1. Introduction

Currently, the massive population expansion and the daily use of polymers for producing and consuming non-reusable objects for different applications (packaging, cosmetics, textiles, detergents, greenhouses, mulches, fishing nets, coating and wiring, trays and bottles, covers, bags, and containers) cause wild waste accumulation, with consequent significant complications owing to its management and disposal [1,2,3,4]. In specific, the municipal solid waste worldly production passed from 1.3 billions of tons in 1990 to 3.81 billions tons after 25 years [5,6]. Even if the waste flow comes from different sources [7,8], plastics represent a substantial portion of the municipal solid waste. In 2016, about 27.1 million metric tons (Mt) of plastic litters were stored in the European Union (EU), of which 31.1%, 41.6%, and 27.3%, were recycled, reused (for energy production), and dumped again in landfill sites, respectively [9]. Among polymer materials, the greatest contribution is provided by thermoplastic polymers, whose consumption (about 80% of all synthetic polymers) is mostly attributable to packaging and containers, as well as the production of textile fibers [10]. Hence, plastics can be considered highly responsible for waste management issues, not only because of their extensive usage but also because of their short service life together with their long (bio)degradation time [11].

In addition, a great universal worry is due to the plastics’ storage in landfills because of their easy accessibility in the environment. In particular, mismanaged plastic waste of polyethylene containers and Poly(Ethylene terephthalate) bottles of beverages, the most common polymers found in urban waste, leads to a huge amount of surface water and seabed marine litter [12].

Poly(Ethylene terephthalate), generally labeled as PET, is a thermoplastic polymer of the polyesters family produced by the reaction of ethylene glycol and terephthalic acid under high temperatures and low vacuum pressure. The resulting polyester polymer is characterized by high strength and stiffness, low density, good creep behavior, high chemical resistance, and low cost [13].

Today, PET is one of the world’s most commonly used and versatile materials. Global Poly(Ethylene terephthalate) production was 30.3 million tons in 2017 [14], while European PET demand was about 4 million tons in 2018, as compared to the global plastics production of nearly 360 million tons [9]. It is used for bottles, food containers, and synthetic fiber production. It is forecasted that 583 billion PET bottles will be produced in 2021 [15], and for this reason it could be considered one of the most responsible polymers in marine pollution [2]. PET is used not only in the food packaging and textile fields but also in agriculture, electrical applications, and several composite applications in combination with reinforcement fibers for various industrial and civil engineering applications that typically require higher strength and/or higher heat resistance [16,17,18]. Recently, the interest in fiber-reinforced PET has increased due to its benefits, as compared with thermoset composites, such as damage tolerance, high impact resistance, chemical and solvent resistance, unlimited shelf life, low storage costs, welding ability, and recyclability [19,20,21].

Plastic pollution in the marine environment has recently been recognized as one of the most impacting threats for the environment, causing numerous hazardous and ecologically negative consequences, such as the entanglement of the marine species within the plastic or their ingestion [22,23]. In particular, juvenile fish, reptiles (i.e., turtles, etc.), and mammals often become entangled in plastic waste with consequent severe damage for the animal growth [24,25] and restriction of movement precluding them from correctly feeding and, in the case of mammals, breathing [26,27]. A wide variety of species have been reported to be harmfully crushed by plastic trash, such as for example marine birds [28,29], sea turtles [30], cetaceans [31], fur seals [24], sharks [25], and filter feeders [32]. Marine birds are very prone to the ingestion of plastic objects that they mistake for food [28,29]. Plastic ingested by these marine organisms remains in the digestive tract and can lead to reduced feeding stimuli, gastrointestinal obstruction, decreased secretion of gastric enzymes, and lower levels of steroid hormones, causing reproduction difficulties [5]. Specific classes of litter found in the oceans, involving the Antarctic [33], have been observed in the sea for at least four decades [34,35,36].

Microplastics (MPs) are generally defined as polymer particles with a regular or irregular shape and a size ranging between 5 mm and 1 µm and are insoluble in water [37], while bigger particles, such as pellets, are called mesoplastics [26,38,39]. However, a clear and accepted terminology and classification is still under discussion, as well as a standardization of the plastic collection and analysis methods [40]. Microfibers (MFs), very fine fibres (approx. 3–10 μm in diameter), spun as endless filaments can be of both synthetic and natural origin. The size to diameter ratio is also quite high, on the order of 10³, which is an additional crucial property of MFs [41]. The most common constituents of MPs include polyethylene (PE), polystyrene (PS), polyethylene terephthalate (PET), polyvinylchloride (PVC), and polypropylene (PP), [34,42,43]. MPs generally arise from the plastic pollution of seaside and beaches, deriving from fragmentation phenomena or from powders employed, for example, in cosmetics [44,45]. Both microplastic and mesoplastic litters can be eaten by marine species and, thus, can reach the marine food network. In contrast to macroscopic plastic litters, MPs on the seaside, seabed, or surface water, frequently combined with sand, are complicated to be stored and, at present, there is not an easy and universal method for the calculation of their amount [46]. Furthermore, the degradation of marine MPs due to prolonged external light exposure, mechanical abrasion, and biodegradation can cause the creation of nanoplastics (NPs) with sizes lower than 1 μm [47,48,49,50,51]. In particular, marine MPs were investigated by several researchers, and their presence has widely been proven in coastal environments [52,53,54]. The freshwater system is also considered a potential sink of MPs [55,56,57,58,59]. Zbyszewski and Corcoran [60] reported for the first time the presence of MPs in the freshwater system during the coastline of Lake Huron, Canada. Very recently, Li et al. [61] evidenced that there are different concentrations of MPs in Australia, Asia, North America, and Europe. The current literature underlines that MPs are found in every sea basin around the world, with higher concentrations occurring in intense human activity areas demonstrating that plastic debris transport can be extremely efficient and that the prediction of the plastics’ fate is of paramount importance [62]. Additionally, the study and modeling of the transport of MPs in the marine environment has attracting increasing interest [63]. MPs have been detected also in urban atmospheres as well as in remote and pristine environments, showing that atmospheric transport of MPs is also very significant [64,65].

This entry is adapted from the peer-reviewed paper 10.3390/jmse9040445