Introduction to environmental problems caused by plastics

The production of plastic has increased dramatically from around 0.35 million tons in the 1950s to 359 million tons in the year 2018 (Govender et al., 2020) . This production rate included majority of the plastic being non reusable. Since this era of the global pandemic is demanding more plastic to produce personal protection equipment (PPE), packaging and medical tools, a huge rise in plastic production is expected (Govender et al., 2020). The rate of plastic entering the environment is higher than the recovery rate therein, coastal, and oceanic systems are suffering from a rapid plastic pollution. Consequently, the probability of microplastic pollution is also increasing in these sites (Govender et al., 2020). Freshwater bodies play a critical role in the transitioning of microplastic from land to sea as they are potential pathways as well as potential hotspots when surrounded by human residency. Factors promoting the plastic accumulation in freshwater bodies such as lakes are: vicinity to human population, water residence time and stagnant water conditions (Iannilli et al., 2020). Majority of studies have been done in the microplastic pollution in coastal and marine environment while research on fresh water is still limited (Iannilli et al., 2020).

2.1.1 Definition and history of plastic/microplastic

The development of rubber technology after 1800s is the primary cause of excessive usage of plastic materials today (Thompson et al., 2009). Bakelite, the very first synthetic polymer, was introduced by a Belgian chemist namely Leo Baekeland in 1907. In the upcoming decades, many more plastic were also produced. Later, after the 1940s and 1950s, the actual mass production of plastic used in everyday life started. Development of polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyurethane (PUR), and polystyrene (PS) occurred in 1930s.

1950s is considered as the era of development of high-density polyethylene (HDPE) and polypropylene (PP). Synthetic plastic materials were produced without using natural resources in 1960s due to the advancement in material sciences (Thompson et al., 2009; Wagner and Lambert, 2018).

World War Ⅱ accelerated the production of plastic and in 1950s annual production was about 5 million tonnes (Napper and Thompson, 2020). The properties of plastic such as light weight, strong, durability, corrosion-resistant, and cheap production cost further promote the production of plastic. Hence, the production of plastic boosted from 30 million tonnes in 1998 to 359

million tonnes in 2018 (Napper and Thompson, 2020). Plastic production will surge in upcoming years due to its benefits and imagining of modern society is not easy. (Napper and Thompson, 2020; Dong et al., 2020).

Smaller pieces of plastic having diameter less than 5mm are termed as microplastics, however globally accepted definition for microplastic does not exist yet (Pan et al., 2020; Hartmann et al., 2019). According to Hartmann et al. (2019) the size of nanoplastics ranges from 1 to <1000 nm, microplastics ranges from 1 to >1000 µm, mesoplastics ranges from 1 to <10 mm, and macroplastics is larger than 1cm. Presence of microscale particles were identified in marine ecosystem in 1970s and plethora of studies have revealed the occurrence of microplastics in marine system since then (Li et al., 2020). Thompson et al. (2004) first introduced the term

“microplastic” and research on microplastic in fresh water bodies was first initiated by (Zbyszewski and Corcoran, 2011). To date, studies on microplastics in freshwater bodies has been done by not more than 23 countries, with the majority of them being in North America and Europe as well as China, India, Mongolia and other countries. (Ma et al., 2019).

2.1.2 Chemical nature

Fossil fuel is mainly used for the synthesis of plastic, but biomass is sometimes used as feedstock (Andrady and Neal, 2009). The most prevalent plastic materials, polypropylene (PP), Polyethylene (PE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polystyrene (PS) altogether make around 90% of overall production of plastic in the world (Andrady and Neal, 2009). These polymers contain high molecular weight and are nonbiodegradable. Therefore, they are also consistently existing in the environment. The usage of these resins; PE, PP, PVC, PS, PET, and PUR is 29, 19, 12, 8, 6, and 7% respectively in the global production (Rani et al., 2015). Nearly all plastics contains carbon and hydrogen as their key component while PVC also contains chloride as main part with carbon and hydrogen. A large variety of additives such as thermal stabilizers, inorganic fillers, plasticizers, UV stabilizers, and fire retardants are also added to plastics to increase their performance (Andrady and Neal, 2009).

2.1.3 Sources of microplastic

Microplastics enter the environment through different sources. Primary and secondary microplastics are two categories of microplastics (Horton et al., 2017). Microplastics which are intentionally produced in micrometre size such as microbeads and pellets are known as primary microplastics. Microbeads are manufactured to use them in personal care products such as

exfoliating scrubs, lotions, toothpaste while pellets are used by plastic industries to produce plastic goods. Secondary microplastics are formed through the degradation of large pieces of the plastic due to the environmental conditions and abrasion of synthetic fibres during laundry (Horton et al., 2017). According to Browne et al. (2011) estimation, one washing cycle can release more than 1900 fibres.

The major mechanisms behind the formation of microplastic are mechanical, chemical, biological and UV degradation of larger fragments (Horton and Dixon, 2018). There are numerous routes for the entrance of microplastics to the environment and they can be different for different regions. For instance, wealthy regions may experience more primary microplastic from personal care products than less wealthy region (Wu, Zhang and Xiong, 2018).

Wastewater treatment plants (WWTPs) are considered as a major pathway for the transport of microplastics since microbeads and synthetic fibres are released into them. Sludge from wastewater treatment plants can also be the source of microplastic pollution in aquatic system through runoff when applied to the agricultural land (Horton et al., 2017). Other sources for microplastic entrance include overflow of sewage system due to heavy rain, abrasion from tyres, and land litter (Eriksen et al., 2013; Andrady, 2011).

2.1.4 Degradation of plastics

Degradation is the chemical transformation of the structure of polymers by decreasing their molecular weight (Andrady, 2011). The most important feature of synthetic polymers is their strong resistance to environmental conditions which increases their residence time and minimize degradation in the environment. Conversion of polymers into smaller molecular units such as oligomers and monomers occur during the degradation process. Degradation of synthetic polymers can be categorised into two types: biotic and abiotic. (Eubeler et al., 2009).

UV-B radiation from the sun is the primary cause of Low-density polyethylene (LDPE), high-density polyethylene (HDPE), PP, and nylons photo oxidative degradation. This degradation process can be further carried on by thermo-oxidation (Andrady, 2011). Extensive degradation causes the embrittlement of the plastics, which results in micro and nano sized plastics. These micro and nano sized fragments further undergo degradation by microorganisms. Polymers containing carbon can be converted into carbon dioxide and integrated into biomass by microorganisms during biodegradation (Anderson, Park and Palace, 2016). Estimated time needed for complete mineralization of plastic is from hundred to thousand years (Barnes et al.,

2009). Plastic degradation is slower in aquatic than terrestrial systems due to the less exposure to UV radiations and oxygen (Corcoran, Biesinger and Grifi, 2009).

Many factors which affect the degradation of microplastics are environmental conditions, polymer properties (density and crystallinity), and the type and number of additivities added to polymers. Crystallinity of polymers is a key feature which influence degradation by affecting the permeability and density of polymers. The addition of additives for instance, antioxidant and antimicrobial agents extends the plastic life. On the other-hand, biological ingredients enhance microplastics degradation (Wagner and Lambert, 2018).