Introduction

The world’s plastic production has increased rapidly in recent decades, it reached 367 million tons in 2020, and China is the largest producer of plastic products, accounting for 32% (Plastics Europe, 2021). While plastic products bring great convenience to the daily lives of human beings, they also caused major environmental problems when plastic wastes are inappropriately discarded (Xiong et al., 2018). Large plastic wastes gradually degrade into smaller pieces under the action of weathering and ultraviolet radiation, and plastic particles less than 5 mm are defined as microplastics (Thompson et al., 2004). Based on the source of the microplastics, they can be divided into primary and secondary microplastics (Waldschläger et al., 2020).

As emerging pollutants, microplastics have become one of the hot topics for global researchers in recent years. Many studies have reported that microplastics are ubiquitous in marine, coastal and estuarine environments (Cole et al., 2011; McEachern et al., 2019; Deng et al., 2020; Mataji et al., 2020). The presence of microplastics in the oceans is a potential threat to the health of marine organisms because microplastics are easily introduced into the food chain due to their small size (Yan et al., 2019). Moreover, microplastics can be carriers for other pollutants, including heavy metals (Wang et al., 2017a; Deng et al., 2020), PAHs (Fisner et al., 2017), and PCBs (Mohamed Nor and Koelmans, 2019), and the combination of these pollutants poses greater hazards to marine organisms (Lin et al., 2021). Previous studies have indicated that microplastics can be ingested by marine organisms, such as zooplankton, bivalves, whales, fish and prawns, and can severely harm them; some of the effects include decreased growth rates, pathological stress, oxidative stress, and reproductive complications (Cole et al., 2013; Lusher et al., 2015; Abbasi et al., 2018; Yan et al., 2019). Given the potential risks from microplastics, legislation to ban the use of microbeads in personal care products has been introduced in the USA, Canada, Australia, China and in some European Union countries (Rochman et al., 2016; Xiong et al., 2018).

Although the research on microplastics in freshwater systems is relatively recent, problems associated with microplastics in inland freshwaters have also received attention. Microplastic pollution in the surface waters of the Laurentian Great Lakes was reported first (Eriksen et al., 2013), and the presence of microplastics was reported in freshwater beaches, lakes or rivers in Africa, Europe, Asia, North and South America (Eerkes-Medrano et al., 2015). In China, the first report on microplastic pollution in freshwaters emerged from the Three Gorges Reservoir in 2015 (Zhang et al., 2015).

Microplastic pollution is closely related to human activities. The abundance of microplastics in freshwaters is lower in areas that have lower levels of human activity, such as the Qinghai-Tibet Plateau (0–2000 n/m3), the Yangtze River headwaters of China (Feng et al., 2021), and the Ganjiang River, with values of 160–720 n/m3 (Hu et al., 2021). In contrast, microplastic pollution in some freshwater environments in China has been severe, especially in large rivers or lakes where anthropogenic activities are common, such as the Pearl River, with values of 3400–25,800 n/m3 (Yan et al., 2019), the Yangtze River along Chongqing city, with values of 46,700–204,000 n/m3 (Fan et al., 2021), Poyang Lake, with values of 5000–34,000 n/m3 (Yuan et al., 2019), and Taihu Lake, with values of 3400–25,800 n/m3 (Su et al., 2016). Although some studies have reported on microplastics in freshwater environments, most of these studies are pollution-level and polymer-type investigations, and microplastic microscopic features are rarely documented.

The Han River is the largest tributary of the Yangtze River; therefore, it has a high degree of economic importance to China for its economically exploitable hydropower and potential water resources (Li et al., 2013). The middle and lower reaches of the Han River along Hubei Province are approximately 920 km long. The Danjiangkou Reservoir, the second largest reservoir in China, is located on the Han River and is the water source for the Middle Route of the South to North Water Diversion Project (SNWDP) (Di et al., 2019). Thus, water quality protection in the Han River is very important. While some studies have reported on microplastics in the Han River, these investigations and analyses were mainly carried out on a small scale (only in the Danjiangkou Reservoir) (Di et al., 2019; Lin et al., 2021). However, information on microplastics pollution in the Han River along the entirety of Hubei Province is not available, and the surface textures and elemental compositions of microplastics are also rarely documented.

