Microplastics in sediment of the Three Gorges Reservoir:abundance and characteristics under different environmental conditions*

Wang LI, Bo ZU,**, Yiwei LIU, Juncheng GUO, Jiawen LI

1 College of River and Ocean Engineering, Chongqing Jiaotong University, Chongqing 400074, China

2 Chongqing Research Academy of Ecology and Environmental Sciences, Chongqing 401147, China

Abstract Freshwater microplastic pollution is an urgent issue of global concern, and research on the distribution in reservoirs is lacking.We investigated the microplastic pollution levels in wet sediments collected from the Three Gorges Reservoir, the largest reservoir of China.Results show that microplastics were ubiquitous in the sediments of the Three Gorges Reservoir, and their abundance ranged from 59 to 276 pp/kg (plastic particles per kg, dry weight).Economic development and total population were important factors affecting the spatial heterogeneity of microplastic abundance, and the contribution of large cities along the reservoir to microplastic pollution should be paid with more attention.Fibrous microplastics were the most abundant type of microplastic particles in reservoir sediments, whereas polystyrene, polypropylene, and polyamide were the main types of polymers.The apparent spatial heterogeneity in morphology and color of microplastics is attributed to different anthropogenic or landbased pollution sources.Moreover, the accumulation of microplastics near the inlet of tributaries reflects the role of potential contributors of tributaries.More importantly, multiple bisphenols (BPs) and heavy metals detected at the microplastic surfaces indicate that microplastics can act as carriers of these pollutants in the environment in the same way as sediments did, which may alter the environmental fate and toxicity of these pollutants.Therefore, we conclude that the Three Gorges Reservoir had been contaminated with microplastics, which posed a stress risk for organisms who ingest them along with their associated pollutants (BPs, heavy metals).

Keyword: microplastics; Three Gorges Reservoir; sediment; bisphenol; heavy metal

1 INTRODUCTION

Because of their good ductility, durability, and low cost, plastics are one of the most widely used materials.However, only a small number of waste plastic products are effectively recycled, and most of them are directly discarded into environment and gradually decomposed, among which plastic particles less than 5 mm in diameter are called microplastics.The environmental fate of microplastics in different media has become an urgent issue for scholars and common public worldwide (Stolte et al., 2015; Yuan et al., 2022).It is estimated that there are currently more than 150 million tons of plastics in the ocean,and without proper waste management, there could be more microplastics in the ocean by weight than fish by 2050 (Tiller et al., 2018).In fact, rivers are the main pathway for plastic transport into the ocean, accounting for more than 80% of the total plastic load in the ocean (Schmidt et al., 2017).However, the number of studies on the occurrence of microplastics in freshwater is substantially lower than that on marine environments, particularly with regard to the transport, distribution, and occurrence characteristics of microplastics.Previous studies have reported the discovery of fairly abundant microplastics in the surface waters of fresh water(Zhang et al., 2015; Galafassi et al., 2019).These microplastics floating in rivers pose a huge environmental threat to entire river basins and the ocean (Sulistyowati et al., 2022).In addition to floating, microplastics are prone to vertical migration under the influence of suspended sediments and microorganisms, and sediments can become a natural microplastics sink (Alimi et al., 2018; Jang et al., 2022).In comparison to other freshwater bodies, the low velocity hydrodynamic conditions in reservoirs can promote the homogeneous or heterogeneous aggregation of microplastics, thus increasing the possibility of these reservoirs becoming microplastic sinks (He et al., 2021).Moreover,changes in hydrodynamic conditions can lead to the resuspension of microplastics deposited in bottom sediments, which can become an additional source of microplastics (Hurley et al., 2018).

