Study On The Effect Of Microplastics On Different Soils And The Migration And Transformation Of Heavy Metals In Soils

Plastics,as polymer materials with the characteristics of portability,durability and strong plasticity,have been widely used in various fields.With the rapid development of social economy and industry&agriculture,the global production of plastics has also increased dramatically.The durability of plastics is attributed to the stable chemical characteristic,which also makes it need a long time to be degraded or even unable to be mineralized after being abandoned in the environment.Thus leading to the white pollution,it brings more and more serious environmental problems and hazards over time.In recent years,the small-sized plastics that be produced by the cracking of plastic products were being focused by more and more researchers.The reseachers define the small-sized plastic that's smaller than 5 mm as“microplastic”.Microplastic pollution has been found in various environments.And its potential hazards and ecological and health risks are paid more and more attention.At present,there are many researches on microplastics pollution in water environment,but the pollution of microplastics in soil environment needs more thorough research.Microplastics have the characteristics of difficult to degradating,relatively large specific surface area.And the environmental behavior after they enter the soil environment is more complex than other pollutants,which may have a more profound impact on the soil environment and the behavior of other pollutants in soil.Heavy metal pollution in soil has been a difficult problem to be effectively solved,and the transformation of heavy metals with different chemical speciations could be effect by soil characteristic.Different chemical speciations have different ecological risk.The bioavailable speciations of different heavy metals are easier to be migrated and transferred with the change of soil environment.According to summary of the current researches,there are few studies focused on the effects of microplastics on soil properties and heavy metal pollution in soil.These problems need further exploration,the study about the effects of microplastics on soil and the soil pollutant is very urgent.In view of the deficiency of the present research on the influence of microplastic contamination in soil,this paper takes red soil,purple soil and cinnamon soil as the research objects,adding different kinds and different proportion of microplastics in soils to study the effect of microplastics on the physi cochemical properties,microbial enzyme activity and nutrient availability of the three soils.On the basis of the above,adding microplastics in the cinnamon soil containing different heavy metals to explore the effect of microplastics on the transformation and the bioavailable coefficients of heavy metals in soil.Then the effect of microplastics on the migration and transformation of bioavailable speciations in the vertical profile of cinnamon soil was further studied.We hope we can have a more comprehensive understanding about the behavior of microplastics in the soil environment and the influence of microplastics on the environmental behavior of different heavy metal pollutants in the soil,and provide some theoretical support for the further study in this field.The specific research works and achievements of this paper include the following six parts.In the first part,the basic properties,morphology,composition and hydrophilicity of different kinds of microplastics were characterized.According to the SEM analysis,the surface of LLDPE(Linear Low Density Polyethylene)microplastics was relatively roughest.And the average particle size of LLDPE microplastics was the smallest,followed by the PU(Polyurethane)microplastic.The LDPE(Low Density Polyethylene)microplastics had the relatively smoothest surface.The FTIR spectra of the five microplastics c ould obtain their corresponding characteristic peaks.The contact angle could be used to test the hydrophobicity of microplastics,the contact angle results showed that the PU microplastics had strong hydrophobicity,and the PE microplastics have the weakest hydrophobicity.The second part was the effect of microplastics on the physic ochemical properties of different soils.Microplastics could affect the physicochemical properties of different soils,and different soils had different effects.The results showed that the physical properties(bulk density,field water holding)of purple soil were more susceptible than the red soil and cinnamon soil.The physical properties(bulk density,field water holding)of purple soil were both significantly affected by the microplastic/soil ratios and microplastic types.And comparing to the other two soils,the red soil had more regular changes with the change of microplastic/soil ratios.In terms of chemical properties(p H,organic matter content,cation exchange capacity),red soil was less affected than the other two types of soil.With the changing of the addition proportion of microplastics,the p H value of three types of soils were all increased firstly and then decreased.But the red soil and cinnamon soil were not significantly impact by microplastic.Some types of microplastics had significant influence on the p H value of three kinds of soil.Different soils had different types of microplastics that