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Cite This: Environ. Sci. Technol. XXXX, XXX, XXX-XXX
Biodegradability of Plastics: Challenges and Misconceptions Stephan Kubowicz† and Andy M. Booth*,‡ †
SINTEF Materials and Chemistry, Oslo, 0314, Norway SINTEF Ocean, Trondheim, 7465, Norway
‡
ABSTRACT: Plastics are one of the most widely used materials and, in most cases, they are designed to have long life times. Thus, plastics contain a complex blend of stabilizers that prevent them from degrading too quickly. Unfortunately, many of the most advantageous properties of plastics such as their chemical, physical and biological inertness and durability present challenges when plastic is released into the environment. Common plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) are extremely persistent in the environment, where they undergo very slow fragmentation (projected to take centuries) into small particles through photo-, physical, and biological degradation processes1. The fragmentation of the material into increasingly smaller pieces is an unavoidable stage of the degradation process. Ultimately, plastic materials degrade to micron-sized particles (microplastics), which are persistent in the environment and present a potential source of harm for organisms.
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For those plastics that are considered truly biodegradable (e.g., polylactic acid, polycaprolactone, polybutyrate adipate terephthalate), the biodegradability of the final product is not solely determined by the properties of its polymer. It is also determined by additives that are incorporated in final consumer products, as well as the environmental conditions in which the material ends up.4 One of the main challenges currently facing society is a lack of plastic materials that rapidly and completely decompose under natural environmental conditions, yet retain their properties for a sufficient duration in consumer products. To assist in the development of biodegradable plastics, internationally recognized standard methods are available that allow biodegradability to be evaluated in a range of common
here have been significant efforts in recent decades toward developing and industrializing so-called “biodegradable” plastics that might have shorter residence times in the environment.1 Oxo-degradable plastics are one class of plastic materials that are commonly promoted as biodegradable. In reality, they are conventional plastics (e.g., PE, PP, PET) containing additives that accelerate the oxidation process, socalled prodegradants.2 The major issue with oxo-degradable plastics is that they rapidly fragment into huge quantities of microplastics when exposed to a combination of sunlight and oxygen. While this speeds-up the first step of the degradation process, making large plastic items ’disappear’ relatively quickly compared to conventional plastics, the generated microplastic is no different to any other type of microplastic. Under natural environmental conditions, microplastic fragments resulting from oxo-degradable plastics still take a long time to completely biodegrade and continue to pose a threat to the environment.3 © XXXX American Chemical Society
Received: August 7, 2017
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DOI: 10.1021/acs.est.7b04051 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
be sufficiently high to make this economically viable. Furthermore, biodegradable plastics are considered an undesired contaminant within the recycling streams of conventional plastics. While conventional plastics may also be present in the composting stream of biodegradable plastics, they do not significantly impact the composting process. They can simply be removed through sieving, together with other noncompostable items, after the composting process is complete. Like other dedicated waste and recycling streams, compostable waste needs to be separated at source from normal household or industrial waste. However, unless collection systems and composting facilities are available to consumers, biodegradable plastic is most likely to end up in conventional waste streams (e.g., incineration, landfill). Ultimately, many of the same challenges appear to exist for biodegradable plastics as for conventional plastics. They need to be contained in existing waste streams to prevent release to the environment and they need to be separated from all other waste materials (including plastics). Importantly, when they are mineralized in industrial composting facilities this represents the loss of a potentially useful resource that fails to meet societal goals for a circular economy. A circular economy aims to keep products, components, and materials at their highest utility and value at all times, emphasizing the benefits of recycling residual waste materials and byproducts.10 The development of recycling approaches for biodegradable materials is therefore going to be necessary if they become high volume production materials.
environmental matrices (e.g., marine, soils, or wastewater treatment plants). Standard specifications and test methods (e.g., ASTM D6400, EN 13432, ASTM D5338, ASTM D5929) also exist for assessing the biodegradability of plastics under optimized industrial and municipal composting conditions.5 Plastics that fully mineralize over reasonable timeframes under the high temperatures and controlled conditions created within industrial composting facilities can accurately be classified as biodegradable or compostable. In addition, the fragmentation occurs quickly in an industrial composter and poses little risk for exposure of microplastics to the environment as the degradation happens within a closed and controlled system. However, there currently appears to be no global data available on the proportion of biodegradable plastics that are composted rather than entering general waste streams. Although individual polymers and plastics can be classified as biodegradable according to test methods designed to assess biodegradability under optimized industrial composting conditions, there is limited control or regulation over how the data is utilized. In recent years, the term “biodegradable” has become an appealing marketing term that is very misleading; in most cases, the biodegradability was tested only under very specific conditions and does not represent a generic property of the material. When plastic materials are promoted as biodegradable or “compostable” it suggests to consumers and companies that they biodegrade in the same way under many different end-of-life scenarios. Yet in the natural environment, these same materials will take much longer to fully biodegrade (often taking decades), and the degradation process still generates large quantities of potentially harmful small particles.6 It is therefore recommended that stronger legislation regarding use of the terms biodegradable and compostable on consumer goods be established. Clearer guidance and terminology for consumers and companies purchasing plastic materials is needed to aid in the selection of the appropriate materials for their respective end-of-life conditions. Plastic recycling represents a successful example of such an approach, where there is legislation regarding minimum recycled content mandates and procurement policies, combined with recycled product labeling.7 As demand for biodegradable plastic materials by consumers is increasing rapidly, we believe a key research need is an evaluation of their environmental and societal benefits. In the 2000s, for example, there was a shift toward the use of oxodegradable plastics for supermarket carrier bags as this was considered a more environmentally friendly alternative to the conventional polyethylene-type carrier bags, since the large plastic fragments would persist in the environment for a shorter amount of time. New knowledge has subsequently led to a move away from oxo-degradable materials, which are designed to rapidly fragment without considering the formation of microplastic,8 toward truly biodegradable plastics and so-called multiple use ’bags for life’ made from conventional, recyclable materials. The available evidence suggests that the residence time of biodegradable plastics in the natural environment is less than that of conventional plastics, but degradation is highly dependent upon environmental conditions and they still undergo processes that generate microplastics.8,9 Biodegradable plastics are also challenging to recycle and they are currently difficult to isolate from mixed plastic waste streams that contain recyclable (PE, PP, PET) and nonrecyclable plastics. Technologies for isolating biodegradable plastics could be implemented, but the volume of biodegradable plastic needs to
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Andy M. Booth: 0000-0002-4702-2210 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This manuscript has been written as part of the Research Council of Norway (RCN) funded project “MICROFIBRE” (Grant Agreement number 268404). We thank the RCN for their financial support.
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REFERENCES
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DOI: 10.1021/acs.est.7b04051 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology (8) O’Brine, T.; Thompson, R. C. Degradation of plastic carrier bags in the marine environment. Mar. Pollut. Bull. 2010, 60 (12), 2279− 2283. (9) Tosin, M.; Weber, M.; Siotto, M.; Lott, C.; Degli Innocenti, F. Laboratory Test Methods to Determine the Degradation of Plastics in Marine Environmental Conditions. Front. Microbiol. 2012, 3, 225. (10) Andersen, M. S. An introductory note on the environmental economics of the circular economy. Sustainability Science 2007, 2 (1), 133−140.
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DOI: 10.1021/acs.est.7b04051 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