The Three Gorges Reservoir is located in the main stream of the Changjiang (Yangtze) River.Owing to the high population density and wide distribution of agricultural and manufacturing zones in the Changjiang River Basin, anthropogenic pollution has substantially increased since the impoundment of the Three Gorges Reservoir (Zhao et al., 2017).As emerging pollutants, microplastics have been widely detected near the Three Gorges Dam (Zhang et al., 2015).Zhang et al.(2017) detected microplastics in surface water and sediments of Xiangxi River, a typical tributary of the Three Gorges Reservoir area, with concentrations of 0.55×105-342×105items/km2and 80-864 items/m2,respectively.Fan et al.(2021) has also found microplastics in surface water and sediments in the Chongqing urban section of the Three Gorges Reservoir, and reported that human activities may play an important role in microplastic pollution in the study area.However, the limited research data is insufficient to clarify the pollution of microplastics in the entire reservoir area.

Thus, in this study, sediments were collected from different areas of the Three Gorges Reservoir to investigate: 1) the abundance and distribution pattern of microplastics in the sediments of the Three Gorges Reservoir and the social and environmental factors affecting microplastic pollution;2) the characteristics (color, shape, and size) of microplastics occurring in sediments and their possible sources; 3) the main types of microplastic polymers in the sediments of the Three Gorges Reservoir and the potential impact of their surface morphological changes on the environment; and 4) the accumulation of several pollutants on the surface of microplastics and sediments.

2 METHOD

2.1 Survey area and sample collection

Reservoir sediments were sampled from May to June 2021.The sampling points were distributed across 13 counties/districts (S1-S13) under the jurisdiction of the Chongqing Municipality (Fig.1),representing different areas of the Three Gorges Reservoir.Sediment was collected with a Van Veen grab (250-cm2sampling surface), then transferred to aluminum foil with a stainless-steel shovel, placed in a resealable plastic bag, and stored away from light.The procedure was repeated in triplicate at each sampling point.

2.2 Isolation of microplastics

In this study, we employed the two-step extraction method proposed by Di and Wang (2018)for the extraction of microplastics from the sediments, albeit with some modifications.In short,the collected wet sediment samples were dehydrated in a drying oven at 60 °C and then passed through a stainless-steel mesh screen with a pore size of 5 mm.Subsequently, each sample (50 g) was weighed and placed in a conical flask, to which 250 mL of saturated sodium chloride solution (1.2 g/cm) was added.The mixture was shaken and stirred thoroughly and then allowed to stand for 24 h.The supernatant was transferred to a beaker for secondary separation,and each sample was subjected to density separation in triplicate, leaving mainly low-density microplastics in the supernatant.After the previous extraction step,the remaining sediment was collected, and then, the second extraction step was performed using a zinc chloride solution (1.5 g/cm), following the same method as the first extraction.The second extraction step was conducted to further extract high-density microplastics.Finally, the separated microplastic particles were broken down with 30% hydrogen peroxide solution to remove organic matter, filtered by suction, and stored in petri dishes for testing.

Fig.1 Sampling area and abundance of microplastics in sediments

2.3 Observation and identification of microplastics

Stereo microscopes with electronic eyepieces(Shenying, China) can be used to quickly and easily identify the color, shape, and size of particles, and they have been widely used in previous studies for the identification of environmental microplastics(Ding et al., 2019; Kim et al., 2021).Microplastic is classified into five shape types based on its appearance: beads, fibers, fragments, film, and foam.At the same time, the color (transparent, black/gray, white, red, green, and others) and particle size(＜0.5, 0.5-1, 1-2, 2-3, 3-4, and 4-5 mm) of these microplastics were also classified according to the actual situation of the sample.The microplastic polymer types were identified through a micro-Fourier translation infrared spectroscopy (FTIR)microscope (Thermo Scientific, America), and their spectra were compared with the Omnic polymer spectra library.Components with a match ≥70%were considered acceptable.Based on previous studies on pollutants in the water and sediments of the Changjiang River basin, we measured the bisphenol (BP) and heavy metal levels on the surface of microplastics using LC-MS (Waters Corp,America) and ICP-MS (PerkinElmer, America),according to the methods previously described by Lo et al.(2021) and Wang et al.(2017).The microplastics used for the analysis of BPs and heavy metals were separated from the sediments using sodium iodide instead of zinc chloride to avoid influencing the extraction and analytical results.

