Distribution,Heavy Metal Adsorption Behaviors And Risk Evaluations Of Microplastics In Upstreams To Baiyangdian Wetland

As a new type of pollutant,microplastics are widely distributed in the world.Due to the large surface area,microplastics can adsorb a variety of pollutants from environment.Meanwhile,microplastics will be aged and attached by microorganisms in the natural environment,which will promote the adsorption effect of other pollutants on microplastics.When microplastics enter the food chain,they will bring potential health risks to human bodies.Baiyangdian Wteland is the largest freshwater wetlandin northern China,where the micropastic pollution exists,and the content of Zn,Pb and Cd in some areas exceed the environmental standard.Therefore,this paper mainly studied microplastic distribution in the eatuary area of upstream to Baiyangdian wetland,and discussed the adsorption characteristics of heavy metals on aging and biofilm-attached microplastics,and evaluated the potential risks of microplastics and combined pollution of microplastics with heavy metals on human body,which is of great significance for protecting the ecological environment security of Baiyangdian Wetland.Taking Fuhe River and Xiaoyi River estuary to Baiyangdian Wetland as the research objects,the microplastics pollution in water and sediment was studied.The results showed that microplastics were ubiquitous in water and sediment.In the water of Fuhe River and Xiaoyi River,the average abundance of microplastics was 4 and 5 n/L,respectively;the main shapes of microplastics were fragment and fiber;0.5-1 mm microplastics accounted for the largest proportion with 50.7%and 58.1%,respectively;the proportion of blue microplastics in the water of Fuhe River was the largest,accounting for 55.2%,and the proportion of transparent microplastics in the water of Xiaoyi River was the largest,accounting for 32.6%;the main type of microplastics was polyethylene（PE）.In the sediments of Fuhe River and Xiaoyi River,the average abundance of microplastics was 558 n/kg and 305 n/kg,respectively;the main shapes of microplastics were also fragment and fiber;the microplastics with particle size of 0.5-1 mm accounted for the largest proportion with 46.3%and 31.8%,respectively;the proportion of transparent microplastics was the largest,accounting for 43.9%and 40.6%respectively;the main type of microplastics was polyethylene（PE）.The microplastic content in Fuhe River sediments showed an exponential function relationship with the depth of sediments,and significantly decreased with the increase of sediment depth（R²≥0.836,p≤0.002）,which was maily related to the formation period of sediments.The particle size of microplastics decreased with the increase of the sediment depth.The grinding-aging microplastics certainly adsorbed Zn,Pb and Cd.The results showed that the adsorption capacity of aged polyethylene（A-PE）for Zn,Pb and Cd was 35.54,28.34 and12.74 mg/kg,respectively,which was 37.1%,44.7%and 48.5%higher than that of polyethylene（PE）.The adsorption capacity of aged polypropylene（A-PP）for Zn,Pb and Cd was 41.19,29.85and 13.65 mg/kg,respectively,which was 39.0%,52.0%and 49.5%higher than that of polypropylene（PP）.The adsorption kinetics of three heavy metals on the aged microplastics can be well fitted by the second-order kinetic model（R²>0.912）,and the adsorption isotherm data can be well fitted by Freundlich model（R²>0.928）.The aging of microplastics significantly increased the adsorption of heavy metals on microplastics.The biofims are easy to grow on the microplastic surface,and the adsorption of heavy metals on microplastics will change after the biofilm is attached on the microplastics.The results showed that the adsorption capacity of biofilm-attached aged-polyethylene（B-A-PE）microplastics for Zn,Pb and Cd was 43.06,30.25 and 13.45 mg/kg,respectively,which was 66.0%,54.5%and 56.8%higher than that of PE.The adsorption capacity of biofilm-attached aged-polypropylene（B-A-PP）microplastics for Zn,Pb and Cd was 27.95,32.46 and 14.75 mg/kg,respectively,which was61.8%,65.3%and 61.5%higher than that of PP.The adsorption kinetics of three heavy metals on four kinds of microplastics can be well fitted by the second-order kinetic model（R²>0.923）,and the adsorption isotherm data can be well fitted by Freundlich model（R²>0.911）.The attachment of biofilms on microplastic surface further promoted the adsorption of heavy metals on microplastics.On the whole,the ecological risk of microplastic pollution in Fuhe River and Xiaoyi River is not significantly different.The ecological risk of microplastic pollution in water of the two rivers was greater than that in sediments.The risk level of microplastic pollution in sediments of Xiaoyi River is gradeⅠ,and the others are gradeⅡ.Polyamide（PA）and Polystyrene（PS）microplastics were the types with the highest hazard score in in the eatuary area of Fuhe River and Xiaoyi River.Taking PP microplastic as an example,the target hazard index（THQ）and total hazard index（HI）to human bodies of aged microplastics and biofilm-attached aged microplastics adsorbing Pb and Cd was less than 1,indicating that there was no apparent health risk to human body.The cancer risk（CR）caused by aged microplastics and biofilm-attached aged microplastics after adsorption of Cd was greater than 10^-4,indicating a high cancer risk.This paper studied the microplastic pollution in Baiyangdian Wetland,and evaluated the ecological risk of microplastics in the water and sediments of Fuhe River and Xiaoyi Rive estuary,which filled the blank of the research on microplastics in Baiyangdian Wetland,and provided data support and theoretical basis for the control of microplastic pollution in the freshwater environment.