Identification of Microplastic
1.4.3.1 Visual Sorting
In most studies, visual sorting is the first step to separate MPs from samples before identification of the polymer type. Large MPs (> 1 mm) can be recognized by the naked eye (Anderson et al. 2017), while smaller particles are identified using dissection microscopes (Faure et al. 2015; Mani et al. 2015) or scanning electron microscopy (SEM) (Eriksen et al. 2013; Su et al. 2016). This step requires experienced researchers and good optical quality of the microscope. However, identification of all particles is difficult if they are smaller than a certain size, if they are unable to be distinguished visually or cannot be managed with forceps due to their minuteness. Thus, visual sorting is time‐consuming and easy misidentification or underestimation of MPs is possible. Recently, another visual identification method using fluorescence was applied to detect and quantify small MPs. In most studies, Nile Red (NR) was used and dissolved in different solvent solution such as acetone, chloroform, and n‐hexane (Crew et al. 2020; Tamminga 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,
|