2.4 Quality assurance and quality control

All sampling tools, including the Van Veen grab,stainless-steel shovel, and aluminum foil bags, were rinsed with ultra-pure water prior to sampling.Three replicates were taken from each sampling point.Direct contact with plastic materials was avoided during the collection of samples.To avoid contamination by external microplastics throughout the experiment, the researchers wore non-textile lab coats and nitrile gloves.All glassware was rinsed with ultra-pure water and dried at 120 °C before use,and the glassware was covered with aluminum foil when not in use.Samples were treated in a clean laminar flow cabinet.

2.5 Acquisition of other data

Data such as gross regional product, population,urbanization rate, and meteorological data for each site were obtained from information published by the National Bureau of Statistics of China (http://www.stats.gov.cn/) and the websites of the county(district) governments and the meteorological administration.The type of land use (such as industrial or agricultural) at each site was determined through on-site visits.

2.6 Statistical analysis

Statistical analysis was carried out using oneway analysis of variance (ANOVA) to verify significant differences between microplastic abundance at different sampling sites (P＜0.05).The difference between the contaminant concentration on the surface of microplastics and sediments was analyzed usingT-tests.The correlation between microplastic abundance and the distance of each sampling site from the Three Gorges Reservoir was tested using the Pearson’s method.The distance from each sampling site to the Three Gorges Reservoir was measured using the Google Earth.

3 RESULT AND DISCUSSION

3.1 Abundance and distribution patterns of microplastics in reservoirs

As shown in Fig.1, microplastics were detected in all sediment samples in amounts ranging from 59 to 276 pp/kg (plastic particles per kilogram, dry weight).There were significant differences in the abundance of microplastics at different sites, and the highest abundance was observed in S4 (276±34 pp/kg),whereas the lowest was observed in S8 (59±11 pp/kg).In general, low-velocity hydrodynamic conditions render reservoir sediments vulnerable to becoming microplastic sinks.In a survey conducted on the Nandoni Reservoir in South Africa, the highest microplastic abundance in sediments was 6 417 pp/kg(Mbedzi et al., 2020), which was substantially higher than that in the Three Gorges Reservoir in this study.Similarly, Vayghan et al.(2022) and Gao et al.(2022) collected and separated sediment samples from the Aras Reservoir in Iran and the Sardis Lake in the United States, and their results indicated microplastic abundances in sediments of 32-528 and 270-950 pp/kg, respectively, which were also higher values than those observed in the sediments of the Three Gorges Reservoir in the present study.Table 1 lists the microplastic abundance in sediments of rivers, lakes, and reservoirs worldwide.Although the differences in microplastic abundance in these studies might be attributed to differences in sampling methods and protocols for particle separation, the microplastic abundance in the sediments of the Three Gorges Reservoir was at a relatively lower level than those in sediments of other freshwater bodies worldwide.

