Distribution Of Microplastics In Different Aquatic Ecosystems In China

As one of the top ten countries in the world that produce and consume plastic products, China's aquatic ecosystem is severely affected by microplastics. Currently, microplastics are widely detected in oceans, freshwater, and wastewater treatment plants. Among them, the main types of microplastics include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET).
marine environment.

China is a major producer of plastics, with an estimated 8.82 million tons of plastic waste being discharged into the ocean each year, accounting for 27.7% of the global total. The current research areas mainly focus on the Yellow Sea, Bohai Sea, East Coast, and South China Sea. Table 1 summarizes some relevant studies on the abundance of microplastics in China's marine environment. Research has found that the content of microplastics varies greatly in different regions. For microplastics ranging from 0.33 to 5 mm, the concentration of microplastics in the Yellow Sea is 0.09 items/m ^ 3, while in the Bohai Sea it is 0.65 items/m ^ 3, which is closer to human activities. Therefore, artificial plastic waste from inland or coastal activities may be the main source of marine microplastics. The abundance of microplastics is also influenced by seasons. The study on the concentration of microplastics in the coastal environment of the the Pearl River Estuary shows that the average abundance of microplastics is 295 items/m ^ 3 in dry season and 545 items/m ^ 3 in rainy season. Due to the lack of unified sampling, processing, and analysis methods, it is currently difficult to scientifically compare microplastic pollution in different oceans.

However, generally speaking, the smaller the mesh size, the higher the detected concentration of microplastics. Small particle size microplastics usually account for a large proportion. For example, in the coastal area of the the Pearl River Estuary, about 53.5-73.9% of microplastics are less than 0.5 mm. In the South China Sea, microplastic particles with a diameter of 0.02-0.3 mm account for 92% of the total detected amount, with an average concentration of 2.57 × 10 ^ 3 ± 1.77 × 10 ^ 3 items/L. The analysis of microplastic composition shows that the main types of microplastics in the Bohai Sea water environment are PE, PP, and PS. In addition, as the particle size decreases, the average concentration of microplastics in the marine environment increases exponentially. This may be due to the influence of ocean waves, such as ultraviolet radiation, causing large particle fragments entering the ocean to continuously split into smaller and smaller particles. Therefore, there are smaller microplastics present in the ocean that are difficult to detect, which also indicates that current research results cannot strictly represent the concentration of microplastics in the actual environment, and the concentration in the actual environment may be higher.

Table 1: Research on Microplastic Pollution in China's Marine Environment

Freshwater environment

Microplastics are not only widely distributed in the ocean, but also found in lakes, rivers, and reservoirs. Due to the settlement of large or high-density microplastics during transportation, research on microplastics in freshwater typically includes surface water and sediment. Lakes can serve as temporary or long-term gathering places for microplastics, while rivers are considered as conveyors for microplastics entering the ocean. Compared with other aquatic environments, China has conducted more research on microplastics in freshwater systems. At present, the characteristics of microplastics in freshwater ecosystems in China are mainly small in size (<1 mm) and fibrous, with PP and PE being the most common types of microplastics. In terms of geographical distribution, the research on microplastics in freshwater environment in China is mainly concentrated in the Yangtze River basin, the Pearl River basin and some urban inland waters.

Table 2 summarizes some relevant studies on the abundance of microplastics in freshwater environments in China. A study was conducted on microplastic pollution in surface water of Wuhan City (1.66 × 10 ^ 3~8.93 × 10 ^ 3 items/m ^ 3), and it was found that the concentration of microplastics increased closer to the city center. A study on the surface water (1.60 × 10 ^ 3-1.26 × 10 ^ 4 items/m ^ 3) and sediment (25.0-300 items/kg wet weight) of the Three Gorges Reservoir found that surface water pollution in urban areas is severe, with low-density microplastics (such as PP and PE) being the main pollutants. The content of microplastics in riverbed sediments in rural areas is relatively high, and most of them are high-density microplastics such as PS. High concentrations of micro plastic particles were found along the the Pearl River coast (1.99 × 10 ^ 4 items/m ^ 3) and in the estuary area (8.90 × 10 ^ 3 items/m ^ 3) in Guangzhou, and industrial parks or logistics parks were found nearby, indicating that human activities may be the most important reason for the increase of plastic particle pollution.

In addition, when comparing the concentrations of microplastics in the lakeshore sediments of the Xizang Plateau (8.00-563 items/m ^ 2) and Qinghai Lake (5.00 × 10 ^ -3-7.58 × 10 ^ -1 items/m ^ 2), it was found that even in areas with relatively low population density, remote lakes may become "hot spots" of microplastics due to tourism and inflow rivers. Microplastics are ubiquitous in aquatic ecosystems. Some high-density microplastics or plastic balls may settle into sediments during transportation along the river. After entering estuaries or oceans, microplastics will disperse into the ocean due to hydraulic forces and be diluted due to the huge volume of the ocean. Therefore, the amount of microplastics in freshwater environments is usually higher than in marine environments. Compared with data from other regions of the world, it has been found that the microplastic content detected in inland waters of China is more abundant, while the microplastic content in sediments is similar or even lower.

Table 2: Research on Microplastic Pollution in Freshwater Environment in China

Sewage treatment plant

The wastewater from sewage treatment plants is one of the potential sources of microplastics entering the natural environment. Primary and secondary microplastics used in daily life are transported to sewage treatment plants in the form of raw wastewater. After undergoing a multi-stage sewage treatment process, although most microplastics are trapped in the sludge, the remaining microplastics are discharged into natural water environments such as rivers and oceans along with wastewater from the sewage treatment plant. A study has found that a large secondary sewage treatment plant in Glasgow, Scotland discharges approximately 65 million microplastic particles into the water every day. In Chinese Mainland, an average of 209.7 trillion micro plastic particles are released into the aquatic ecosystem every year, about 306.9 tons of which are more than 80% from sewage treatment plants. Compared with international research progress, China's research on microplastics in sewage treatment plants started relatively late. Current research mainly focuses on several cities, including Beijing, Wuhan, and Xiamen.

Research in Xiamen shows that 79.3-97.8% of microplastics can be removed by WWTPs. However, based on daily sewage discharge, it is estimated that approximately 6.50 × 10 ^ 8 microplastic particles are released into Xiamen Bay every day. Although WWTPs have a high retention rate of microplastics, they have become an important source of microplastics in natural water environments due to their enormous emission load. The main types of microplastics discharged from sewage treatment plants are PP, PE, PS, and PET, which are consistent with microplastics found in natural water environments. In addition, particles, fragments, and fibers are the main shapes of microplastics found in WWTPs. Table 3 summarizes some relevant studies on the abundance of microplastics in sewage treatment plants in China. At present, the database of microplastics discharged from sewage treatment plants in China is not comprehensive. The concentration and properties of microplastic pollution are the main research topics, but technologies to reduce microplastic pollution are still lacking.

Table 3: Microplastics detected in Chinese sewage treatment plants