Research Progress of Microplastic Pollution Status and Risk Assessment of Typical Rivers in China

Microplastics (MPs) have received a lot of attention since they were recognized as an emerging pollutant. Rivers transport 80 % of the land’s MPs to the oceans. With so many rivers in China, the problem of MPs pollution cannot be ignored. This paper collects data related to MPs in typical rivers in China, summarizes the sources of river MPs, and analyses the status of river MPs pollution and risk evaluation. The results show that riverine MPs mainly originate from sewage treatment plants, surface runoff, atmospheric deposition and plastic breakage. MPs abundance is closely related to population density, with urban rivers having higher MPs abundance than other regions. MPs less than 1 mm in size account for more than 60 % of most rivers. At present, the risk assessment of river MPs is mainly based on three methods, namely, the ecological risk index (PHI), the pollution load index (PLI) and the potential ecological risk index (PERI), and the results of the assessment are mostly low risk. There are differences in the results of the assessment models, which need to be applied jointly with each other for continuous improvement.


Introduction
Since their invention in the 20th century, plastics have been continuously used in various fields [1], agriculture, medicine, aerospace and scientific experiments due to their properties such as lightness, simplicity and durability.In 2022, China will produce nearly 80 million tons of plastic products and rank first in the world in terms of recycling and utilization of plastics, but the recycling rate is only 31%.Some European countries still do not pay much attention to the problem of plastic pollution and treat it as general waste to be shipped to landfills [2].Plastics can be turned into microplastics with particle size less than 5 mm, defined as Microplastics (MPs) [3], through a series of physical and chemical actions, which is also used as an emerging pollutant [4].The source of MPs can be categorized into primary and secondary MPs [5], primary MPs are mainly from care products, made from 100 nm PP and PE materials [6], and secondary MPs are made from larger particles of MPs that have been cracked by weathering, light, or UV irradiation.
MPs are found in the atmosphere, water and soil environments, and humans have even found them in the North and South Poles, glaciers and Mount Everest [7].MPs have a high adsorption capacity and a specific surface area [8], which can create more hazardous composite pollutants when combined with heavy metals, antibiotics, and organic pollutants [9], which also attach to biological surfaces and

Wastewater Treatment Plants
Cheung et al. [13] studied that over 80% of MPs in urban streams come from the effluent of sewage treatment plants.The production and manufacturing process of plastic products will generate large amounts of MPs that are not utilized, and these MPs enter the sewage treatment plant along with the wastewater pipes.Although MPs are removed in the treatment process, the sewage treatment plant handles about 200,000 tons of MPs every day, and many MPs will still enter the river.In addition, sludge in domestic wastewater treatment plants traps most of the MPs, Meng et al. [50] in their study of the Hefei wastewater treatment plant found that the sludge removed 83.5% of MPs, the sludge was eventually landfilled or incinerated, and some of the sludge with high organic content was utilized by farmland, and these treatments lead to secondary entry of MPs particles into the water column up to the river.Nearly 60 per cent of sludge from wastewater treatment plants is used in landfills and enters runoff through leachate [15].

Surface Runoff
Surface runoff is an important pathway for the transfer of pavement MPs to rivers.Road dust is the main source, and MPs in the dust come mainly from the use of plastics in daily life, which can cause abrasion and breakage into the aquatic environment.Car tyres are the most common, the driving speed of the car is an important parameter to cause MPs shedding, driving process, the faster the speed and the ground friction is more intense, the greater the degree of wear and tear, the more the number of shedding MPs will also be more and more, with the rainwater into the ground [16].Road marking paint and asphalt pavement materials contain high levels of MPs.Li et al. [17] studied MPs in street dust and detected abundance values up to 1526 n/kg.

Atmospheric Deposition
Atmospheric deposition is also a particular way in which MPs enter rivers..It has been shown that the amount of atmospheric deposition of MPs is positively correlated with the size of the watershed.Large quantities of MPs granules are contained in the exhaust gases emitted from industrial production.Some dust and fumes are transported horizontally by wind up to rivers.Dris et al. [18] found that fibrous form occupies more than 90% of MPs produced by atmospheric deposition, which possibly related to the volume and density of MPs.Liu et al. [19] found MPs abundance of (1.42±1.42)m -2 •d -1 by examining atmospheric deposition in various regions of Shanghai.