In this study, microplastics were investigated in the surface waters of the Han River along Hubei Province. The morphological characteristics, surface textures, elemental compositions and polymer types of microplastics were analyzed, and potential sources were inferred. To the best of our knowledge, this is the most detailed study aimed at analyzing the abundance and characteristics of microplastics in the Han River along Hubei Province. This study contributes to water quality management and conservation in the Han River and lays the foundation for future studies.

Materials and methods

Study area and sampling sites

The Han River falls between 106° and 114°E, 30°and 34°N and is the largest tributary of the Yangtze River, with a total length of 1,577 km and a drainage area of 1.51 × 105 km2. Danjiangkou and Zhongxiang divide the main stream of the Han River into the upper, middle, and lower reaches. The middle and lower reaches of the Han River along Hubei Province are approximately 920 km long. The river enters Hubei Province from Yunxi County, Shiyan city, flows through the cities of Laohekou, Xiangyang, Jingmen, Zhongxiang, Tianmen, Xiantao and Hanchuan, and finally empties into the Yangtze River from the Longwang temple in Wuhan city (Fig. 1) (Zhang et al., 2022). The middle and lower reach regions of the Han River are dominated by a subtropical monsoon climate with an annual average temperature of 15–17 °C and an annual total radiation of more than 448 kJ/cm2. The middle reach receives lower amounts of precipitation, with an average annual precipitation of 750–900 mm. The lower reach flows into the Jianghan Plain, and rainfall is abundant, with an annual average precipitation of over 1100 mm.

Fig. 1
figure 1

Locations of sampling sites in the middle and lower reaches of the Han River

Full size image

In this study, 11 sampling sites (Fig. 1) were set based on river water quality monitoring sections set up by the Hubei provincial department of ecology and environment at cross-city boundaries in the middle and lower reaches of the Han River. The basic information for each sampling site is listed in Table S1.

Sample collection and pretreatment

The sampling work was conducted in early March 2021. Retractable stainless-steel water collectors (3.5 m long, 2 L volume) were customized to collect river surface water. A 500-mesh (mesh size 25 μm) stainless-steel screen was used on site to filter the collected water and to collect entrapped substances; this approach prevented the need to transport multiple water samples over long distances. After the investigators reached the predetermined site locations, the water collector and stainless-steel screen were rinsed with ultrapure water three times. Then, the telescopic rod was extended, the water collector was used to collect surface water samples far from the shore, and the samples were filtered through a stainless-steel screen; this process was repeated five times. The materials trapped by the sieve were repeatedly rinsed into a brown glass bottle with ultrapure water and stored in a sealed container. Three parallel water samples were collected, and a background control group was set at each sampling site. Ultrapure water was added to the collector to repeat the sampling steps to exclude airborne microplastics from entering the water sample and interfering with the experimental results. The basic parameters for water quality, including water temperature, pH, dissolved oxygen, conductivity, salinity, and total dissolved solids, were measured with a YSI at each sampling site. The preserved samples were transported to the laboratory for storage at 4 °C for subsequent processing.

Separation of microplastics

Microplastics were separated from water samples based on the direct filtration method reported by (Su et al., 2016), with some improvements. The specific steps were as follows: each water sample was transferred into a clean 500 mL beaker, the bottles were rinsed at least three times with ultrapure water, and the water was poured into the beaker. Then, 30% H2O2 (AR) was added to degrade the organic matter at room temperature for 24 h. After digestion, using a vacuum pump, the water samples were directly filtered through a nitrocellulose mesh filter membrane (Sartorius, diameter 5 cm, pore size 0.45 μm). The filtration device was also rinsed with ultrapure water three times and filtered. Ultimately, the filter membranes obtained above were stored in clean Petri dishes and placed in a drying oven at a constant temperature of 50 °C for at least 48 h. Three blank control groups were set for the overall experimental process, and the experiment was operated in a closed environment to prevent microplastic contamination from airborne particles on the filter membrane. A total of 47 filter membrane samples were obtained after the filtration process.

