Basics Marine biodegradable plastics A solution option to the Marine Litter problem? Marine litter is an insidious form of environmental pollution. Currently, more than 150 million tonnes of plastic garbage pollute the oceans, with between an estimated 4.8 to 12.7 million tonnes added each year [1]. 15 % of marine trash floats on the surface, 70 % lands on the sea bottom and another 15 % eventually reaches the coast [2]. Marine litter has different sources. About half of all ocean waste plastic consists of single-use plastic items such as bottles, lids, cigarette filters, toiletries, plastic bags, disposable tableware, cups, and food packaging. These mainly originate from land-based sources. A study in Lower Saxony, Germany concludes that 36 % of marine waste comes from fishing and 7 % from shipping [3]. Coastal and offshore aquaculture and fisheries also suffer material losses due to, among others, storms. The lost materials include net and rope balls, fish traps, net bags, fish boxes, buoyancy aids and adhesive tapes. The durability of conventional plastics, which offers such advantages during the life of these products, becomes an ecological disadvantage at the end of these same products’ useful life, as most plastics are not degradable and therefore able to cause substantial ecological problems. There are various different approaches which can be used to reduce marine litter. The potential countermeasures can be divided into primary and secondary measures. Primary measures include directly preventing plastic from entering the marine environment through the further improvement of waste and wastewater management, in combination with the creation of knowledge and awareness of the need for the careful handling of products and the resulting waste. However, completely eliminating the input, especially from certain product groups for which loss to the environment during or after use is intrinsic or very likely to occur, is infeasible. Therefore, in addition to primary measures, appropriate secondary measures are also required to combat marine litter. In the research project MabiKu [4], the IfBB and partners are working to identify critical products that have a high loss rate or where loss is difficult to avoid. The primary goal is to further develop bio-based plastics that degrade under marine conditions and fulfil the required functionalities (Fig. 1). These critical products include items from aquaculture and fisheries, such as feeding pipes, fish boxes and other fishing gear. To assess the capability of substituting persistent plastics with biodegradable ones, their marine degradability must be investigated under standardised conditions. This presents a number of challenges. Although the investigation and certification of the industrial compostability of bioplastics is state of the art, the methods applied are not transferrable to marine degradation, since the environmental conditions are completely different. First of all, the temperature is, in general, lower (from below 0°C up to 32°C) than the over 60°C in an industrial composting facility. This means that plastics that are certified as industrial compostable according to DIN EN 13432, such as PLA, are not implicitly biodegradable under marine conditions. Furthermore, the ocean is much more diverse in terms of light, sediment contact and varying temperatures with season, depth and latitude. Thus, depending on where the plastic item ends up, the microorganism composition differs strongly from one place to the other. Predicting the biodegradability of materials in the marine environment is therefore difficult. To approach these challenges, tests are conducted at three different scales: at lab scale (in bottles), mesocosm scale and in field tests (Fig. 2). The final degradation to the end product - CO 2 - can be analysed and verified only in closed bottles. However, the volume of a bottle is small, which means that only a small part of the environment can be represented. Hence, mesocosms can be used to model a more realistic but still controlled environment, in which e.g. the mechanical changes during degradation can be monitored. In the field the full spectrum of the environment is present with the influence of hydrodynamics, UVradiation and all present organisms. Assessing degradation quantitatively in the field remains a bottleneck. A reduction in weight is difficult to assess due to the biofilm growing on the plastic item, which falsifies the result. That’s the reason why in the current project further measurement techniques are being developed to quantify biodegradation in the field. Furthermore, different environments should be tested such as the benthic zone (contact with sediment) and the pelagic zone (the open ocean). This is reflected in the two teststandards: ISO 18830 and ASTM D6691-17, respectively. Whether or not toxic by-products emerge is dependent on the composition of the utilized plastic and needs to be considered when evaluating the marine degradability. Previous studies have shown that polyhydroxyalkanoate (PHA), some types of cellulose acetate and polycaprolactone (PCL) are degradable under marine conditions. The degradation rate is, besides the monomeric structure of the polymer, influenced by factors such as temperature, habitat and surface/volume ratio of the product and is therefore difficult to generalise. However, Dilkes-Hoffman et al. (2019) have determined a degradation rate of 0.04 - 0.05 mg/day and cm² for PHA in a meta-analysis [5]. A water bottle, for example, would need 1.5 - 3.5 years to degrade completely. Currently, further material development is being conducted to improve the mechanical characteristics and marine degradability of several plastic types to make them suitable for sensible applications. www.ifbb-hannover.de | www.sea-art.org 64 bioplastics MAGAZINE [03/20] Vol. 15
Basics By: Carmen Arndt and Andrea Siebert-Raths Institute for Bioplastic and Biocomposites (IfBB) Hannover, Germany References [1] European Parliament (2018), https://www.europarl.europa.eu/news/ de/headlines/society/20181005STO15110/plastik-im-meer-faktenauswirkungen-und-neue-eu-regeln [2] NABU (2016). Müllkippe Meer. Plastik und seine tödlichen Folgen. https://www.nabu.de/imperia/md/content/nabude/naturschutz/ meeresschutz/nabu-broschuere_muellkippe_meer.pdf [3] Dau, K.; Millat G.; Brandt, T.; Möllmann, N. (2014). Pilotprojekt Fishing for Litter in Niedersachsen. Final report 2013 – 2014 [4] The MabiKu-project is funded by the Federal Ministry of Food and Agriculture (BMEL) via the project management agency Fachagentur Nachwachsende Rohstoffe e. V. (FNR). [5] Dilkes-Hoffman, L. S.; Lant, P. A.; Laycock, B.; Pratt, S. (2019): The rate of biodegradation of PHA bioplastics in the marine environment: A meta-study. In: Mar. Pollut. Bull. 142, p. 15–24. A. Lab-scale in a bottle to test for final biodegradation to CO 2 Material development B. Mesocosm scale to survey mechanical characteristics under more realistic conditions Marine biological degradation H 2 O Enzymes Bacteria CO 2 C. Field-scale with the full spectrum of the environment to verify and correlate the results with the other two test-scales Critical products Bacterial cell mass Method development bioplastics MAGAZINE [03/20] Vol. 15 65