Plastic Breakage
Breakage of plastic products is also a source of MPs in rivers.Most of the city's pipes are made of plastic, which breaks down due to ageing and enters the river with the drainage system.A plastic with a particle size of 200 mm will crack into 6.25 × 10 5 MPs particles of 0.8 mm [20].A single laundry wash generates at least 2,000 MPs particles.Wear and tear of plastic products of fishing gear and nets is common in fishing activities.For agriculturally developed regions, such as Xinjiang, where plastics are heavily used in agricultural activities, things like greenhouses, mulch films and pesticide bags can be sources of MPs.Qiantang River 3.9n/L < 1.0 mm (60%) PET, PVC [30] Meishe River 3-10n/L 0.1-0.5 mm (48.56%), 1.0-5.0mm (＞20%), 0.5-1.0mm (20%)

Microplastics Pollution in Typical Rivers in China
Rivers serve as a source of drinking water on which people depend for their livelihoods.The abundance of MPs detected in Chinese rivers varied significantly, with distributions ranging from dozens to hundreds, and the MPs detected in most rivers were dominated by PP and PS.Abundance is an important indicator of the extent of MPs contamination.The current status of MPs pollution in typical rivers in China revealed that human activity and population densities are important factors affecting the abundance of MPs.MPs abundance values in Xinjiang and Tibet in the western region were low relative to those in the eastern region, and MP abundance shows a regional pattern of higher abundance in the south than in the north.This is closely related to population density, with the population in the west much smaller than the population in the east, and the population in the central and southern regions exceeding the population in the north by 4%.The Min River, an urban river with high population density, has abundance values that are 3-4 orders of magnitude higher relative to other rivers.Zhang et al. [32] detected abundance values of 27.84 ± 11.81 n/L for small-scale rivers in the mega-city of Shanghai, where pollution levels were much higher than those in cities with low population densities.The Wei River has higher abundance values than the mainstream Yellow River.Yuan et al. [33] found that large quantities of MPs in the sampling area was positively related to the population density by studying MPs in waters of the Yangtze River.Li et al. [34] used a mathematical model regression analysis and calculated that population density indicates a favorable correlation with the abundance value.(P < 0.05).Qinghai Lake, located on the Tibetan Plateau, has a closed lake with no external water inputs, and MPs external inputs were detected from tourists.China's river sediment MPs abundance performance, in the same river, concentration of MPs in the sediment is often greater than the surface water, the sediment as a MPs "sink", organisms attached to the surface of the MPs, the density increases, easier to settle, the abundance of values increased [35].The upper river sediments had the highest MP percentage and high percentage of MPs was found in the lower river water samples.Hydrological characteristics influence abundance variability, with different cross-sectional layers in the watershed showing significant differences in MPs abundance in the water column, influenced by channel and flow rate [36].Microplastics tend to accumulate as the channel narrows; MPs accumulate in the sediment as the flow slows.Meteorological conditions are also an important factor in modifying abundance.According to numerous studies, the concentrations of MPs on land are two to twenty-three times greater than those in the ocean, and that the rainy season washes MPs from land into rivers with surface runoff.Liu et al. [37] studied MPs concentrations during the dry and flood seasons separately, which increased from 3 × 10 -4 -2.5 × 10 -3 items/L to 4 × 10 -5 -9 × 10 -4 items/L in surface water.In addition, MPs are degraded by light and ultraviolet rays into secondary MPs.Fan et al. [38] studied that abundance was positively correlated with the area of close cropland, precipitation and UV intensity, and negatively correlated with dissolved oxygen, redox sites and wind speed.
It has been shown that size also serves as an important indicator of MPs contamination.The study found that more than 60 % of the MPs in the country's large rivers are less than 1mm in size.This is consistent with the Qingdao nearshore MPs data studied by Yin et al. [39].MPs are generally dominated by small particle sizes.On the one hand, it is from the natural environment subjected to environmental action into a small pellet; another aspect, it is by the large particle size in the transport process occurs in the mechanical movement or physical and chemical effects lead to breakage, MPs transported in the water flow with the riverbed and riverbank collision led to become smaller [40].The relative specific surface area of MPs increases as they become smaller, leading to easy adsorption of microorganisms to the surface and degradation of MPs.Smaller MPs are also more likely to be ingested by microorganisms or to be attached to organisms, and the more harmful they are to the environment.It has been shown that large particle sizes occupy the largest amount of surface water, which correlates strongly with the density of MPs [41].Small particle sizes tend to make up the largest portion of the sediment.MPs in surface water degrade to smaller sizes under UV irradiation and eventually accumulate in the sediment.In general, particle size reduction is due to photochemical degradation, thermal oxidation and bioerosion.