A recent study highlighted that microplastic abundance in the surface water of the Changjiang River generally increased in the downstream direction (Yuan et al., 2022).However, this spatial trend did not apply to sediments.In this study, the Pearson correlation analysis showed that the microplastic abundance in sediments was not significantly correlated with the distance of each sampling point to the Three Gorges Dam (P＞0.05).However, there were significant differences in the spatial distribution, with higher microplastic abundance at points closer to the main urban areas(S2-S4; Fig.1).According to previous studies,tourism, improper domestic sewage treatment,and discarded garbage are important sources of microplastics (Yonkos et al., 2014; Vaughan et al.,2017; Peng et al., 2018).These factors may be the main reasons for the high abundance of microplastics in the S2-S4 samples, especially considering the relatively high population density and economic development in Chongqing’s main urban area and its developed tourism industry.Further analysis verified a significant positive correlation between the microplastic abundance in sediments and the gross regional product (GRP), GRP per unit area,GRP per capita, population size, population density,and urbanization rate (Fig.2).Previous studies on the ocean around South Africa also reported that microplastics along the coastline tended to accumulate near industrialization centers of coastal cities (Collins and Hermes, 2019).Bashir et al.(2021) reported that densely populated areas in Macao are important primary sources of microplastics and are associated with the release of tens of billions of plastic microbeads into the environment every year.In areas with developed roads and traffic, the wear of automobile tires also generates a large amount of microplastics, which are transferred to the surrounding environment through runoff (Luo et al., 2021).Some studies on the distribution of microplastics reported that population density can significantly affect the abundance of microplastics in surrounding water bodies, whereas total population does not present significant effects(Zhang et al., 2022).However, we found that microplastic abundance in sediments was correlated with local population size (R=0.73), which suggested that sediment microplastic pollution results from long-term accumulation and is closely related tototal population size.Conversely, microplastics in surface waters are more likely to be directly affected by high population densities in the short term(Zhang et al., 2022).Notably, although S7 and S11 presented lower economic development and population size, their abundance of microplastics was higher than those of their adjacent sites.This may be associated with their location at the inlet of the tributaries (Wujiang River and Xiaojiang River)of the Changjiang River.A recent study also confirmed the ubiquity of microplastics in Wujiang River sediments, with an abundance of 310-2 620 pp/kg(dry weight; He et al., 2022), which substantially exceeded the microplastic abundance of this study.These results suggest that tributaries might be important contributors to microplastic pollution in the Three Gorges reservoir.In addition, the results of this study showed a significant negative correlation between microplastic pollution and annual mean temperature.A possible explanation is that lower temperatures slow the diffusion of microplastic particles, thereby increasing particle sedimentation (Migwi et al., 2020).

Table 1 Microplastic concentration in sediments of freshwater ecosystems

3.2 Characteristics of microplastic occurrence in reservoirs

Fig.2 Pearson correlation between microplastic pollution and local socio-economic, climatic, and environmental factors

To ensure standardized and comparable results,we sub-divided microplastics into five classes based on their shape: beads, fibers, fragments, films, and foams (Lambert et al., 2017).Fibrous microplastics were dominant in all sampling sites, accounting for 41.1%-69.3% of all microplastics, whereas microbeads (4.3%-13.2%) and foams (2.2%-13.9%)were detected at smaller proportions (Fig.3).This is consistent with the results of previous studies, in which fibrous microplastics are the most common microplastic shape distributed in surface water and sediments in the Chongqing urban section of the Three Gorges Reservoir (Fan et al., 2021), and Xiangxi River, a typical tributary of the Three Gorges Reservoir area (Chen et al., 2022).In general, wastewater from household washing is a major source of fibrous microplastics in freshwater(De Falco et al., 2018).Napper and Thompson(2016) reported that a single wash of 6 kg of clothes in a household washing machine can produce 1.4×105-7.3×105microplastic fibers.Sites S2-S4 were located within the main urban area, in a high urbanization rate and population density.Therefore,the high proportion of fibrous microplastics in these sites (Fig.3) further suggests that urban domestic sewage is a main source of microplastic pollution.After fibers, fragments (19.0%) and films (11.3%)represented the next largest microplastic proportions.These microplastics are usually associated with landbased sources, such as packaging, containers, and building materials, which gradually decompose into smaller fragments after being discarded (Song et al.,2015).Debris from tire wear can be further transferred to rivers through surface runoff (Luo et al., 2021).Similarly, plastic films can also be derived from the degradation of plastic packaging,whereas agricultural activities, especially the use of agricultural mulch and coated fertilizers, are important sources of thin-film microplastic pollution in surrounding water bodies and sediments (Cao et al., 2021).Except for the main urban area(S2-S4), most districts and counties in Chongqing have abundant agricultural resources, which explains the relatively high proportion of thin-film microplastics in the S5-S13 samples.In addition, small amounts of spherical particles and foam detected in the sediments may have originated from cleaning products and construction materials present in the sewage from municipal areas.Although the use of cosmetics or personal care products can lead to tens of billions of plastic microbeads entering the sewage network daily (Cheung and Fok, 2016), this study showed that the proportion of microbeads in the sediments of the Three Gorges Reservoir was relatively small.One possible reason is that plastic microbeads are more likely to be intercepted by sewage treatment systems than fibrous microplastics and are transferred to the sludge (Yuan et al., 2022).

