Plastic and Microplastic in the Environment. Группа авторов

secondary sources. Primary MPs are produced in micro sizes, such as microbeads for personal care products and plastic resins. Secondary MPs are generated from the degradation of large plastics (Andrady 2011). Textile laundering facilities and sandblasting are other sources of MPs (Browne et al. 2011; Napper and Thompson 2016). The pollutants are washed down the drain along with wastewater and enters sewage systems, where they are too small to be removed by WWTPs and so end up in the river systems and finally in the oceans.

      1.2.1 Primary Sources

      1.2.1.1 Microplastics from Personal Care Products

Plastic materials have been utilized in the cosmetic industry for decades since the 1960s. The particles of plastic used in personal care products can be large enough to see with the naked eye (50–1000 μm), fine particles (lower μm‐range), or very fine particles (< 2.5 μm) (Leslie 2014). MPs used in the industry may be spherical or irregular morphology as shown in Figure 1.1 (Godoy et al. 2019). MPs used as ingredients in cosmetic products include the |two main categories that are typically made from petroleum carbon sources: thermoplastics (e.g. polypropylene (), polyethylene (PE), polytetrafluoroethylene (Teflon), polystyrene (PS), polymethyl methacrylate (PMMA); and thermoset plastics (e.g. polyester, polyurethanes)) (Napper et al. 2015). According to Gouin et al. (2015), around 93% of the microbeads applied in cosmetics are PE, since their smoothness reduces redness and impact on the human skin compared to other polymers.

Figure 1.1 (a) Micrograph of a microbead found in Lantau Island, Hong Kong

      Source: Cheung 2016, p. 02 / With permission from Elsevier. (b), (c) Scanning electron microscopy (SEM) of microbeads utilized in cosmetic products

      Source: Napper 2015, p. 05 / With permission from Elsevier; Godoy 2019, p.06 / With permission from Elsevier.

      1.2.1.2 Microplastics from Plastic Resins

      Another major source of primary MPs are plastic resins (pellets or powders). The pellets or powder resin (≤ 0.5 mm) are generally cylindrical or disk shaped (Bergmann et al. 2015). These plastics are transported to factories for re‐melting and molding into a wide range of commercial plastic products. The plastics are released into the environment due to improper handling, such as accidents during transport or runoff from production processes.

      1.2.2 Secondary Sources

      1.2.2.1 Microplastics from Degradation of Plastic Debris

Large plastic debris in freshwater environments gradually degrades into smaller particles when exposed to some factors in the environments (Figure 1.2). In the degradation of plastic, the average molecular weight of polymers is drastically reduced (Andrady 2011). The degradation can categorize as agents causing this process: biodegradation is an action of microorganisms; photodegradation is an action of light and sunlight; thermal degradation is the action of high temperatures; thermooxidative degradation is an oxidative breakdown at medium temperatures; hydrolysis degradation is a reaction with water. In the environment, UV radiation from sunlight is the major factor of plastic degradation, which speeds up the oxidative breakdown of polymers (Andrady et al. 1996). Polymers such as high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), PP, and nylons are degraded mainly by the UV‐B radiation in sunlight as they are exposed to environments. As degradation is initiated, the thermooxidative process can continue without further exposure to UV radiation. The autocatalytic degradation may occur as long as the existence of oxygen in the system (Andrady 2011). During the degradation process, plastic wastes typically discolor, evolve surface features, and become weak and brittle, gradually. Other forces such as waves, wind, and human and animal activities can easily crack the embrittled plastics into small particles. The degradation and fragmentation of plastics is the major process for the formation of secondary MPs in aquatic environments (Kershaw 2015).

Figure 1.2 (a) Degradation and fragmentation of plastic under environmental factors; (b) and (c) the cracks seen at the surface are caused by photochemical degradation

      Source: Ter Halle 2016, p. 15 / With permission from American Chemical Society.

Figure 1.3 SEM of typical fibers: (a) polyester‐cotton blend; (b) polyester; (c) acrylic

      Source: Napper 2016, p. 03 / with permission from Elsevier.

1.2.2.2 Microplastics from Textile and Domestic Washing

Fibers from synthetic textiles are another source of secondary MPs in the environments (Figure 1.3). Synthetic fibers are made from petroleum through polymerization, polycondensation, or polyaddition processes (Astrom 2016). In 2010, total synthetic fiber production was 49.6 million tons, accounting for 60% of the world's fiber production (Essel et al. 2015). Synthetic textiles can be used in clothes, furniture, geotextiles, cloth, sports, packing, toys, construction, and agriculture. Shedding of textiles relies on the textile types, the yarn and texture type, and the fiber types involved. According to Astrom (2016), most fibers are shed from synthetic fleece and microfleece. A synthetic fleece coat can shed about 1900 fibers with each wash (Browne et al. 2011). These authors also concluded that using detergents causes more shedding compared with using only water. The average size of fibers ranged from 5.0 to 7.8 mm in length, and 11.9 to 17.7 μm in diameter (Napper and Thompson 2016). Compared to other MPs detected in environments, such as pellets or fragments, fibers have a higher surface‐to‐volume ratio; therefore, they can attract more chemicals than other MPs (Astrom 2016).

1.3 Pathways of Microplastics into Freshwater Environments

MPs can enter freshwater environments by several pathways due to the bulk of plastic wastes in the environment (Figure 1.4). A pathway of MPs entering may be important in one region but less important in another (Lambert and Wagner 2018). For example, MPs applied in personal care products are likely more important in urban than agricultural regions (Lambert et al. 2014). Potential environmental release pathways of MPs can be separated by their primary or secondary sources.

      A pathway of primary MPs input to freshwater environments has been found through WWTPs and the utilization of sludge from WWTPs to agricultural lands. Previous studies depicted that 90% of MPs in domestic wastewater are retained within sludge (Magnusson and Norén 2014; Talvitie and Heinonen 2014). In Europe, sewage sludge is normally composted to produce agricultural fertilizer as well as dispose of the sludge to land. The EU countries apply about four to five million tonnes of sludge to agricultural lands, annually (Willén et al. 2017). MPs that cannot be removed in the treatment process will reach the freshwater environments via effluent (Horton et al. 2017). Another pathway of primary MPs could be from the release of industrial products or processes.