Microscopic inspection and identification

The treated membranes were observed under a microscope with a scale (Olympus STM6, Japan), beginning at the top and moving in a “zigzag” pattern from left to right. Each square was observed to find microplastics, and the microplastic particles were counted, measured and photographed. The microplastics were visually identified following the same rules as (Deng et al., 2020) and were classified by size, shape and color. Three shapes were observed, namely fiber, film and fragment, and six colors were found: transparent, white, blue, red, black and yellow. Note deep blue, light blue and green were included in the blue category. In the red category, deep red, light red and pink were included. Brown was included in the yellow category. Purple was included in the black category.

Some representative (approximately 50 items) and all suspected (93 items) microplastics were identified by a Raman microscope (HORIBA Evolution, France) and by a Fourier transform infrared spectrometer (FTIR, Thermo Scientific iS5, USA) to determine polymer types. The Raman microscope was equipped with a 785 nm laser diode, and the detector spectral range was 100–3200 cm−1. Spectra were collected with two accumulations of an integration time of 90 s each (Frère et al., 2016). FTIR was used to identify relatively large microplastics (> 3 mm), and smaller microplastics that were not convenient to transfer were analyzed directly by Raman microscopy.

SEM/EDS analysis

The surface texture and elemental compositions of the microplastics were further studied by a scanning electron microscope equipped with an energy-dispersive spectrometer (SEM/EDS, Phenom proX, Netherlands). SEM was performed at 10 keV, and EDS was used to determine the qualitative elemental composition on the microplastic surface. Because eroded samples were heterogeneous, visualization was repeated at different surface locations at least three times (Wang et al., 2017a).

Data analysis

The map of sampling site locations was illustrated in ArcGIS 10.5. Statistical analysis was performed in SPSS 22.0, and one-way analysis of variance (ANOVA) with the LSD post hoc test and Duncan’s test were performed to compare significant differences between the microplastic abundance of sampling sites. All figures were plotted with OriginPro 9.0.

Quality assurance and quality control

To avoid external microplastic contamination during the experimental process, quality assurance and quality control (QA/QC) procedures were applied during all steps from sample collection to microplastics detection to reduce potential influences (Lin et al., 2021). Stainless-steel tools and glass containers were used instead of plastic during the entire process. The bottles were rinsed with ultrapure water and dried in the laboratory before sample collection. The water collector and stainless-steel screen were rinsed with ultrapure water before water collection at each site. Before use, all ultrapure water and solutions were filtered with a 0.45 μm filter membrane (Mu et al., 2019). A separate, closed laboratory was used for the experimental analysis of microplastics. The samples were kept covered in aluminum foil during settling to decrease airborne contamination sources. Blank control groups were set for all sample collection and experimental procedures. The mean microplastic particle counts on the filter membranes of all field blank water samples and in the blank control groups for the experimental process were five fibers and two fibers, respectively. These mean microplastic particle counts were subtracted from all sample counts for data analyses and reports (Lin et al., 2021). In addition, microscopic inspection and identification on all filters was essential during filtration.

Results and discussion

Microplastic pollution level in the middle and lower reaches of the Han River

Microplastics were detected at each site, and their abundance ranged from 2315 ± 603 to 8406 ± 2055 n/m3 (shown in Fig. 2), with an average of 4218 ± 806 n/m3. The microplastic abundance compared with freshwater areas in other countries was higher than the surface waters of the Vistula River (range 1600–2550 n/m3) in Poland (Sekudewicz et al., 2021) and the Han River (range 0–42.9 n/m3, mean 7.0 ± 12.9 n/m3) in South Korea (Park et al., 2020) but lower than the plain and mountainous lakes (range 4000–26,000 n/m3, mean 11,000 ± 7000 n/m3) in Siberian Russia (Malygina et al., 2021). In comparison with other inland freshwater areas in China (Table 1), the abundance was lower than that in large freshwater lakes such as Taihu Lake (range 3400–25,800 n/m3) (Su et al., 2016) and Poyang Lake (range 5000–34,000 n/m3) (Yuan et al., 2019) but higher than that in Dongting Lake (range 616.67–12,611 n/m3) (Jiang et al., 2018). Large rivers with more frequent human activities usually have higher average abundances than the Han River, such as the Yangtze River along Chongqing city (104,000 ± 52,000 n/m3) (Fan et al., 2021) and the Pearl River along Guangzhou city (19,860 n/m3) (Yan et al., 2019). Thus, the microplastics in the surface waters of the middle and lower reaches of the Han River were at moderate levels.

