*2.2. ML Sylt*

In the ML Sylt sample, the nets fraction made up the highest proportion with 55 wt% (Table 3). As shown in Figure 3D, the nets consisted of different types of plastic such as polypropylene (PP) and high-density polyethylene (HDPE). 3D plastics made up 15 wt% of the total weight. In contrast to ML Norderney, there were no bottles within the sampled material. This fraction mostly comprised fragments of boxes, caps and canisters made of polystyrene (PS), PP, polyvinyl chloride (PVC), HDPE and acrylonitrile butadiene styrene (ABS) (Figure 3A). 8 wt% were rubber and elastomers, like the glove shown in Figure 3E and 6 wt% films consisting of PP, PVC, PA, HDPE and PS. Foamed plastics made up 5 wt% of the total weight including polyethylene terephthalate (PET), PS, HDPE and polyurethane (PUR) foams. Non-polymer materials of the sample made up 9 wt% and consisted of glass, textiles, bones or feathers. Metals, like the spray can shown in Figure 3C, represented 2 wt% of the sample.

**Figure 3.** ML fractions Sylt: (**A**) 3D Plastics; (**B**) Films; (**C**) Metals; (**D**) Nets; (**E**) Rubbers & Elastomers; (**F**) Foamed Plastics and (**G**) Others. The scale amounts to 1 cm per section (Picture: Jacqueline Plaster, Johann Hee).

### *2.3. ML Norderney*

For the ML Norderney sample, the 3D fraction made up the highest proportion with 88 wt%, comprised of plastic bottles (Table 3 and Figure 4A). The materials from this fraction consist mainly of PET, PP and HPDE but also PS and PUR. Films made up 10 wt% of the total quantity including PVC, HDPE, PS, PP, ABS (Figure 4). The remaining portion (1 wt%) consisted of metals, others (1 wt%) and foamed plastics with less than 1 wt% in relation to the total weight. In contrast to the material from Sylt, no nets were found.

**Figure 4.** ML fractions Norderney: (**A**) 3D Plastics; (**B**) Films; (**C**) Metals; (**D**) Foamed Plastics and (**E**) Others. The scale amounts to 1 cm per section (Picture: Jacqueline Plaster, Johann Hee).

### *2.4. Selection of A Comparative Material*

For the thermochemical conversion experiments investigating the behaviour of ML in thermal waste treatment processes, mulch foil was chosen as a reference material for the two ML fractions. Mulch foil is often used in agriculture for vegetable production to enhance the soil temperature and to protect crops from unfavourable growing conditions, such as weed growth, evaporation of soil moisture and the spread of soil-borne diseases. The material usually consists of PE from fossil resources which is hardly biodegradable. Due to its benefits and leading to a higher harvest yield the use of mulch foil increased rapidly over the last decades. At seasons end the material requires collection and disposal. Instead, because of wear and tear, photodegradation, lack of waste collection at seasons end and its durability the material continuously accumulates in the environment and thus becomes an increasing contamination [13,14].

Recent developments in recycling processes focus on the biotechnological degradation of PE and subsequent enzymatical conversion of decomposition products into precursor chemicals. The most promising results in precursor yields are currently observed at microbial fermentation experiments on pyrolysis condensate from PE. [15] A possible aim for future so called biotechnological upcycling is to include a pyrolytic pre-treatment to provide access to a suitable carbon source. Consequently, the mulch foils are a suitable reference material for evaluating the behaviour of a homogeneous fraction in thermal waste treatment processes in comparison to heterogenous ML.

### *2.5. Chemical Composition of Material Samples*

For the analytical determination of their chemical composition and thermogravimetric analysis (TGA) trials the respective material fractions were mixed again, shredded and sampled in accordance with LAGA PN 98 [13]. After separation of metallic fractions, the samples were milled to <1 mm grain size.

Table 4 depicts the results of laboratory analyses of the raw materials used in thermochemical conversion trials. The two types of marine litter ML Sylt and ML Norderney show similar results both in proximate and elemental composition. Apart from the ash contents of 13.52 and 10.55 wt%, they mainly consist of volatile matter of 84.6 and 91.0 wt% respectively. Both present a similar relation of carbon and hydrogen while the ML Sylt sample additionally shows significant amounts of nitrogen, sulphur and chlorine as potential pollutant precursors. Comparatively the virgin nature of the mulch foil reference sample becomes apparent, containing no detectable pollutant precursors and negligible amounts of ash. Subsequently it delivers the highest values for C, H and energy content.


**Table 4.** Characteristics of ML fractions and mulch foils (as received) (\* n = 2; \*\* n = 3).
