Classification of Fragmented Plastics

Fragmented plastics are usually classified into microplastics and nanoplastics depending on their size.

Microplastics

Microplastics are fragments of synthetic polymers having the upper limit of 5 mm. There is no official lower limit of size. They usually have variable shapes but mostly present as fibers. Their chemical composition, color, density and other characteristics are also variable [3,4]. Microplastics are further classified into primary and secondary forms.

Primary microplastics are usually manufactured in industries as scrubbers and are used to blast clean surfaces. They are also manufactured in the form of plastic powders and used in molding and as microbeads in cosmetic formulations [5,6]. Secondary microplastics are the predominant form which are formed by the fragmentation of plastic debris in the oceans. This fragmentation process occurs owing to the exposure of plastics to ultraviolet radiation and by physical abrasion. They can originate from both sea and land-based sources. Land sources of microplastics include packaging materials, polyethylene bags and industrial wastes. Sea sources are mainly the sewage released from humans, industries, ships, fishing materials and equipment. Biofouling of these plastic fragments make them to sink to the depth of the sea floor. Microplastics from terrestrial sources consist of wastes including personal-care products such as cleaning agents, toothpastes, textile fibers. They get transported to the sewage system. The current sewage system is not as potent to clean

Secondary microplastic emission by different sources

FIGURE 6.1 Secondary microplastic emission by different sources.

these fragmented plastic forms, and thus, they eventually get deposited into the marine ecosystem [6,7].

Zooplankton, invertebrates and vertebrates are exposed to microplastics through biomagnification. As per an estimate [7], the amount of secondary microplastic emission to the marine ecosystem falls between 68,5000 and 275,000 tonnes per year. These data can be further classified into coastal, inland and marine emission (Figure 6.1). Among all polymers, the most commonly found are polyethylene, polystyrene and polypropylene. Additives of both organic and inorganic nature such as phthalates, bisphenol A and alkylphenols and inorganic additives such as barium, sulfur, titanium dioxide and zinc make up to 4% of the weight of plastics [8,9].

Microplastics can travel long distances from their place of origin through air currents and get deposited on land and water. Although their impact on human and marine life is obvious, there is no ecological or toxicological threat identified so far from the chemical and physical composition of microplastics. Although indirect threats are quite evident, microplastics are usually hydrophobic in nature [3] and thus adsorb harmful organic and organochlorine pesticides such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and dichlorodiphenyltrichloroethane (DDT). In the food chain, these microplastics are bioaccumulated.

Nanoplastics

As per EFSA, nanoplastics measure from 0.001 to 0.1 pm (i.e., 1-100 nm). Different theories are present about the formation of nanoplastics. It is assumed that fragmentation of plastic debris over the years may lead to formation of nanoparticles [10,11]. In an experiment conducted by Lambert and Wagner, nanoplastics were formed by degradation of polystyrene disposable coffee cup lids [12]. Microbial degradation is also expected to play its role in hydrocarbon degrading, as several such microorganisms have been identified. They used to thrive in the plastic debris environment in the oceans [13]. Continuous fragmentation of microplastics into nanoplastics also occurs in nature [14]. Industrial nanoplastics also accumulate in oceans as effluents.

 
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