Fig. 2
figure 2

Microplastic abundance and distribution in surface water of middle and lower reaches of the Han River (data were presented as mean ± SD, n = 3) The lowercase letters (a, b, c, d, e and f) indicate nonsignificant differences (p > 0.05)

Full size image
Table 1 Microplastics abundance in inland freshwater areas of China
Full size table

The microplastic concentration of the Chenjiapo monitoring section (R1) upstream of the Danjiangkou Reservoir was higher than that of the Shenwan monitoring section (R2) downstream. Combined with the research results, the average abundance of microplastics in the Danjiangkou Reservoir area was 6081 n/m3 (Lin et al., 2021), which was higher than that in the upstream Sect. (4108 n/m3). This result indicated that the Danjiangkou Reservoir might be a sink for upstream microplastics; the water flow slows upon entering the reservoir area, and thus microplastics readily accumulate and the microplastic concentration of the reservoir area increases. The microplastic abundance generally showed an increasing trend along the flow direction of the middle and lower reaches of the Han River (from R2 to R11). The highest and lowest abundance values appeared in the Zongguan monitoring section of Wuhan city (R11) and the Baijiawan monitoring section of Xiangyang city (R3). The abundance of microplastics indicated high spatial heterogeneity at different sites, and there were significant differences between microplastic abundance among most surface water samples (p < 0.05), but nonsignificant differences occurred in the Shenwan monitoring section of Xiangyang city (R2) and Luohanzha monitoring section of Jingmen city (R7), Huangzhuang monitoring section of Jingmen city (R6) and Yuekou monitoring section of Tianmen city (R8) (p > 0.05). Previous studies have reported that the distribution of microplastics in surface waters can vary due to many factors, such as source loading, plastic properties, and meteorological and hydrodynamic conditions (Zhao et al., 2015; Xiong et al., 2018). Microplastics migrate downstream under the action of hydrodynamics, resulting in increased downstream concentrations. Previous studies have revealed that human activities are related to microplastic pollution and that production and human activities were the main sources of plastic wastes in rivers (Wang et al., 2017b; Xiong et al., 2018). Large transport ships are frequently docked in a shipping wharf near the Zhuandou monitoring section (Fig. S2), and agricultural production activities are intense near the Hannancun monitoring section. These human activities may be the reasons for the significant increase in microplastics in the two sections. The Zongguan monitoring section was located in Wuhan city, the megacity and center of China, and was surrounded by highly populated residential areas. In addition, the abundance observed in the Zongguan monitoring section was essentially consistent with the those observed in inland city lakes in Wuhan city, Beihu Lake (8925 ± 1591 n/m3) and Huanzi Lake (8550 ± 990 n/m3) (Wang et al., 2017b).

Morphological characteristics of microplastics

Some typical optical microscope images of different shapes of collected microplastics on the filter membranes are shown in Fig. 3. The morphological characteristics of microplastics found in the surface waters of the Han River are presented in Fig. 4. The microplastics were divided into four size groups: < 200 μm, 200–500 μm, 500–1000 μm, and 1–5 mm. The majority (56.90%) of the microplastic particles in this study were < 200 μm in size, 31.68% were in the 200–500 μm range, and only 7.54% and 3.88% were in the 500–1000 μm and 1–5 mm ranges, respectively (Fig. 4a). Among all microplastics found in the water samples, fiber was the most prevalent shape (59.27%), fragment accounted for 39.22%, and film accounted for only a small portion, 1.51% (Fig. 4b). When classified by color (Fig. 4c), transparent was the most abundant (53.45%), followed by blue (14.22%) and red (11.64%), and white, black and yellow contributed only 8.84%, 6.68% and 5.17%, respectively.

