into consideration during the environmental risk evaluation of degradable agricultural films.