Effect Of Polyethylene Microplastics On Phytoremediation Of Phenanthrene Contaminated Soil By Ryegrass

Phytoremediation is widely used in the remediation of organic pollutants due to its simple operation,low cost,and environmentally friendly characteristics.In recent years,the emerging pollutant of microplastics has attracted more and more attention from scholars.Microplastics are formed by crushing large pieces of plastic through physical,chemical,and biological processes.Research has found that the surface of microplastics can attach organic pollutants,heavy metals,and antibiotics.As a carrier for the transport of these pollutants,they coexist with microplastics in the soil,forming combined pollution.Currently,scholars have studied the content related to the adsorption mechanism of microplastics and organic pollutants,and explored the toxic effects of microplastics and organic pollutants combined pollution on animal genetics,growth and development,as well as on plant growth.However,few studies have been conducted on the impact of combined pollution on soil properties,plant growth and microbial community structure on the effectiveness of phytoremediation of organic pollutants.This experiment explored the effect of PE on phytoremediation of organic contaminated soil.Under the condition of planting ryegrass,two different particle sizes（10μm,1 mm）of original and aged（60 d）polyethylene microplastics（PE）and 100 mg·kg^-1 of phenanthrene（PHE）were used as experimental materials.Control group（CK）,single PHE,PE and PE-PHE combined pollution group were set up to investigate the effects on soil physicochemical properties and enzyme activities,ryegrass growth and physiological indicators,and plant inter-root soil microbial community structure,further explore the effects of PE on ryegrass remediation of phenanthrene-contaminated soil.The main conclusions were as follows:（1）The 10μm PE compared to 1 mm PE,had a rougher surface,significant grooves and folds,larger specific surface area and crystallinity,stronger absorption peak intensity,and more carbon and oxygen elements.Compared to the original PE,the aged 10μm and 1 mm PE had an increase in specific surface area of 9.1%、11.0%,and were broken into smaller plastic particles with significant cracking.The intensity of the absorption peak of the functional group increases significantly after ageing,and no significant change in crystallinity,exposure to air reduced carbon and increased oxygen.（2）Compared to CK,the single PHE treatment significantly increased soil p H by 8.9%,and significantly reduced soil CEC and urease activity by 33.8%、20.1%respectively,which had no significant effect on soil EC,organic matter content,dehydrogenase and peroxidase activities.After entering the soil,the surface of the microplastics will adsorb alkali metal ions,resulting in an increase in p H,PE has different effects on soil organic matter content depending on particle size.The addition of single PE contamination reduced the CEC,with the highest reduction in CEC for 1 mm aged PE,45.8%lower compared to CK,PE significantly increased soil catalase and urease activities,with a stronger promotion of their activities by large particle size.non-significant changes in soil dehydrogenase activity.Combined pollution synergistically promoted soil p H and organic matter content,decreased soil CEC,catalase,dehydrogenase,and urease activities.Single factors such as PE particle size,aging,and PHE were the main influencing factors for soil p H,organic matter content,catalase,and urease activities.There was an interaction between PE particle size and PHE that affects soil p H,while there was an interaction between PE aging and PHE that affected soil conductivity.There was an interaction between two factors such as PE particle size,aging,and PHE that affected soil CEC,but there was no interaction between two or three factors that affected soil enzyme activity.（3）PHE can stimulate plant growth and increase biomass,PE had an inhibitory effect on plant growth,reduced plant height,root length,leaf length,and dry weight,1 mm PE inhibited the plants more than 10μm PE,and combined pollution reduced the inhibitory effect of single PE pollution on ryegrass growth.Compared to CK,the addition of a single 1 mm PE increased the stress level of the plant,resulting in a significant increase in MDA content.Compared to a single PE pollution,the MDA content of PE-PHE combined pollution increased,the results indicated that combined pollution synergistically increased plant damage,PE particle size,aging,and PHE are the main factors that affect MDA content in plants.There were interactions between PE particle size and aging,and between PE particle size and PHE.Compared with CK,adding a single PHE pollution relatively reduced the antioxidant enzyme activity of ryegrass,while adding a single PE pollution significantly increased the CAT and SOD activities,while reducing the POD activity.PE-PHE combined pollution decreased the activity of antioxidant enzymes compared with single PE pollution.Single factors such as PE particle size,aging,and PHE were the main factors that affected the activities of POD and SOD in plants.Single factors such as PE aging and PHE significantly affected the activities of CAT in plants,with no interaction between the other factors.（4）The presence of PE reduced the degradation rate of phenanthrene in the soil,with the addition of 1 mm aged PE showing the least degradation rate of 73.9%,indicating that the presence of PE inhibited the remediation effect of ryegrass on phenanthrene.At the same time,PE also reduced the accumulation of PHE by ryegrass roots and stems and affected the phytoremediation effect.Microbes colonizing the surface of PE increased the microbial community number,while PHE could have a toxic effect on microorganisms.Under PE-PHE combined pollution conditions,the two act antagonistically,reducing community numbers relative to single PE pollution.The species of dominant bacteria in different samples at the phylum level did not change,but the abundance of dominant bacteria was changed,and the microbial diversity and abundance also changed.The correlation analysis of soil phenanthrene degradation rate showed that soil p H,CEC,microbial evenness,and Gemmatimonadota were directly and significantly correlated with soil phenanthrene degradation rate.The greater the soil p H and CEC,the lower the soil phenanthrene degradation rate.The higher the microbial evenness and Gemmatimonadota abundance,the higher the soil phenanthrene degradation rate;Soil dehydrogenase activity,plant leaf length,water content,microbial diversity,and abundance indirectly affected the degradation rate of phenanthrene through influencing soil properties and microbial communities.The degradation rate of phenanthrene in soil and the abundance of Gemmatimonadota significantly affected the absorption and accumulation of phenanthrene by plants.The higher the abundance of Gemmatimonadota,the higher the degradation rate of phenanthrene,resulting in a lower content of phenanthrene absorbed and accumulated by plants.