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Traversing the footprints of carbon in waste decomposition

By Sayan Basak posted 07-05-2020 08:59


Tracing carbon is one of the pivotal components if we want to address the sustainability challenge. Although we often aim to recycle the used macromolecular chains, a quantified study defines the scope of the recycling and peeks into the areas of improvement. For instance, recycling a material that loses half of its carbon content from collecting the waste material till feeding it into the recycling pathway is of no economic value. Therefore, it is essential to track carbon for each molecular building block, including its source and destination. Dr Sander, from ETH Zurich, experimented with poly (butylene adipate-co-terephthalate) (PBAT) to analyze the decay and the kinetics of the process degrading the same in soil. In the recent realms, PBAT is often used as an alternative to the conventional polyethylene, especially in the food packaging industry owing to its biodegradable traits. However, we do not know how much of the material is bio-degraded after being used or what is the amount of carbon lost into the ‘non-destined’ zones.

Various pieces of the literature suggest that a part of the two million tonnes of plastic films that are produced percolates into the environment. Although we label a product as ‘100% biodegradable’, the outcome frequently doesn’t prove its accurate label. And the worst part is that the part which is lost can never be traced back, questioning the efficacy of the recycling process. The assertion arises how can a material tagged as completely recycle can’t be recycled completely? Let’s assume a plastic A which has been devised to be decomposed by a microbe B in a humid environment. However, unfortunately, if the year experiences a slight variation of climate change, it shall hinder the optimum humidity, thereby preventing the microbe from decomposing the plastic. Apart from the climactic drift, we also have a genetic mutation in the microbial genomic sequences, which prevents the microbes from breaking down the molecular chains. These chains, which cant be degraded, instead of funneling into the recycling channel lands tunnels into the environment, thereby deteriorating it.

So, how to trace the carbon content to contrast the efficacy of a recycling process? Dr Sander’s research group integrated 13C isotope on the monomers of PBAT and allowed the system to degrade in the soil (PBAT is composed of three monomers comprising of 1,3-butanediol, adipic acid, and terephthalic acid). The group intended to use analytical tools such as mass spectrometry to trace the degradation of these labeled’ monomers. Since the consumers, in almost all the cases, disposes of the films in open fields after their respective agricultural usages, the experimental set up was devised by the research group was perfect to revisit the fate of the carbon atoms during recycling.

The good part of the project was that the results infer quantitatively that the material is completely biodegradable, with almost one-tenth of the carbon isotope releasing as carbon dioxide. All the studies which were conducted prior to this labeling experiment were reinforced on weight loss or carbon dioxide emission, thereby failing to provide any quantitive insight on the process. A previous study by Dr Bertocchini claimed that caterpillars (Galleria mellonella) often nibbled holes into the waste plastic, which was a qualitative study with no such insights on the process of the polymer chains degradation. However, the research was supplemented with various spectroscopic analytical tools inferring that these insects might help in the depolymerization process by converting the polyethylene into polyethylene glycol.

Interestingly, the results seemed dubious when Dr Opatz showed the exposing the plastic bags to moieties apart from the caterpillar (for instance, egg yolk) resulted in comparable spectroscopic data, raising doubts on the role/potential of microbes/living organisms to degrade polymer chains. Therefore, the need to analyze a process via a quantitive perspective is needed, yielding the data on how unique and efficient a depolymerization strategy is. The research, as conducted by Dr. Sander, widens the scope to answer the question ‘Where and how much are the carbon footprints being funneled?’