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✍️Ellen Loft | BSc Natural Sciences student at the University of Leeds
What is the 'Plastic Problem'? 🤦🏽♀️
With hundreds of millions of tonnes of plastic waste accumulating globally each year, the need for new recycling and waste management solutions are growing.
The accumulation of plastic waste is not slowing down. With only 9% of plastic waste recycled and an additional 22% mismanaged, the remaining plastic waste is dumped in landfill or incinerated, having a detrimental effect on the environment and future ecosystems.
The only waste plastic processing method used on a large scale is mechanical recycling. This comes with drawbacks, for example:
Contamination with non-recyclable materials. These contaminants can be food waste, small particles of glass and other non-plastics and additives absorbed in plastic production that cannot be removed using standard cleaning procedures.
Limited Scope. Mixed plastics and those contaminated with food debris or grease cannot be recycled this way
Quality degradation. Each time a plastic is mechanically recycled, its quality decreases due to degradation of polymer chains and impurities. This means there is a finite number of cycles a plastic can endure before it is deemed unusable.
Why is polyethene a stubborn plastic? 😵💫
Polyethene makes up 30% of all synthetic plastic production, but it is a long inaccessible chain that requires a lot of processing.
Polyethene is a long hydrocarbon chain, meaning it is formed of just hydrogen and carbon atoms joined by strong covalent bonds. Polyethylene is a crystalline, dense structure in which oxygen must be introduced before the chain can be broken into smaller molecules. It also repels water, making it difficult to mix and spread across surfaces, and is generally inaccessible, making it more difficult to treat than other waste.
What is the first stage of polyethylene degradation? ✂️
'Oxidation' is the first stage of polyethylene biodegradation which can take years.
The oxidation step involves adding oxygen to the long hydrocarbon chains before they can be cleaved into smaller chains.
This is usually the bottleneck step in biodegradation, because it cannot be carried out at the rate that is required for plastic breakdown. It can be aided by non-living factors like light and temperature, and then continued by living organisms like bacteria and fungi.
However, this is where an additional difficulty becomes clear. For these organisms to perform biodegradation of polyethylene, a pre-treatment of UV light or heat is required. This adds time, money and extra facilities.
Which enzymes have been discovered? 🔍
Two enzymes within waxworm saliva, namely Demetra and Ceres, can carry out the oxidation step much more rapidly.
These two enzymes within the Galleria mellonella waxworm saliva were found to oxidise polyethylene within a matter of hours at room temperature and under neutral pH.
In fact, these two enzymes are thought to be the first animal enzymes with the capacity to oxidise polyethylene without pre-treatment. They can carry out this step within an hour - the fastest agents from living organisms to modify polyethylene and aid its biodegradation.
How have they been discovered? 🔬
Using experimental procedures, scientists in a Spanish Research Centre isolated the two key enzymes in the saliva that could carry out the crucial oxidation step.
An initial observation Plastic was appearing to break down and form debris when it came in contact with waxworm cocoons and saliva.
Analysis of the saliva After this initial observation, a technique similar to sieving separates molecules based on size (Gel Permeation Chromatography) showing the long plastic chains had been cut into smaller molecules.
Identifying the enzymes Using analysis techniques, they found a handful of enzymes belonging to the specific group of enzymes called the phenol oxidase family. Further experiments at room temperature found two of these enzymes had the capacity to carry this out very quickly. These enzymes were Demetra and Ceres as mentioned previously.
Why do waxworms have this ability? 💡
Waxworms commonly live in beehives and feed off beeswax. This could be a related function to their ability to degrade plastic.
One theory is that there is similarities between beeswax and plastic, and the enzymes discovered in their saliva were originally evolved to digest beeswax and utilise it as an energy source.
An alternative argument is that these enzymes are found in waxworms as part of their first defence immune system. This is because the enzymes are found in the same family as enzymes known for having defensive properties and protecting insects against disease-causing organisms and plant defences.
What now? 🌍
This initial discovery could be the stepping stone in accelerating plastic upcycling, a crucial improvement needed in plastic waste management in the wider circular economy frame.
Despite more research being crucial in unveiling a deeper understanding of the steps of biodegradation by these enzymes, there is hope in these biological agents. One scientist imagines litres of aqueous solution could be poured onto plastic and quickly break down the plastic ready for re-use.
A circular economy frame is one in which materials are not wasted, but instead are kept in circulation, and nature is regenerated. Plastic upcycling plays a part in this system and optimistically, this alternative method could provide an answer.
Edited by: Olivia Laughton | Content Editor | BSc Microbiology, University of Leeds
Great interesting read and easy to understand!
Very interesting read!