## 1 Introduction  
One of the most concerning problems of global pollution is marine debris containing plastic [1]. A total of 370 million tonnes of plastic waste were produced in 2019 [2], and it is estimated that this number will double by the year 2040 [3]. It is estimated that 12 million tonnes of plastic waste, about 3% of the total amount generated, is discarded into the ocean [4, 5]. Plastics represent between 60-80% of the total amount of marine debris [6], of which an estimated 20% comes from marine sources [7] such as buoys and fishing fleets (including nets, lines and abandoned vessels). The remaining 80% comes from land-based sources, where the most common waste is plastic bags, bottles and packaging [8]. Furthermore, primary microplastics (plastics discharged directly into the environment in small particles) are frequently found in freshwater lakes and rivers; these include scrub particles from cosmetic products, fibers released from synthetic textiles during the washing process, abrasives used in water-blasting for cleaning, and powders for injection molding [9]. This huge amount of plastic is disposed of in different environments: on the ocean surface [10], on plastic islands [11], in the deep ocean [12], on shores [13] and even in Arctic sea ice and seawater [14]. Researchers address the marine plastic waste problem from different points of view. Some focus their research on the origins of plastic waste, such as industrial activities [6], maritime industries [15], etc. A review of methods for characterization and identification of the type of plastic found in marine litter can be found in the literature [16], where the most relevant are spectroscopic techniques [17]. A significant amount of plastic waste is not properly managed, either because of its urban waste nature [5, 18] or because it is discharged into rivers [19]. The rapid accumulation of plastic waste stored in landfills that are not managed can lead to the contamination of water sources, which eventually reaches the marine environment [18]. Between 1.15 and 2.41 million tonnes of plastic waste flows out of rivers, mainly as a result of poor waste management, among other factors [19; 20]. Other studies focus on marine debris depending on plastic size, including megal plastics [11], macroplastics [21; 22], mesoplastics [23], microplastics [24; 25] or nanoplastics [26]. A common definition of plastic size ranges is as follows: mega (>1 m), macro (<1 m), meso (<2.5 cm), micro (<5 mm) and nanoplastic (<100 nm). The valorization of marine plastics is also a relevant research issue, with different approaches proposed; these include incineration and energy recovery [27; 28], landfilling [29] and reuse/recycling [30; 31]. This is consistent with plastic generation in Europe in 2019: 57.9 million tons of plastic were generated, of which 29.1 million tons were collected for treatment. These materials were placed in landfills (24.9%), used in energy recovery techniques (42.6%) and recycled (32.5%) [2]. Ocean plastic waste has also become a hot topic in the media, public opinion and policy strategies. Several organizations have released new policies and recommendations to reduce the global impact of plastic on the environment. A review of these policies can be found in [32; 33], which highlights the reduction in consumption, greater collection of plastic, regulation of the entire plastic value chain, and recycling. Recycling is one of the most essential actions to avoid increases in environmental pollution [34]. Recycling involves different agents throughout the whole management chain for the recycling of marine plastic waste, including collection, sorting, cleaning, volume reduction and separation [35]. Within the European Union (EU) framework, there has been promotion of a large number of measures and projects that revolve around the reduction of plastic waste in the marine environment. In 2018, the European Commission released a new strategy to change the way plastic products are designed, used, produced and recycled in the EU [36].