Fig.3 Distribution of color, shape, and size of microplastics in various sediments

Color can be used to identify potential sources of microplastics and analyze their potential hazards.Some studies assert that aquatic organisms are more likely to ingest microplastics that are similar in color to their prey (Xiong et al., 2019).Moreover, the inhibitory effect of microplastics on algal growth might also be related to their color (Chen et al.,2020).In this study, the colors of microplastics in the sediments of the Three Gorges Reservoir were mainly transparent, black/gray, white, red, and green, and the abundance of colors represented the diversity of sources.Transparent and black/gray were the most common colors (Fig.4).In general,fishing activities (nets and lines) are more likely to release transparent and black/gray fibers into rivers and reservoirs (Xue et al., 2020; Gao et al., 2022).Zhang et al.(2018b) reviewed research on microplastic pollution on inland freshwater systems in China, and they reported that colored microplastics were dominant in most samples from urban rivers,whereas transparent microplastics were more common in water bodies of regions with lower population densities.This finding is consistent with our results.Furthermore, the color of microplastics is not attributed to the polymer itself, but to chemical dyes added during manufacturing processes.Therefore, the color of microplastics is likely to fade owing to longterm physical, chemical, or biological effects (Gao et al., 2022), which may increase the stock of transparent microplastics in the environment.

Fig.4 Proportion of color, shape, and size of microplastics in sediments of the Three Gorges Reservoir

In addition, among the microplastics detected in the sediments of the Three Gorges Reservoir, small microplastics (＜0.5 mm) accounted for the vast majority (Fig.4).Microplastics of such small sizes potentially pose a serious threat to riverine food webs (Windsor et al., 2019) and freshwater ecosystems (Naqash et al., 2020).Compared to larger plastic particles, smaller particles are more likely to be ingested by organisms (Zheng et al.,2019).Furthermore, owing to their relatively large specific surface area, microplastics with smaller particle sizes have higher adsorption capacity for environmental pollutants (Zhang et al., 2019).Although sewage treatment plants can remove most microplastic contaminants, the removal of microplastic particles smaller than 0.5 mm via ordinary treatment processes is difficult (Mintenig et al., 2017).This might be the reason for the wide presence of smallsized microplastics in reservoir sediments.Moreover,most small microplastics detected in this study were in the form of fibers, fragments, and film, which suggests that microplastics in the sediments of the Three Gorges Reservoir derive mainly from secondary microplastic pollution rather than primary plastic microbeads.D?browska (2021) also reported that microplastics of diverse forms in the environment are mainly produced by the fragmentation of larger plastic particles after long-term physical, chemical,or biological weathering; and most primary microplastics, such as microbeads from personal care products, can be filtered by wastewater treatment systems (Murphy et al., 2017).

3.3 Polymer type and surface morphology of microplastics

Previous studies have shown that microplastics in the environment tend to derive from different polymer types (Lambert et al., 2017).The potential sources of microplastics and their potential risk to the environment can be clarified by analyzing their chemical composition.In this study, 100 randomly selected samples were analyzed using FTIR spectroscopy.We identified 87 samples as plastic polymers, including acrylonitrile-butadiene-styrene(ABS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polyamide (PA), polystyrene (PS),polyethylene (PE), polypropylene (PP), and polyester(PES), among which PS, PE, and PA were the main polymer types.These polymers are commonly detected in water bodies (Lin et al., 2021), soils(Chia et al., 2021), and sediments (Doyen et al.,2019).Moreover, they are the most produced types of plastics worldwide and are widely used in various forms of textiles, packaging, building materials,fishing gear, and agricultural mulch (Chia et al.,2021).The diversity of microplastic polymers further reflects the complexity of microplastic sources.In addition, the long-term existence of plastics in the environment is related to their chemical composition and stability of molecular structure.Mechanical wear in the environment significantly accelerates the fragmentation of microplastics, but it does not lead to molecular degradation.Researchers have reported that some microorganisms can degrade plastics, and the degradation of polyethylene terephthalate or polyamide is catalyzed by the secretion of hydrolytic enzymes(Miri et al., 2022).However, the degradation efficiency of these microorganisms toward plastics is low, and many types of plastics are difficult to degrade (Zhang et al., 2020).