could have significant influence.The p H value of red soil appeared significant increasement after adding the PU microplastics,the p H value of purple soil appeared significant increasement after adding the LDPE microplastics,and the p H value of cinnamon soil appeared significant decreasement after adding the PA and PS microplastics.The organic matter contents of cinnamon soil were less affected than red soil and purple soil,and the change of different proportions h ad no significant effect on cinnamon soil.The cation exchange capacity of red soil was least affected.The changes of different proportions and different types of microplastics had no significant influence on red soil but all to cinnamon soil.And the purple soil showd a significant decrease after adding LLDPE and PA.According to the correlation analysis,we could find that there was a different degree of correlation between the properties of three different soils containing microplastics,and some of them had a relative higher degree of correlation.And the field water holding capacity of soils had significant correlation with bulk density(red soil),CEC(purple soil),and organic matter content(brown Soil).Microplastics,as polymer materials,had high hydrophobicity on the surface,and the characteristics of the surface were also different from that of soil colloidal particles.After adding to the soil,microplastic was incorporated into the soil system and further affected the microenvironment of the soil.The effects on soil physical properties could cause changes in the chemical properties.Diverse physicochemical properties of different soils may lead to different changes after adding the microplastics.The third part was about the influence of microplastic on the activities of different microbial enzymes in different soils.The addition of microplastics can affect the enzyme activities of three types of soils,and the degree of influence varies with the microplastic/soil ratios and microplastic types.The addition of different proportions of LDPE microplastics could decrease the activity of urease and sucrose in purple soil and cinnamon soil,and the enzyme activity decreased as the proportion of microplastic increased.But the the enzyme activity of red soil increased irregularly.The catalase activity of cinnamon soil increased after adding different proportions of LDPE microplastics,and the red soil and purple soil had less catalase activity.Different types of microplastics had different infl uence on different soils.The LLDPE,PA,PU and PS(Polystyrene)microplastics could siganificance increase the activity of urease in red soil.The LLDPE,PU and LDPE microplastics could siganificance effect the activity of urease in purple soil.The LLDPE microplastics could siganificantly decrease the activity of urease in cinnamon soil.The activity of catalase in red soil was siganificantly affected after adding the all types of microplastics.The activity of catalase in purple soil siganificantly decre ased after adding the LLDPE,PA,PS and LDPE microplastics.The activity of catalase in cinnamon soil siganificantly increased after adding the PU microplastics.The LLDPE,PA and LDPE microplastics could siganificantly increase the activity of sucrase in re d soil.The PS microplastics could siganificance decrease the activity of sucrase in purple soil.All types of microplastics siganificantly decreased the activity of sucrase in cinnamon soil.According to the correlation analysis,the changes of soil physi chemical properties and soil enzyme activities had different correlation.The existence of microplastics might disturb the correlation between soil physichemical properties and enzyme activit ies.The differences between microplastics and soil particles make it possible to influence the physicochemical properties of the soil after entering the soil,which will lead to changes in microbial population and activity in the soil,and then affect the activity of various enzymes.The differences of different kinds of soil,the difference composition and surface characteristics of different kinds of microplastics led to the difference influence on enzyme activities.The fourth part was about the influence of microplastics on the contents of available nutrients in different soils.The addition of microplastics changed the contents of available nutrients in three types of soils.After adding different proportions of microplastics,the contents of available potassium and available phosphorus in red soil were less affected than available nitrogen.The content of available potassium and available phosphorus in red soil were not siginificantly accfected,while the available nitrogen content of red soil showed a significant decrease of nearly 50%.The content of available potassium in purple soil was significantly reduced and had larger influence than the other two nutrients.The contents of available phosphorus and available nitrogen in purple soil were increased firstly and then decreased with the proportion increasing.Different proportions of microplastic had the largest effect on the content of available nitrogen in cinnamon soil.And the content of available potassium in cinnamon soil decreased significantly under different proportions of microplastic.The existence of different kinds of microplastics could affect the available nutrients content in three types of soil.The LDPE,PA and PS microplastics