Fig. 3
figure 3

Optical microscope images of different shapes of microplastics on the filter membranes a red fiber, b transparent fiber, c black fiber, d transparent film, e blue film, red fragment, f yellow fragment

Full size image
Fig. 4
figure 4

Morphological characteristics (size, shape and color) of microplastics. a Size, b shape, c color

Full size image

Small (diameter < 500 μm) microplastics (ratio 88.58%) were dominant in the surface waters in this study, which agreed with the results from the Danjiangkou Reservoir (Lin et al., 2021). A possible reason is that the source of microplastics entering the river, such as the wastewaters discharged from most sewage treatment plants, contains mainly small fibers, which are less than 500 μm (Li et al., 2018). Furthermore, larger microplastics continue to degrade into smaller particles under the action of external forces, and smaller particles more easily enter rivers with rainfall (Zhang et al., 2015). It is worth noting that microbeads or pellets were not found in the present study unlike observations from the study conducted in the Danjiangkou Reservoir in August 2019. This might mainly be because the production and use of daily-use chemical products with microbeads were banned by the Chinese government in 2020, as microbeads are an essential source of personal care product pollutants (Rochman et al., 2016).

Surface textures and elemental compositions of microplastics

SEM images of some typical microplastics are presented in Fig. 5. The images show that the plastic fibers were slender and some had linear cracks of different widths on the surface (Fig. 5c, d). At the same time, some were smooth and contained no visible cracks on the surface (Fig. 5a, b). The surfaces of film-type microplastics were relatively large and had substantial specific surface areas, which easily allowed them to adhere to other substances. They were light and soft, with different degrees of wrinkling (Fig. 5e, f, g). Fragment surfaces were rough and usually showed deep cracks. SEM–EDS was used to analyze the elemental compositions of the microplastics. EDS spectra demonstrated that the constituent elements of the microplastic polymers were C, O, Ca, K, Al, and Mg, in addition to heavy metals such as Zn, Ni, and Cu, which pose risks for environmental contamination and poisoning (Fig. 6).

Fig. 5
figure 5

Typical SEM images of microplastics. a, b, c, d Fibers, e, f, g films, h fragment

Full size image
Fig. 6
figure 6

EDS images of elemental compositions of microplastics

Full size image

The SEM‒EDS analysis results illustrated that different microplastics in the surface waters of the Han River experienced different levels of mechanical erosion and chemical weathering, which resulted in fracturing, avulsions, pits, flakes and adherence of other matter. These common degradation-induced textures allowed the microplastics to potentially serve as effective adsorption vectors for heavy metals. Similar analyses and results for microplastics have been reported in some coastal areas, mainly in soil media, such as in Caofeidian beach soils (Zhou et al., 2016), in Jinjiang estuarine wetland sediments (Deng et al., 2020), in the surface waters and sediments of the southern Caspian Sea (Mataji et al., 2020), and in the Beijiang River littoral zone surface sediments (Wang et al., 2017a), but few studies have been reported in freshwater environments. Hodson et al. (2017) revealed that microplastics could act as vectors for heavy metals, and Maršić-Lučić et al. (2018) also reported that plastic pellets could sorb metals from the marine environment. A simulation study indicated that aging could change the surface structure of microplastics, resulting in more cracks and ravines, thereby increasing specific surface area and creating more developed pore structures that provide more sites for the adsorption of heavy metals (Lang et al., 2020). Although metals are usually added as catalysts, pigments or stabilizers during plastic manufacturing (Fries et al., 2013; Wang et al., 2017a), the observed metals on different sites of individual microplastics varied in this study, which provided evidence that the metals carried by microplastics did not originate from the manufacturing process but were derived from the environment. This might be related to the microplastic migration process and the level of metals in the medium (Deng et al., 2020). However, further research is necessary to determine whether the heavy metals loaded on the microplastics originated from the waters of the Han River.

Potential sources of microplastics

Human activities and industrial and agricultural production are considered the primary sources of plastic wastes in natural water bodies (Wang et al., 2017b). Microplastics enter the freshwater system through swage discharge, rainfall runoff, tributary input, anthropogenic input and atmospheric deposition (Lin et al., 2021). It is challenging to trace the origins of microplastics due to their complex migration, transport and conversion patterns in the environment. Nevertheless, their sources can be inferred based on their morphological characteristics and chemical compositions (Yan et al., 2019; Deng et al., 2020). Polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE) and polyamide (PA) (Fig. S1) were observed most frequently in the polymer types of microplastics according to identification by Raman and FTIR spectroscopy in this study.

