Microplastics: The Evolution of Plastic Pollution
Apart from plastic litter, MPs (microplastics) have undoubtedly been present in the environment for many years. Concern has been raised that microscopic plastic debris may also be harmful to the environment and to human health . The term microplastic is used to describe the tiny plastic pieces in freshwater systems around the globe including lakes and rivers. In the literature on freshwater habitats, few studies on microplastics existed prior to the 21st century; Hays and Cormons (1974)  and Moore et al. (2011)  documented plastic particles (<5 mm) in the rivers of North America, while Faure et al. (2015)  reported on microplastics in Lake Geneva.
The term “microplastics” commonly refers to plastic particles with a diameter <5 mm. It has been suggested that the term microplastics be redefined as items <1 mm to include only particles in the micrometer size range [22,23], and the term “mesoplastic” introduced to account for items between 1 and 2500 mm . Lambert et al.  described macroplastics as >5 mm, mesoplastics as <5 to >1 mm, microplastics as <1 mm to >0.1 pm, and nanoplastics as <0.1 pm.
Generally, MPs are divided into categories of either primary or secondary MPs. Primary MPs are manufactured as such to be used either as resin pellets to produce larger items or directly in cosmetic products such as facial scrubs (known as microbeads; see Figure 11.1) and toothpastes or in abrasive blasting (e.g., to remove lacquers). Compared to this restrained use, secondary MPs are formed from the disintegration of larger plastic debris. They include fragments arising from plastic bottles, bags and other packaging materials .
Sources of Plastics and Microplastics into the Freshwater Environment
There are various ways and sources in which plastic waste enters into the freshwater environment. Land littering is an important environmental and public issue, and measures are now needed to reduce damage to the environment .
MPs have different environmental release pathways (Figure 11.2).
• Primary MPs like polyethylene, polypropylene and polystyrene particles in cleaning and cosmetic products enter aquatic systems through household sewage discharge [28-30].
FIGURE 11.1 Microbeads.
FIGURE 11.2 Conceptual diagram of microplastic sources and flows among anthropogenic, terrestrial, freshwater and marine environmental compartments. (Source: Horton AA, Walton A, Spurgeon DJ, Lahive E, and Svendsen C. Microplastics in freshwater and terrestrial environments: Evaluating the current understanding to identify the knowledge gaps and future research priorities, Sci Total Environ 2017, 586, 127-141.)
- • Beaches receive primary MPs from spills at inland factories that made their way to coastlines via rivers, streams, and storm-water drains . In recent freshwater literature, large amounts of virgin plastics have been found near plastic production and processing plants along the Rhine River  and Danube River .
- • Secondary microplastics arising from litter such as a discarded drinking-water bottles, fishing gear or plastic film can disintegrate into multiple smaller items over long timescales, and even if all inputs of plastic to aquatic environments were to cease with immediate effect, an increase in microplastic particles would still exist as a result of the fragmentation of large plastic litter already present in the environment.
A 2017 study documented that as a result of slow degradation rates of plastics and the lake’s long hydraulic residence time, some microplastics that exist in Lake Erie are likely fragments from some of the very first plastic products that entered the consumer market .
• Microplastics that come from washing clothes are mainly polyester, acrylic and polyamide [35,23].
A study reports maximum fiber loads released from washing of clothes as >700,000 fibers from a 6 kg wash of acrylic clothes .
- • Plastic films used for crop production are considered to be one of the most important sources of plastic contamination of agricultural soils [37-39].
- • A wide assortment of microplastics includes particles from plastic mulching, abrasion from car tires or abrasion of synthetic paints . While these plastics may be defined as secondary source microplastics, they are still different from particles that arise owing to environmental weathering, and so, they can safely be categorized separately.