Читать книгу Plastic and Microplastic in the Environment - Группа авторов - Страница 46

Plastic or synthetic polymers are artificially made from petroleum products, and due to their versatile nature of being lightweight, strong, durable, transparent, and waterproof they are now part of everything we human beings use. Thus, they are widely distributed to the entire segment of human life, from waking up in the morning until going to bed at night; modern humans are surrounded by plastic. Plastics have proliferated into food packaging industries; stationary, electric, and electronic goods; vehicles, private and public transport; medical appliances; fishing nets and more. In today's era, one cannot imagine a day without plastic. Toothpastes, shaving creams, and soaps all have synthetic polymers in the form of microbeads (Sun et al. 2020). Plastics are highly resistant to microbial degradation as they are of artificial (human‐made) petroleum‐based products; therefore, their remediation by natural processes is difficult enough and takes a long time that can vary in the range of hundreds of years (Wierckx et al. 2018). Due to chemical (acid rain) and physical (temperature, pressure, moisture) processes, they degrade and break down into smaller fragments and, in smaller forms which do not degrade completely and pose negative impacts on the environment, these smaller forms have high potential to enter into the water or air matrices in invisible forms. MPs have a size ranging from 100 nm to 5 mm (Zhang et al. 2020). The microbeads and nanoparticles that are intentionally added into facewash, shaving creams, soaps, etc. are termed “primary” MPs; however, those which are added to the environment after the fragmentation of larger size plastic particles by impact of natural phenomenon, as well as anthropogenic activities in the environment, are considered “secondary” MPs (Lei et al. 2017). Due to poor waste management practices and lack of knowledge about proper disposal of such wastes among the consumers of such plastic‐based products, plastic debris is dumped directly into the ocean every year in huge quantities. Varying sizes of plastic particles, including larger and smaller beads of MPs, ultimately reach the water column of the ocean, and their presence is significantly reported in sediments (Harris 2020). They are found on the deep floor of the sea, in underground water tables, and in soils. Global plastic production has significantly boomed from 300 to 360 million metric tons in the last five years (Deccan Herald, 2020). As they are highly resistant to microbial degradation, they persist for a longer time in the ecosystem. MPs are a matter of great concern because of their high potential to make any organism unfit, as they have huge impacts on their metabolism.

According to Boucher & Friot (2017), approximately 1.53 million tons/year of primary MPs enter the ocean via different pathways. These pathways include flushed water from our washroom's containing the MPs in microbead form from face scrubs, toothpaste, detergents, facewash, shampoo, cosmetic cream, etc., which goes into rivers through the drainage systems, and later become part of the ocean, as sewage treatment plants are not made for such efficiency in developing countries like India. MPs are extensively distributed throughout our ocean ecosystem; from zooplankton, bivalves, crustacean, fish, and seabirds, and ultimately reach humans due to their extensive consumption and dependency on seafood (Santillo et al. 2017). In aquatic organisms, the uptake pathways of MPs are through their gills and gastrointestinal tracts (Franzellitti et al. 2019). The organisms are often confused as these colorful MP fragments look similar to plankton species on which they feed, and thus a significant portion of MPs reach and are accumulated in consumer organisms (Setälä et al. 2014). Sometimes these MPs are deposited on seaweed or algal blooms, and become part of the food to the organisms, where they enter the gastrointestinal tracts of organisms (Walkinshaw et al. 2020). Once these MPs are mistakenly consumed by smaller organisms in confusion of phytoplanktons, they make their way to successive trophic levels, as predators consume prey already having MPs in their guts. Polymers of rope, usually old fishing gear left deliberately or mistakenly in the ocean, can entangle marine creatures, suffocating them by restricting their mobility and unintentional killing them; this is referred to as “ghost fishing” (Gilman 2015). These polymers may also undergo reduction in size due to natural forces acting on them in the ocean, such as waves, water temperature, contact with other floating debris, or larger marine creatures nibbling the pieces into smaller fragments, which then contributes to MPs. Recent studies show the presence of MPs of an array of shapes and sizes in various organs of different organisms such as gills, liver, gut, muscles, etc. They cause disturbances in processes of metamorphosis, metabolism disorder, behavioral change, oxidative stress, genotoxicity, etc. (Rahman et al. 2021). This is becoming a food safety threat as these organisms are heavily consumed as food by humans as seafood, and contribute to a significant enough proportion of the daily diet of people in coastal areas; for example, India has a huge coastline of 7516.6 km.

MPs have a high density, and become settled on the ocean floor, both after entering into the oceans and through the feces of organisms. There it causes oxygen‐deficient conditions like anoxia and hypoxia, in which there is less availability of oxygen and nutrients. In this way, it causes harm to seaweed, corals, and planktons (Seeley et al. 2020). Deposition of layers of MPs on body surfaces of corals (in the tropics) cause their degeneration by lowering the absorption of essential nutrients from the surrounding environment.

However, plastics play an important role in food safety and security by providing safe and durable packaging, and is a great contribution to the pharmaceutical and medical industries for packing medicine and providing disposal medical equipment (Hui et al. 2020). Due to a lack of proper waste management, over 250 000 tons of plastic pieces are dumped into the oceans (Hahladakis 2020). Larger plastics sizes are undergoing slow degradation by integrated physical–chemical and biological processes. They mainly degrade due to photo‐ and thermo‐oxidative processes (Mierzwa‐Hersztek et al. 2019).

MPs are generally found in the form of pellets, fragments, or fibers. Some of them are denser than seawater and settle at the seafloor like polyamide, polyester, polyvinyl chloride (PVC), and acyclic, etc. In contrast, those found throughout the water column and floating on the sea surfaces are a lighter density than the sea surface, e.g. polyethylene, polypropylene, polystyrene (Hidalgo‐Ruz et al. 2012). Although plastic is treated as non‐toxic because of its less reactive nature (Hwang et al. 2020).