**1. Introduction**

Plastics are a versatilely and controversially discussed topic. Due to the many advantages of the various materials concerning durability, low-costs in production, light weight and the possibility to fulfil numerous requirements (formability, heat resistance, insulation, etc.) it is ubiquitous [1]. Historic production is estimated at 8300 million metric tons (Mt) of virgin plastics with a waste generated of 6300 Mt [2].

While the materials themselves have a high added value, the problem of their material characteristics arises when getting into the environment. Here, there are different ways of entry which can be divided into land-based including river transport and ocean-based including oil and gas platforms [3]. Causes are littering, industries, natural storm events, wear and tear and defective waste management. The plastic leakage of land-based, mismanaged waste which is entering the oceans from coastlines is calculated to an annual

**Citation:** Hee, J.; Schlögel, K.; Lechthaler, S.; Plaster, J.; Bitter, K.; Blank, L.M.; Quicker, P. Comparative Analysis of the Behaviour of Marine Litter in Thermochemical Waste Treatment Processes. *Processes* **2021**, *9*, 13.

https://dx.doi.org/10.3390/pr9010013

Received: 30 November 2020 Accepted: 18 December 2020 Published: 23 December 2020

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amount of 4.8 to 12.7 Mt taking into account solid waste, economic status, population density and coastal access [4]. After entering the environment, plastics are transported mainly in riverine systems, temporarily accumulate on sediments, in soils and on water surfaces and eventually deposit on the sea floor as a final sink [3].

Based on an oceanographic model which was additionally calibrated with data from numerous expeditions, Eriksen et al. [5] estimated the total count of plastic pieces and its weight floating in the oceans worldwide (Table 1). Thereby, the results showed a total count of 525.0 × <sup>10</sup><sup>10</sup> plastic particles with a total weight of 2689.4 × <sup>10</sup><sup>2</sup> tons. The results do not consider the plastics that sank to the ground.

**Table 1.** Estimated count of plastic pieces (all size classes) and its weight floating in the oceans worldwide: North Pacific (NP), North Atlantic (NA), South Pacific (SP), South Atlantic (SA), Indian Ocean (IO), Mediterranean Sea (MED) (data from Eriksen et al. [5]).


Research on plastic in the environment started in 1972 [6]. Since then, many terms and definitions have been used describing this contamination focusing different aspects, such as size (nano-, micro-, meso- and macroplastic, macro litter), shape (plastic debris) or origin (anthropogenic litter, marine litter, marine plastic, plastic litter) [3]. These different terms make it particularly difficult to compare results of already published studies and thus to derive further prevention and waste treatment strategies.

A frequently used term also applied on a political level (United Nations, European Commission, Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection) is 'marine litter' [7–9]. According to the United Nations Environment Programme marine litter is defined as 'any persistent, manufactured or processed solid material discarded, disposed of or abandoned in the marine and coastal environment' and thus includes not only plastics but also other possible waste fractions and emphasises the heterogeneity of the material [7].

The ocean surface can be seen as temporary sink within the life cycle of marine litter, as there are already different technical solutions to collect these waste items: extraction by vacuum, skimmer, air barrier, sand filters, drones and robots, waterway litter traps, detection aids, boats and wheels, river booms, large-scale booms and miscellaneous capture [10]. After the collection of marine litter, a further application in terms of recycling is required. In general, there are different treatment possibilities for waste plastics (Figure 1).

Although these methods are proven as treatment processes for plastics, especially the recycling routes are mainly restricted to pre-processed, sorted and separated polymer fractions. Considering marine litter in particular, additional points of conflict occur since the material is not only very heterogenous but also strongly weathered by various environmental influences. These external influences include mechanical degradation based on wave movement and sandy shores as well as degradation due to UV radiation, oxidation and the general process of biofouling [11]. Leaching of chemical additives (e.g., plasticisers) within the plastic fraction due to external influences additionally changes the material properties [12].

There are no standardised treatment processes for marine litter to date caused by missing recovery and recycling strategies. Furthermore, research on the applicability of treatment and recycling processes for marine litter is still lacking. This study is the first to analyse thermochemical waste treatment processes in application for marine litter comparing the methods of pyrolysis, gasification and incineration on a laboratory scale (Figure 1).

**Figure 1.** Life cycle of plastics with focus on the investigations of this study. (Figure: Peter Quicker) In terms of environmental input, the figure does not represent the entire life cycle of plastics, which is shown in Reference [3].

## **2. Materials and Methods**

The material used in this study consists of three batches. The batches one and two were procured by Pacific Garbage Screening and originate from the North Sea where it was collected on Sylt and Norderney (Table 2). The sampling sites are shown in Figure 2. The materials were washed up on the beach and due to their location, origin and appearance the materials meet categorisation criteria for marine litter (ML). Manual collection was carried out randomly. Therefore, the material cannot be considered representative for the total waste load of the respective beach. The third batch used as reference material was commercial mulch foil for agricultural usage purchased from a hardware store.


**Table 2.** Material characteristics of the two batches of marine litter and mulch foil.

**Figure 2.** Overview map of the sampling sites (marked black). (**A**) Norderney: the material was samples within a radius of approx. 250 m in a chain of dunes south of the "Schlopp" (garbage hotspot), (**B**) Sylt: the sampled section reached from "List Weststrand Strandsauna" to "List Westfeuer am Ellenbogen" and additional material was sampled at "List Oststrand Wattenmeer." (Figure: Simone Lechthaler, Johann Hee).
