- Zakaria Ahmed
Bioplastics: source, biodegradability, and recycling
Many when they hear about bioplastics think that bioplastics should readily decompose in the environment and disappear automatically. It is important to make distinction between three key concepts: bioplastics, bio-based, and biodegradable plastics as these terms are usually confused or misunderstood. So, what are bioplastics and what is biodegradation? And what is the fate of bioplastics after their use?
The increased production of plastics from fossil fuel resulted in increased pollution and climate change impact because of plastics resistance to degradation and being a source for greenhouse gases. Bioplastics has come into the discussion around sustainable solutions for plastics waste/pollution alongside chemical and mechanical recycling. Bioplastics are plastic materials made from more renewable sources, such as from agricultural waste and have applications in packaging and biomedical materials. Bioplastics include plastics sourced from bio-based sources such as bacteria, soy, lignin, cellulose, and fibres; as well as plastics sourced from fossil fuels – look at diagram below. The current fraction of bioplastics is still under 1% of total plastics produced globally. It is estimated that in 2.1 million tonnes of bioplastics were produced in 2020 and the amount is expected to increase to around 2.8 million tonnes in 2025. Bioplastics have better degradation properties than conventional petroleum-based plastics and their carbon dioxide emissions footprint is lower as well. However, bioplastics are more expensive to produce, and they have less favourable mechanical properties than petroleum-based plastics as biopolymers are generally brittle, limiting their wider use.
Coordinated system of bioplastics (Source: European Bioplastics )
The term 'bioplastics' commonly refers to plastic materials that are either bio-based or biodegradable or both. The term bioplastics is meant to indicate the desired attributes of sustainable sourcing and non-persistence in the environment. However, 'bio' does not indicate 'green' simply because shifting to bio-based raw materials gives rise to other set of concerns in the bigger picture. Such concerns include increasing cost of food due to competition on land use, use of genetically modified bacteria for biological processing, intensified farming, deforestation and reducing, or loss of biodiversity. On the other hand, advantages of bio-sourcing of plastics include savings in energy consumption and air pollution. For instance, sourcing plastics from starch (like PLA biopolymer) results in reduced energy consumption (50% reduction) and less production of greenhouse gases (60% reduction) compared to polystyrene packaging.
'Bio-based' refers to sourcing the plastics from renewable sources; but it does not indicate biodegradability. This means bio-based polymers can be biodegradable or non-biodegradable as the term refers only to the raw material of plastics being biomass rather than oil. Examples of bio-based plastics products include rigid packaging such as containers/bottles, film packaging for food, catering products (trays, cups, plates etc), biodegradable mulch film, compostable waste bags and carrier bags for organic waste.
'Biodegradable' refers to the susceptibility of plastics to decomposition by biological activity of organisms in and on the plastic material. The process of biodegradation is described as the process in which plastics become food for the micro-organism. Biodegradation starts by growth of the microorganism in a on the plastic material. This then causes fragmentation of the plastics material to smaller parts. These smaller parts are more readily digestible by the microorganism; and their digestion results in producing water, carbon dioxide and biomass eventually. Therefore, biodegradable plastics are classed from view-point of biodegradability regardless of their source. In reality, biodegradation occurs under specific conditions at specific environments for specific period of time and these conditions have to be met to realise the biodegradation of these plastics. Their degradation depends on factors related to the environment the material is in such as amount of oxygen, and how acidic or basic the environment is. Another factor is the material's physical and chemical structure as plastics with less ordered structure and smaller molecular size (or shorter polymer chain) are more readily degradable.
Current options for management of bioplastic waste include biodegradation as mentioned above, physical/mechanical reprocessing, and biological waste treatment. An example of biological treatment is composting in which microorganism turn the organic matter in the plastic into carbon dioxide and soil-like material called humus). Besides these options, bioplastics end up in landfills and in marine environments. Bioplastics in landfills and marine environments have less favourable conditions for biodegradation to occur fast enough, although in soil and compost there is better microbial degradation than in marine environment. Industrial composting can provide a solution to avoid accumulation of bioplastics in landfills, whereas in oceans bioplastics are still a source of pollution due to limited degradation.
With restricted biodegradation conditions, research is carried out to build customised industrial recycling processes (especially chemical and biological recycling processes) for bioplastics benefiting from their biodegradation characteristics in a more circular approach. Example processes include pyrolysis, and decomposition in solvents such as, water, alcohols, and glycols. These processes aim to recapture the value of the bioplastics rather and achieve decomposition faster rather than accumulating bioplastic waste in landfills and waiting for their slow biodegradation. Researching potential degradation processes for bioplastics comes in response to increased use of bioplastics in applications such as packaging. With this increased trend, soon large quantities of bioplastic waste will be generated and will need to be recycled sustainably. With this in mind, developing chemical and biological recycling processes will prevent bioplastics waste accumulation in the surroundings; and avert a plastic waste crisis similar to that caused by accumulation of fossil fuel based plastic waste in environment.
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