Study On The Distribution And Transport Of Microplastics In The Yangtze River(Chongqing Main City Section)

Microplasstics,as a type of emerging pollutant,may pose huge potential risks to the water environment and even the entire ecosystem because of their small size,ubiquitous distribution and easy to be ingested by mistake,which have rasied worldwide concern.Up to now,the status quo of microplastics pollution in freshwater rivers in China is not clear,and the research on microplastics in the Yangtze river is mainly focused on the middle and lower reaches and the estuary,while the research data of microplastics in the upper reaches are extremely scarce.The paper takes the Yangtze river of Chongqing main city section as the research area,investigates the distribution characteristics of microplastics in surface water and bank soil at different periods.At the same time,aiming at the problem of the fate of microplastics,the paper carried out a laboratory simulation experiment,and explored the transport mechanism of microplastics in porous media under different conditions.There are great significance to the study of microplastics pollution,ecological risk and control in freshwater watershed in China.The results of the study are as follows:(1)By investigating the distribution characteristics of microplastics in surface water at 10 sampling sites in the study area,we found that when the flood season,the abundance of microplastics varied from 43.55 to 125.00 n/L,average abundance was71.84 ± 23.93 n/L in surface water.when the non-flood season,the abundance of microplastics was 46.67 ～ 204.17 n/L,average abundance was 104.54 ± 55.44 n/L.There was no significant difference between the abundance of microplastics in the flood season and in the non-flood season(p = 0.112 > 0.01).We found that whenther in flood season or non-flood season,the type proportion of microplastics in the surface water from large to small was: fiber > film > fragment > microbeads > foam,of which the fiber accounted for the majority(the average proportion was 76.84%).The statistical analysis of fiber in different particle size groups showed that the amount of microplastics increased gradually with the decrease of particle size.The chemical composition of microplastics was identified and four chemical components were found:polypropylene,polyester,o-xylene acid polyvinyl chloride and high-density polyethylene.(2)By investigating the distribution characteristics of microplastics in bank soil at 10 sampling sites in the study area.When the flood season,the abundance of microplastics varied from 40.00 to 420.00 n/kg dw,average abundance of microplastics was 193.33 ± 131.95 n/kg dw in bank soil.There was no correlation between the abundance of microplastics in surface water and in the bank soil during the flood season(r=0.274,p>0.05).When the non-flood season,the abundance of microplastics was 100.00 ～ 583.33 n/kg dw,average abundance was 231.67±138.39n/kg dw.There was no correlation between the abundance of microplastics in surface water and in the bank soil during the non-flood season(r=-0.197,p>0.05).There was no significant difference between the abundance of microplastics in bank slope soil in flood season and that in non-flood season(p=0.843>0.05).Five types of microplastics were found,of which fiber(28.85%)and film(33.10%)microplastics were the main components.With the decrease of particle size,the amount of microplastics increased gradually.There was no significant correlation between the abundance of microplastics and the concentration of Dissolved Organic Carbon(DOC)in soil.(3)By taking polystyrene microplastics of 0.2μm,2μm and 4μm as the experiment object,vertically and horizontally fixing the plexilass column filled with quartz sand,the indoor soil column experiment showed the following transport mechanism of microplastics: In general,With increasing flow velocity,concentration or decreasing particle size,the peak value of the breakthrough curve increased gradually,which promoted the transport of particles in porous media.However,when the concentration of the working solution exceeded a certain critical concentration value,increasing the concentration of the working solution inhibited the transport of particles in the porous media,and the increase of the flow velocity made the critical concentration value larger.Moreover,when the concentration of the working solution was high enough,the increase of the flow velocity inhibited the transport of particles in the porous media.As the flow velocity increased gradually,the larger the particle size was,the greater the influence of gravity on particle transport would be,while the influence of gravity on particle transport would be gradually weakened with the increase of working solution concentration.Analysis of the transport of particles under the vertical and horizontal condition,found that there exists a critical level,when the working solution concentration was less than the critical concentration,microplastics deposited less under the horizontally fixing condition.If the working solution concentration was greater than the critical concentration,they deposited less under the vertically fixing condition.The increase of flow velocity or particle size made the critical concentration value increase.The particle size increased to a certain degree,there was also a critical concentration value.If the concentration of the working solution was less than the critical concentration value,the increase of the concentration played a leading role in the transport of particles in the porous media.while the concentration was greater than the critical concentration value,the gravity played a leading role in the transport of particles in the porous media.