Small-sized (< 500 μm, 88.58%) fibers (59.27%, Fig. 4) were the most dominant shape of microplastics in surface waters, and some of them were made of primary microplastics. PET, PP and PA are widely used in textiles and clothing, which suggests that clothes laundering might be an important source of microplastics in the freshwater environment of the Han River. These results were consistent with Peng et al.'s (2017) findings that the source of microplastics in the Changjiang Estuary was mainly from clothes laundering. Browne et al. (2011) found that a single garment could produce up to 1900 fibers during a domestic washing process. Li et al. (2018) also proved that sewage sludge discharge is an important source of microplastic pollution. The washing wastewater generated by the residents in the cities along the middle and lower reaches of the Han River enters the sewage treatment plants through the drainage system and is discharged into the Han River or its tributaries after treatment, which results in microplastic fibers convergence in the river. The results of a study on the Chin Ling-Wei River also revealed that tributaries were a leading source to trunk streams, which showed a convergent effect (Bian et al., 2022). Furthermore, some residents wash their clothes directly on the shores of the Han River, which directly introduces plastic fibers into the water. In addition to primary slender-fibered microplastics, elongated fiber microplastics were found, some of which were aggregated together, and their surfaces were damaged (Figs. 3, 5). Their polymer types were mainly polyethylene (PP) and polypropylene (PE), which indicated that the microplastics might have originated from agricultural packing textiles. PP and PE are widely applied in commonly used agricultural packing textiles, such as woven bags, fishing nets or strapping ropes (Xiong et al., 2018; Deng et al., 2020; Fan et al., 2021). Consequently, this type of plastic waste was observed along the shore of the Han River (Fig. S2).

Plastic waste from human activities and agricultural production may be another source of microplastics in the middle and lower reaches of the Han River. Fragment- and film-shaped microplastics accounted for 39.22% and 1.51% (Fig. 4b) in this study, and they likely originated from the degradation of large plastic wastes. As a common raw material for plastic products used in daily life, PP is widely used for household products, such as food packaging and containers, straws, and woven plastic products. PE is widely applied in film products, such as food bags and packaging films (Peng et al., 2017; Fan et al., 2021; Feng et al., 2021). Industrial production activities have been prohibited near the Han River in Hubei Province since 2020 when the water environment protection regulations of Hubei Province for the Han River basin were issued, but agricultural production is still active. Disposable plastic bags, packaging bags or films for food, seeds, pesticides, bottles for drinks and pesticides, etc., were observed along the Han River during the survey (Fig. S2). According to a study of microplastics on the shores of the river Rhine-Main in Germany, riverine input was an essential source of microplastic pollution (Klein et al., 2015). Plastic wastes from land sources can be transported to rivers via rainfall-induced surface runoff (Xiong et al., 2018), and large plastic pieces are readily broken and decomposed into microplastics under the action of ultraviolet radiation or water (Wang et al., 2018). In addition, many cargo ships in the Han River have been found to be covered with transparent plastic films. If the film is not recycled or handled correctly, it may also enter the water environment.

Conclusion

The microplastic abundance ranged from 2315 ± 603 to 8406 ± 2055 n/m3 in the middle and lower reaches of the Han River surface water and showed a moderate pollution level. The microplastic abundance was significantly different (p < 0.05) among most sampling sites and generally showed an increasing trend along the river flow direction. The highest amount occurred in the Zongguan monitoring section in Wuhan city. Small-sized (< 500 μm, 88.58%) and transparent (53.45%) fibers (59.27%) were the most dominant microplastics found. Polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE) and polyamide (PA) were the main materials for these microplastics according to identification by Raman and FTIR spectroscopy. SEM analysis showed that fractures, avulsions, pits, flakes and adherence of other matter resulted in the common microplastic degradation textures observed, revealing that these were secondary microplastics and had experienced different levels of mechanical erosion and chemical weathering. However, some fiber microplastics with smooth surfaces were also present, and there made of primary microplastics. EDS analysis found that heavy metals such as Zn, Ni, and Cu were absorbed on the surface of microplastics, which illustrated that microplastics might be adsorption vectors for heavy metals. Based on the morphological and microscopic characteristics and the polymer types of the microplastics found, clothes laundering, plastic wastes from human activities and agricultural production may be the main microplastic sources in the Han River water. Therefore, treatment of microplastics from domestic sewage and management of plastic waste around the Han River basin could reduce microplastic pollution from the main sources.