The performance of the biodegradable plastic materials polyhydroxybutyrate (PHB), polybutylene sebacate (PBSe) and polybutylene sebacate co-terephthalate (PBSeT), and of polyethylene (LDPE) was assessed under marine environmental conditions in a three-tier approach. Biodegradation lab tests (20\u00b0C) were complemented by mesocosm tests (20\u00b0C) with natural sand and seawater and by field tests in the warm-temperate Mediterranean Sea (12\u201330\u00b0C) and in tropical Southeast Asia (29\u00b0C) in three typical coastal scenarios.<\/p>\n
Plastic film samples were exposed in the eulittoral beach, the pelagic open water and the benthic seafloor and their disintegration monitored over time. We used statistical modeling to predict the half-life for each of the materials under the different environmental conditions to render the experimental results numerically comparable across all experimental conditions applied.<\/p>\n
The biodegradation performance of the materials differed by orders of magnitude depending on climate, habitat and material and revealed the impreciseness to generically term a material \u201cmarine biodegradable.\u201d The half-life t0.5 of a film of PHB with 85 \u03bcm thickness ranged from 54 days on the seafloor in SE Asia to 1,247 days in mesocosm pelagic tests. t0.5 for PBSe (25 \u03bcm) ranged from 99 days in benthic SE Asia to 2,614 days in mesocosm benthic tests, and for PBSeT t0.5 ranged from 147 days in the mesocosm eulittoral to 797 days in Mediterranean benthic field tests. For LDPE no biodegradation could be observed. These data can now be used to estimate the persistence of plastic objects should they end up in the marine environments considered here and will help to inform the life cycle (impact) assessment of plastics in the open environment.<\/p>\n
Introduction
\nGlobal plastic production is growing exponentially. It has almost doubled since the beginning of this century (European Bioplastics, 2020) to 400 Mt in 2020 (including fibers) and is estimated to reach 800 Mt in 2050 (Rouch, 2019). Biodegradable polymers are a small, but growing segment in this market with a share of 0.3% (1.227 Mt) in 2020 (European Bioplastics, 2020). The amount of plastic entering the natural environment is augmenting rapidly, accumulating at an exponential rate (Geyer et al., 2017). If introduced to the environment, e.g., as litter, it can be assumed that most items made from biodegradable plastic materials have similar pathways and sinks as conventional non-biodegradable plastic items. Plastic pollution is found almost anywhere in nature it has been looked for, including air (e.g., Dris et al., 2016), high-mountain and polar ice (e.g., Ambrosini et al., 2019; Kanhai et al., 2020), terrestrial soil (e.g., review by Helmberger et al., 2019), freshwater (e.g., Wagner et al., 2014) and marine systems (e.g., Weber et al., 2015; Figure 2 and references therein) with effects on ecosystem level, organism level and on humans still to be fully understood.<\/p>\n
Biodegradable plastics are used as alternative materials to conventional plastics, e.g., for agricultural films (e.g., Sintim and Flury, 2017) and\/or as substitutes required by legislation such as fruit and vegetable bags (Journal Officiel de la R\u00e9publique Fran\u00e7aise, 2016; Gazzetta Ufficiale della Repubblica Italiana, 2017). Biodegradable polymers are discussed as a mitigation strategy against environmental plastic pollution (Republic of Indonesia, 2017). The European Commission in their European Plastics Strategy (European Commission, 2018) stated that new plastics with biodegradable properties bring new opportunities but also risks. It was also pointed out the importance to make sure that biodegradable plastics are not put forward as a solution to littering.<\/p>\n
For some applications where the unintentional loss of plastic to the environment is intrinsic to its use (e.g., fishing gear, boating gear, and beach tourism items) or which are prone to unavoidable input (e.g., abrasion of tires, shoes, textiles, paint) and thus are continuously introduced to the environment, biodegradable polymeric alternatives might be the only solution from the material side (Albertsson et al., 2020).<\/p>\n
There is no universal definition, yet several descriptions and definitions of the term \u201cbiodegradation\u201d exist, which might lead to decision-making based on wrong assumptions and even to misuse, false claims, and disinformation. Following the biogeochemical point of view, we define \u201cbiodegradation\u201d of a carbon-based polymer as the mineralization to carbon dioxide (and in the absence of oxygen also methane), water, and the incorporation of its breakdown products into new biomass by naturally occurring bacteria, archaea, and fungi, leaving no residue behind.<\/p>\n
Hence, \u201cbiodegradability\u201d describes the capacity of a polymeric material to be broken down by microorganisms in the considered receiving environment. As the abundance, diversity, and activity of microorganisms vary in nature as do the environmental conditions, also the specific biodegradation at a given place and time will vary. To be truly meaningful, the term \u201cbiodegradable\u201d must therefore be clarified and linked not only to a duration in time, compatible with a human scale but also to the conditions under which biodegradation occurs, and to be seen as a system property (Albertsson et al., 2020).<\/p>\n
As reliable, comparable, and verifiable information is needed and officially asked for (The European Green Deal, European Commission, 2019) the claim \u201cbiodegradable\u201d of a certain material should be sufficiently specified and reliably proven. Therefore suitable tests are needed (Harrison et al., 2018).<\/p>\n
Here, we tested the performance of biodegradable plastic in the marine environment applying laboratory methods proposed by Tosin et al. (2012) and field and mesocosm methods developed during the EU project Open-Bio (Lott et al., 2016a, b, 2020). In a 3-tier approach, we answered the questions whether the tested materials are biodegradable at all, whether biodegradation does take place under real natural marine conditions, and at which rates in the different environmental settings.<\/p>\n
To enable a numerical comparison of the experimental results we applied a statistical model to the experimental results to mathematically describe the biodegradation over time with a specific half-life. This number can further be used as a material property specific for defined environmental conditions and used to set thresholds, estimate environmental risk, and be fed into lifecycle assessment.<\/p>\n
In this study, we tested three biodegradable polymers with natural marine seawater and sediments in lab and mesocosm tests, and in field tests in three coastal marine scenarios in the Mediterranean Sea and in tropical Southeast Asia. We focused on three easily accessible habitats in coastal shallow water: the intertidal beach, the open water, and the sandy seafloor.<\/p>\n
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