could siganificance decrease the available nitrogen content in red soil.The LLDPE and PA microplastics could siganif icance increase the available nitrogen content in purple soil.The LLDPE microplastics could siganificance increase the available nitrogen content in cinnamon soil.The PA microplastics could siganificantly increase the available phosphorus content in red soil.The PU and PS microplastics could siganificantly decrease the available phosphorus content in purple soil.The PU microplastics could siganificantly decrease the available phosphorus content in cinnamon soil.All types of microplastics siganificantly decreased the cintents of available potassium in red soil and cinnamon soil.The PA and LDPE microplastics could siganificantly decrease the available potassium content in purple soil.At the same time,the correlation analysis showed that the available nitrogen content of three soils had a negative correlation with urease activities.The cinnamon soil had the highest correlation and purple soil had the least correlation,which indicated that the exsitance of microplastics could have a higer effect on cinnamon soil than purple soil.The two other available nutrient content and soil properties also exsited different correlation.Hydrophobic microplastics are very different from mineral particles in the soil,various organic matters and so on.After entering the soil,the microplastic affected the physical properties of the soil.With the increase of time,they further affect the chemical and biological properties of the soil.The above effects on the soil properties further affect the existence of available nutrients in the soil.The fifth part was about the effect of microplastics on the chemical speciation and bioavailability of different heavy metals in cinnamon soil.Microplastics could influence the chemical speciation proportion of Cd²⁺,Pb²⁺,Cu²⁺and Zn²⁺in the soil,and has a certain relationship with different addition proportion and different types of microplastics had different affect.The high addition proportion of microplastics had a relative large influence on the chemical speciations of Cd²⁺and Zn²⁺,while the chemical speciations of Pb²⁺and Cu²⁺was greatly affected by the relative low concentration of microplastics in soil.The LLDPE and PU microplastics had a relative higher effect on the chemical speciations of different heavy metals than other microplastics.The LLDPE microplastics could improve the bioavailability of Cd²⁺,Pb²⁺,Cu²⁺and Zn²⁺in cinnamon soil.The PU microplastics could improve the residue speciation proportion of Cd²⁺,Pb²⁺,Cu²⁺and Zn²⁺in soil.After adding microplastics,the changes of physicochemical and biological properties in cinnamon soil may lead to the chemical speciation changes of different heavy metals.Combined with the analysis of various microplastic properties in chapter II and the results of chapter III to IV,it can be seen that after the addition of microplastics,the properties of the physical and chemical biology of cinnamon soil have been affected,this phenomenon may be one of the reasons for resulting in the changes of chemical speciation and bioavailability of different heavy metals.Meanwhile,microplastics as a synthetic polymer material,their surfaces had higher degree of hydrophorate.They were different from the soil particles.The sixth part was about the effect of microplastics on the migration and transforamation of bioavailable speciations of Cd²⁺,Pb²⁺,Cu²⁺and Zn²⁺in different layers of cinnamon soil.The presence of microplastics affected the migration and transformation of the bioavailable heavy metals in different layers of soil,and the distribution trend and content of different bioavailable heavy metals in different layers were both changed,but the changes of different heavy metals were not the same.After adding microplastics,the distribution trend of bioavailable Cd²⁺,Pb²⁺and Cu²⁺was more likely to be affected than that of and Zn²⁺.The C1 to C4 ratio and LLDPE,PU,PS,and LDPE were all affect the distribution trend of Cd²⁺,Pb²⁺and Cu²⁺.The distribution trend of exchangeable Cd²⁺,Pb²⁺and Cu²⁺changed from middle enrichment to surface or middle-surface enrichment after adding PU and PS.And the existence of LLDPE,PS,and LDPE microplastics changed the distribution trend of available Cd²⁺,Pb²⁺and Cu²⁺from deep enrichment to middle enrichment or from middle-deep enrichment to surface,middle and deep enrichment.While the distribution trend of exchangeable Zn²⁺was not affected.The distribution trend of available Zn²⁺changed under C3 ratio and LLDPE.The contents of bioavailable heavy metals in different soil layers after the add ing microplastics were all changed.The contents of exchangeable Cu²⁺,Pb²⁺and Zn²⁺in the surface layer were larger than the control group.The PA microplastic increased the contents of exchangeable Cd²⁺and Pb²⁺in the middle layer.The contents of available Cu²⁺and Zn²⁺in the surface layer were decreased,while the contents of available Pb²⁺in the surface layer were increased.And the contents of available Zn²⁺and Cd²⁺in the deep layer were largely decreased.The existance of microplastics may change the soil microenvironment and change the filtering ability of soil in vertical layers for bioavailable heavy metals,and thus leading to the change of the distribution trend and contetns of bioavailable heavy metals in different soil layers.