Ecological Risk Assessment of Microplastics in Typical Rivers in China
The problem of MPs pollution in rivers is becoming more and more prominent, MPs can directly harm aquatic organisms, and human beings often use aquatic products or even consume them in their lives, and ecological risk assessment has attracted a lot of attention since MPs has become a hot topic.The special high hydrophobicity and large specific surface area of MPs are favorable carriers for toxic chemicals and harmful microorganisms, which become an important cause of harm to aquatic organisms.There are no well-established evaluation criteria for ecological risk assessment of MPs in rivers.In many studies, three main evaluation methods have been used, an ecological risk index (PHI) evaluation method based on the chemical toxicity of polymers; a pollution load index (PLI) evaluation method on the basis of calculation of MPs abundance, and a potential ecological risk index (PERI) that analyses the environmental impact of MPs.

Ecological Risk Index (PHI)
Pn is the proportion of different polymer kinds at the sample location, and Sn is the risk score for each type of polymer [42].
) CF i is the ratio of the MPs abundance (C i ) to the minimum abundance (C oi ) at a single sample point.PLI and PLI zone are the individual sample point pollution load index and total pollution load index, respectively [43].

Potential Ecological Risk Index (PERI)
T r i is each MP's risk factor, E r i is each MP's ecological risk, and CF i is the pollution factor [44].The higher the risk evaluation level (Table 2), the more harmful it is to the environment.MPs risk evaluation methods are also expanding.Sun et al. [45] used the ecological risk characterization ratio (RCR) combining MPs abundance and polymer toxicity to assess surface water risks in China's terrestrial area, and the findings indicated that there wasn't a meaningful relationship between MPs concentration and RCR.Xiao et al. [46] used MATLAB software to determine the size of overlap of MPs exposure levels and information on toxicity in their study of MPs risk assessment in Chinese surface water, with the larger area being the higher risk.Different assessment models can lead to differences in evaluation results and need to be federated with each other to refine the accuracy of the data.

Conclusion and Prospect
As a hot topic of research, this paper the risk assessment and MPs pollution status of common Chinese rivers.The current level of MPs contamination is mainly judged by the abundance value and size distribution of MPs.Abundance values were strongly related to population density.Geographically, MPs abundance was higher in urban rivers than in non-urban rivers.The size distribution of <1mm is the largest, which may be caused by factors such as the large size becoming smaller by external forces and physical and chemical effects.There is still no well-established ecological risk assessment model for MPs risk assessment, mainly because there are no clear background values.Future research focuses on the following areas: (1) The study of MPs contamination in the environment is based on efficient extraction and detection of MPs.The status of microplastic pollution cannot be based solely on its endowment characteristics as the main research object and should be more combined with the pollution caused by other pollutants to comprehensively evaluate the pollution level.
(2) Determine uniform sampling and analysis methods.Reduce data diversity and improve risk evaluation models under uniform background values.
(3) MPs are difficult to remove in nature, the current level of technology is still insufficient, and future research on the treatment of MPs pollution in rivers needs to be strengthened.

Table 1 .
Distribution of MPs abundance, size and species in typical rivers in China.

Table 3 .
[56]ogical risk assessment of MPs in domestic river waters.Rivers transport 1.2 to 2.4 million tons of MPs to the sea each year.MPs have polymers consisting of monomer polymers with special hydrophobic properties that combine with organic pollutants to form more toxic mixtures.Most of the rivers are still evaluated for ecological risk using the composite hazard indices (PHI, PLI), which are generally at a low level and have not yet caused great harm (Table3).Studies have shown that the higher evaluation of MPs risk in the Yangtze River Basin may be due to the distribution of population and urbanization rate on both sides of the Yangtze River.MPs is increasing at a rate of 4.5% per year, and Zhang et al.[56]predicted that the evaluation of MPs' ecological risk in 2100 found no outbreaks of contamination but posed a huge threat to the environment.At present, there are still many deficiencies regarding the evaluation of MPs' ecological risk and a lack of unified standard models.Existing ecological risk assessment models are affected by a variety of factors, such as different sampling methods, resulting in different detected MPs abundances affecting the final calculation results of the PHI model.Dusacuy et al.[57]in using different sampling and analyzing methods, which ultimately led to the detection of diverse MPs data.One study collected MPs samples with a trawl and found them to be several orders of magnitude lower than samples collected by other filtration methods.For instance, because the PHI only takes into account the possible ecotoxicological effects of individual polymers and ignores their interactions with other pollutants or environmental factors, it does not offer a comprehensive assessment of the environmental risks associated with polymers.Therefore, ecological risk assessment models need to be continuously improved.