The surface morphology of the microplastics was observed under a scanning electron microscope(Fig.5).The images show that fragmented and film microplastics had prominent corners, uneven surfaces,cracks, and physical damage.Fibrous microplastics presented irregular protrusions with embedded filamentous residues.In contrast, the surface of foam microplastics was clearly torn, with visibly damaged internal pore structures.The rough surface and damage to microplastics indicated that they had undergone various phenomena, such as aging,oxidation, or mechanical weathering.These processes can increase the specific surface area of microplastics,thereby increasing their adsorption capacity for pollutants.Zhang et al.(2018a) explored the adsorption properties of oxytetracycline on polystyrene microplastics, and their results indicated that compared to the original microplastics, weathered microplastics presented stronger adsorption capacity and higher specific surface area, micropore area, and degree of oxidation.In addition, under suitable environmental conditions, these adsorbed pollutants can be re-released into the environment, thereby affecting the ecological environment (Bao et al., 2021).

Fig.5 Surface morphology of microplastics: fragment (a), film (b), fiber (c), and foam (d)

3.4 Types and concentrations of environmental pollutants on microplastic surfaces

Fig.6 Total content of bisphenols (a) and heavy metals (c) on the surface of microplastics (MPs) and sediments in the Three Gorges Reservoir; and proportion of various types of bisphenols (b) and heavy metals (d) on the surface of microplastics

Bisphenol A (BPA) is an endocrine disruptor commonly detected in the environment.It can affect the endocrine function of animals and reduce the immune activity of cells in the body (Simonelli et al., 2017).Bisphenol B (BPB), Bisphenol F(BPF), and Bisphenol S (BPS) are structural analogs of BPA, and they have been widely used as BPA substitutes in recent years.An increasing number of studies have confirmed that they share the same endocrine disrupting activity as BPA (Ijaz et al.,2020).These substances have often been detected in the riparian zones and surface waters of the Three Gorges Reservoir (Hu et al., 2018).Figure 6a shows the total concentration of BPs adsorbed to the surface of sediments and microplastics at each survey site of the Three Gorges Reservoir.The total concentration of BPs in the sediments ranged from 15.6±5.4 to 203.7±41.5 μg/kg (dry weight), which was consistent with the concentrations detected in sediments of other freshwater bodies in China (Yan et al., 2017).The total concentration of BPs in microplastics ranged from 52.3±19.8 to 313.6±3.4 μg/kg (dry weight), which was slightly higher than their concentrations in sediments.Most studies emphasize the importance of sediments as a sink of BPs, but the results of this study showed that high concentrations of BPs also accumulated on the surface of environmental microplastics.With the exception of Bisphenol Z (BPZ), all of the BPs listed in Fig.6b were detected on microplastic at all sites.Although BPA was the main BP pollutant on the surface of reservoir microplastics, the total concentration of BPA analogs exceeded that of BPA at some sites.Therefore, the pollution status of BPA substitutes cannot be ignored.From the perspective of spatial distribution, the concentrations and types of BPs at different sites exhibited clear differences.The BP pollution levels on microplastic surfaces at sites S2, S6, S7, S10, and S13 were relatively high,and these sites were mostly near industrial cities.For example, there is an industrial park of approximately 31.3 km2near site S6, and chemical raw materials, chemical manufacturing, synthetic material manufacturing, and other chemical companies account for more than 90% of the industrial park.In contrast, there were many textile printing and dyeing enterprises near sites S10 and S13.Transportation, processing, and wastewater discharge of products are the main sources of BPs to surrounding environments (Vermeirssen et al.,2017).The concentrations of heavy metals (Cu, Cd,Pb, Zn, and Cr) in microplastics and sediments were also investigated.These heavy metals have been widely detected in the surface waters of the Three Gorges Reservoir, and they can substantially affect the environment and biological health (Liu et al.,2015).In this study, heavy metals accumulated on the surface of microplastics, with an accumulation amount comparable to that of sediments.Generally,the background concentration of heavy metals in primary microplastic particles is only several micrograms per gram.However, degradation, aging,or fragmentation can increase the ability of microplastics to accumulate heavy metals in the environment (Wang et al., 2017).The detection rates of Cd and Zn on the surface of microplastics at different sampling sites were 100%, whereas those of Cu, Pb, and Cr ranged between 61.5% and 92.3%(Fig.6d).A higher abundance of heavy metals was often detected in sampling sites near industrially intensive areas.The concentrations and types of heavy metals in microplastics varied according to their location, which further suggested that metal pollution is an endemic problem closely related to the surrounding environment.

