Effect Of Typical Microplastics On Soil Heavy Metals Speciation And Its Association With DOM In Water-level-fluctuation Zone Of Slow-flowing Area Of The Reservoir

Microplastics are difficult to degrade and easy to accumulate,especially in slow-flowing areas with low velocity and weak hydrodynamic conditions,such as bays of reservoir and backwater area.The water-level-fluctuation zone formed by periodic water level change in reservoir is the sink and source of pollutants.The entry of microplastics into the water-level-fluctuation zone of slow-flowing area of Reservoir may have a complex impact on properties of soil and the behavior of heavy metals in soil.As the most active part of organic matter in soil,dissolved organic matter（DOM）in soil is an important factor affecting the behavior of migration and transformation of heavy metals in soil.An indoor incubation with water-level change flooding experiment was conducted under the background of weak hydrodynamic conditions.The effects of two types of typical microplastics with different addition concentrations（1,10 and 100mg/g）,high density polyethylene（HDPE）and polyethylene terephthalate（PET）,on speciation of heavy metals in the soil of the water-level-fluctuation zone of slow-flowing area of the Three Gorges Reservoir were analyzed,and the relationship between dissolved organic matter（DOM）and forms of heavy metals in the soil under the influence of microplastics was analyzed combined with UV-Vis absorption spectrum.The main conclusions are as follows:（1）The presence of each of the two kinds of microplastics could affect the bioavailability and migration of heavy metals in soil by causing changes of the chemical forms of heavy metals.The presence of HDPE could increase the content of exchangeable,carbonate-bound,and organic-bound Cu in soil,while decrease the content of above three forms of Zn.The content of exchangeable and carbonate-bound Cd decreased,while the content of organic-bound Cd in soil had no significant change.The content of Fe-Mn oxide-bound fractions of the three elements in soil could be reduced under the treatment of HDPE.The residual fractions of the three elements increased when the HDPE concentration was low and decreased when the HDPE concentration was high.The content of exchangeable and carbonate-bound fractions of the three metal elements in soil were reduced under the treatment of PET.The content of residual Cu decreased,while the content of the form of Zn and Cd increased with the addition of PET.The content of Fe-Mn oxide-bound Cu and Cd decreased.The content of Fe-Mn oxide-bound Zn in soil decreased by 12.53%with the treatment of 1 mg/g PET,and increased by 8.30%when 100 mg/g PET was added in soil.The presence of PET could lead to the increase of the content of organic-bound Cu,while decreased the content of the form of Zn in soil.The content of organic-bound Cd in soil increased by60%with the treatment of 10 mg/g PET,and decreased by 40%when 100 mg/g PET was added in soil.The change of the forms of Cu,Zn and Cd with the presence of each of the two kinds of microplastics eventually led to the decrease of the content of Cu,Zn and Cd in soil,and the higher the concentration of microplastics,the deeper the reduction degree.（2）The presence of each of the two kinds of microplastics could lead to the changes of characteristics of DOM in soil.Overall,the content of DOM（characterized by content of DOC）and CDOM（characterized byα（355））both increased.The aromaticity(characterized by SUVA₂₅₄)and hydrophobicity(characterized by SUVA₂₆₀)of DOM both decreased.The molecular weight（characterized by E2/E3）of DOM both increased.The exogenous（characterized by E2/E4）of DOM both increased,and the ratio of fulvic acid（characterized by E3/E4）of DOM both decreased.（3）Correlation analysis showed that the properties of DOM in soil such as the content of DOM and CDOM,aromaticity,hydrophobicity,molecular weight,source and components were significantly correlated with the forms and total content of heavy metals in soil with the presence of each of the two kinds of microplastics.The same characteristics of DOM may have similar correlations with the same form of different heavy metal elements and different forms of the same heavy metal.In the presence of HDPE,forms of Cu in soil were closely related to the characteristics of DOM.With the presence of each of the microplastics,DOM in soil could potentially affect the speciation of heavy metals in soil by affecting the properties of soil,such as p H.The results provide a scientific basis for understanding the environmental behavior of heavy metals in water-level-fluctuation zone of the Three Gorges reservoir under the combined action of microplastics and water-level change.