Microplastics In Sediments Of The Three Gorges Reservoir And Its Toxicity Effects As Contaminant Vector

As a new pollutant,microplastics are ubiquitous in aquatic environment.Microplastics often enter river systems through runoff,sewage discharge,and atmospheric subsidence,and eventually make their way to the ocean.However,water projects like the Three Gorges Reservoir have kept plastic waste from rivers that would otherwise have flowed into the ocean trapped in artificial barriers,and accumulate in the sediment.Furthermore,the large surface area of microplastics makes it easy to act as the carrier of other pollutants in the reservoir and enter the food chain through biological ingestion,thus posing a potential threat to the ecological environment of the reservoir.However,there has been no specific study on the characteristics of microplastics in the sediment of the Three Gorges Reservoir,and the environmental behavior of these microplastics and whether they pose a potential threat to reservoir organisms are largely unknown.In this study,the characteristics of microplastics pollution in the Three Gorges Reservoir area were investigated,and the role of microplastics as carriers of environmental pollutants and their potential toxicological effects on aquatic organisms were explored.（1）The results showed that microplastics were ubiquitous in the sediments of the Three Gorges Reservoir,with an abundance ranging from 59 pp·kg^-1 to 276 pp·kg^-1（dry weight）,with an average abundance of 159 pp·kg^-1.Economic development level and total population were important factors affecting the spatial heterogeneity of microplastic abundance.Microplastics in the shape of fibers are the most common,while polystyrene,polypropylene,and polyamide were the main polymer types.It is speculated that municipal sewage and past fishing activities may be the primary potential sources.In addition,the color of microplastics in the sediments of the Three Gorges Reservoir showed significant spatial differences,indicating that the distribution of microplastics in the sediments of the Three Gorges Reservoir was uneven,which may be affected by different anthropogenic or terrigenous pollution.It should be noted that the proportion of microplastics in sediments of the Three Gorges Reservoir increased with the decrease of particle size.Microplastics with a particle size of less than 500μm were mainly fibers,fragments,and films,indicating that the microplastics in the sediments of the Three Gorges Reservoir were mainly secondary microplastics produced by the crushing or degradation of large plastics.（2）The accumulation of two kinds of pollutants（bisphenol and heavy metal）on the microplastic surface in the Three Gorges Reservoir area was investigated.It was found that bisphenol and heavy metal accumulated on the microplastic surface to different degrees,and showed high variability and environmental correlation.Microplastics in the environment may continuously accumulate these pollutants and act as a stressor for organisms to ingest these environmental pollutants.（3）Polymer type,surface area,and surface functional groups of microplastics were the main intrinsic factors affecting the adsorption process.The concentration,hydrophobicity,and dissociation ability of organic pollutants also have an important effect on their adsorption behavior.From the perspective of hydro-chemical environmental conditions,the adsorption capacity of bisphenol analogs on microplastics decreased with the increase of temperature.Salt ions enhanced the distribution of bisphenol analogs in the solid phase in the solution,but this enhancement effect was not significant when the concentration of salt ions was high.The adsorption of bisphenol analogs on microplastics was pH-dependent,and the increase of electrostatic repulsion caused by the increase of pH was an important reason for the decrease of adsorption capacity.In addition,the adsorption of Pb²⁺,Cd²⁺,and Cu²⁺on microplastics was controlled by both membrane diffusion and intra-particle diffusion.The adsorption process was mainly monolayer saturation adsorption,and physical adsorption may be the main driving force of adsorption.The increase of pH could promote the adsorption of heavy metals on microplastics,which could affect the interaction between them by changing the form of heavy metal ions and the surface charge of microplastics.The organic matter in the solution could inhibit the adsorption of heavy metals on microplastics,which may be related to the electrostatic interaction and the formation of bidentate complexes.（4）Silver carp tend to ingest microplastics,and some microplastic particles could be stored in the silver carp for a long time.The particle size and concentration of microplastics greatly influenced the ingestion of microplastics by silver carp.The presence of food interfered with silver carp’s recognition of inedible particles and increased the probability of ingesting microplastics of larger sizes.After ingestion of microplastics,the intestinal tract of silver carp exhibited significant oxidative stress and inflammation,and the permeability was increased.The composition of intestinal microflora at phylum and genus levels changed significantly,and the microbial diversity was greatly reduced.（5）It was found that BPA-loaded microplastics exhibited strong desorption capacity in simulated digestive juices,and the desorption strength was related to intestinal pH,salt ion concentration,and temperature.In the combined exposure environment of microplastics and BPA,the synergistic effect of microplastics and BPA enhanced the concentration of BPA in zebrafish,and the changes of plasma sex hormones and gonadotrophin were significantly enhanced in zebrafish.In conclusion,microplastics were ubiquitous in the sediments of the Three Gorges Reservoir,but their distribution was uneven,and they were easily affected by different anthropogenic or terrigenous pollution.These microplastics could act as effective carriers of organic pollutants and heavy metals in natural water bodies,thus acting as a"sink"for these pollutants.What is more,they could be ingested by edible fish such as silver carp,which not only pose a threat to the survival of these aquatic creatures but may eventually be transmitted through the food chain to humans.This study enriches reservoir microplastic pollution data and supports the hypothesis that river sediments act as a sink for plastic accumulation.Furthermore,the interaction mechanism between environmental pollutants and microplastics is further revealed,and new evidence of microplastics’ecological hazards and health risks is provided.These results help to expand our understanding of microplastic pollution and support further exploration and evaluation of the ecological and environmental risks of microplastics pollutants.