Additives are used in the production of plastics to improve their plasticity and stability, and these additives often contain heavy metals and BPs.However, the concentrations and types of pollutants detected on the surface of microplastics at different sites showed significant environmental correlations.This indicated that substances transported by microplastics were not entirely derived from additives used during plastic production, and the adsorption and accumulation of pollutants in the external environment is likely an important source of BPs and heavy metals observed on the surface of microplastics.Studies have demonstrated that microplastics in the environment can strongly adsorb some pollutants and become sources and sinks of these substances (Ateia et al., 2020).In addition, the standard deviations of pollutant concentrations in each group of samples in this study were high, which was attributed to the differences in the physicochemical properties and environmental conditions between different microplastic samples.Factors such as polymer structure, degree of microplastic fragmentation, water pH, and temperature can strengthen or weaken the interaction between microplastics and pollutants (Naqash et al., 2020).Moreover, the concentration of surface pollutants was not significantly different between microplastics and sediments in most sites, which suggested that the accumulation of pollutants on microplastics was similar to that in sediments.However, considering the wide distribution of microplastics in the environment and their ingestion by organisms, their bioavailability may be substantially higher than that of sediments because they can function as a medium for the transfer of other pollutants along the food chain.Therefore, it is important to further explore the interaction between microplastics and environmental pollutants, and the function of microplastics as carriers that mediate exposure to other pollutants.

4 CONCLUSION

This study investigated the abundance and characteristics of microplastics in the sediments of the Three Gorges Reservoir.The results showed that the abundance of microplastics in these sediments was relatively low compared to that in the sediments of other freshwater environments worldwide.The economic development and total population size were important factors affecting the heterogeneity of microplastic abundance and distribution characteristics.From a morphological perspective, microplastics in the sediments of the Three Gorges Reservoir were mostly secondary microplastics, obtained after longterm aging, oxidation, and mechanical weathering,which affect their environmental behavior.Additionally,we found a variety of plastic polymer types,including PS, PE, and PA, in the sediments of the Three Gorges Reservoir.However, it should be noted that only 100 samples were identified, which is insufficient to extrapolate microplastic pollution in sediments to the whole Three Gorges Reservoir area, for which more data need to be collected.In addition, microplastics can accumulate environmental pollutants such as BPs and heavy metals from the surrounding environment in amounts comparable to those of sediments.Furthermore, considering possible ingestion of microplastics, they may be an important stressor for the ingestion of other environmental pollutants by organisms.Therefore, further assessment of the bio-accessibility of pollutants on microplastics is needed to more accurately understand the potential ecological risks of microplastics in the environment.

5 DATA AVAILABILITY STATEMENT

The datasets generated during the current study are available from the corresponding author upon reasonable request.