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et al. 2017). Suspected MPs are stained with the NR solution and analyzed with a fluorescence microscope. This technique is inexpensive, can utilize available instruments, and can be semi‐automated for large amounts of sample analysis.

      1.4.3.2 Identification of Microplastics by Chemical Composition

       Pyrolysis‐GC/MS

      Pyrolysis‐gas‐chromatography/mass spectrometry (Pyr‐GC/MS) can be used to determine the polymer types and additives. In this method, the samples are combusted, and the thermal degradation products of the polymers are used to detect MPs (Fries et al. 2013). The pyrolysis results provide characteristic pyrograms of MPs samples that can be identified by comparing with reference pyrograms of known polymer types. Particles must be inserted in pyrolysis tubes manually, and the technique can analyze only one particle per run; thus the method is unable for analyzing large amounts of samples (Dris et al. 2018; Klein et al. 2018).

       Infrared Spectroscopy

The spectroscopic identification techniques consist of Raman spectroscopy and FTIR. These techniques are based on the energy sorption by characteristic functional groups of polymeric materials. Thus, samples must be dried before analysis because water strongly absorbs IR radiation. The purification of samples is important because organic, inorganic–organic, or inorganic may affect the IR spectra of samples. Larger particles (>0.5 mm) may be investigated by FTIR with an attenuated transverse reflection (ATR) unit (Doyle et al. 2011; Klein et al. 2018; Zhang et al. 2015). MPs are manually transferred to the crystal of the ATR unit. The FTIR can be combined with an IR microscope to enhance the identification of small MPs (>10 μm) (Sadri and Thompson 2014). In Raman spectroscopy, the sample is exposed to monochromatic laser radiation. The reaction of the molecules and atoms with the laser radiation cause differences in the frequency of the backscattered light compared with the initial laser frequency. This can be computed to create substance‐specific Raman spectra. Raman spectroscopy is also able to be connected with microscopy. The Raman micro‐spectroscopy can identity very small MPs of sizes down to 1 μm (Yan et al. 2019). Until now, FTIR is widely used in MPs studies.

1.5 Occurrence of Microplastic in Freshwater Environments

      In contrast to research in marine environments, MPs in freshwater environments have received less attention, but in the last few years, research on MPs in freshwater are advancing. This helps to reveal the occurrence of MPs in freshwater environments of several continents.

      1.5.1 Microplastic in Lakes

MPs have been reported in lakes of Europe, Africa, Asia, and North America (Table 1.1). In North America, many studies have focused on the Great Lakes system. These studies are clarified by watershed populations, lake areas, and industrial activities. (Anderson et al. 2017; Corcoran et al. 2015; Eriksen et al. 2013). The average number of MPs at the Great Lakes was as high as 193 000 items/km² (Anderson et al. 2017). In Europe, research on MPs usually relates to population sizes. It varies from the densely populated lakes in Swiss Lake Geneva (Faure et al. 2015) to the less populated lakes such as Italian Lakes Bolsena, Chiusi, and Garda, and Swiss Lake Brienz (Fischer et al. 2016; Imhof et al. 2013). To the extent of our knowledge, the greatest number of MPs in Europe has been reported in Lake Geneva, Switzerland, with 220 000 items/km² (Faure et al. 2015). Most MP research in Asia is conducted in lakes in China. MPs were found in all study areas from the isolated lakes in Tibetan Plateau (Zhang et al. 2016) to lakes in the most developed areas such as the Taihu Lake (Su et al. 2016). The MPs pollution in China is reported to be more serious than in other regions. Su et al. (2016) found MPs contamination in Taihu Lake with the abundance ranging from 0.1 × 10⁶ to 6.8 × 10⁶ items/km².

      1.5.2 Microplastic in Rivers

The number of MP studies in rivers is mainly in North America and European countries. MPs have been found in rivers in most European countries. MPs were reported in the Tamar River of UK (Sadri and Thompson 2014); in Ofanto River of Italy; in the Rhine River traversing Germany, Netherlands, and Switzerland (Mani et al. 2015); in Swiss rivers (Faure et al. 2015); and in the Seine and Marne rivers of France (Dris et al. 2015). In North America, MPs studies have been conducted in rivers of the United States, such as the Los Angeles basin watershed (Moore et al. 2011), the North Shore Channel in Chicago (McCormick et al. 2014), and the Detroit and Niagara Rivers (Cable et al. 2017), as well as the St. Lawrence River of Canada, (Crew et al. 2020). In Asia, most studies are carried out in the East region of the continent. MPs were found in many river systems of China such as Yangtze and Pearl (Yan et al. 2019; Zhang et al. 2015), and the Tamsui River of Taiwan (Wong et al. 2020). Similar to results from lake studies, the abundance of MPs found in river systems of China was much higher than in other areas. At the Yangtze River, the mean number of MPs was reported at 8.5 × 10⁶ items/km². Another study at Pearl River found 19860 items/m³. In river studies, the abundance of MPs is reported in different units. Some studies presented the number of MPs per filtered volume of water (items/m³), while others presented it per area of water surface (items/km²). Thus, the comparison between studies is difficult.

Table 1.1 Summary of selected studies on MP contamination in natural freshwater systems.

Types	Study location	Sampling method	Sample process and analysis	Study finding	References
Lake water	Taihu Lake, China	Plankton net: 333 μm	H2O2 (30%); Visual, subset by micro‐FIR or SEM/EDS	Max: 6.8 × 106 items/km2 Min: 0.1 × 106 items/km2	Su et al. (2016)
	Lake Hovsgol, Mongolia	Manta trawl: 333 μm	Density separation (saltwater, 1.6 g/cm3), wet peroxide oxidation; Stereomicroscope (visual), subsample with DSC.	Max: 44 400 items/km2 Mean: 20 264 items/km2	Free et al. (2014)
	Lake Winnipeg, Canada	Manta trawl: 333 μm	Subsample, wet peroxide oxidation; Visual, subsample with SEM/EDX	Max: 748 000 items/km2 Mean: 193000 items/km2	Anderson et al. (2017)
	Great Lakes,USA	Manta trawl: 333 μm	Sieved, hydrochloric acid; Subsamples SEM/EDX	Max: 466 000 items/km2 Mean: 43 000 items/km2	Eriksen et al. (2013)
	Lake Bolsena, Italy	Manta trawl: 300 μm	Sieved, density separation (NaCl, 1.2 g/cm3), HCl digestion, NR staining. Fluorescence microscopy, SEM.	Max: 4.42 particles/m3 Min: 0.82 particles/m3	(Fischer et al. 2016)
	Lake Geneva, Switzerland	Manta trawl: 333 μm	Sieved, wet peroxide oxidation; Stereomicroscope (visual), subsample with ATR‐FTIR	Mean: 220 000 items/km2 Скачать книгу В начало < 6 7 8 9 10 11 12 13 14 15 > В конец