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New research suggests a revolutionary new way to tackle the planet’s growing plastic problem. A new nanoparticle catalyst that mimics natural processes could ‘down-cycle’ plastics for re-use.
Plastic pollution is one of the most pressing ecological problems faced by humanity. Since its first widespread use in the 1950s, Greenpeace estimates that 8.3 billion tons of plastic have been produced.
The Problem with Plastic Waste
One of the elements that make the use of plastic so desirable is exactly the quality that makes it such a threat to the environment—its hardiness. When placed into landfills, it is estimated that plastics will take around 1000 years to naturally degrade.
Despite current drives to recycle and reuse plastics, estimates of just how much plastic gets recycled are shockingly low. Greenpeace estimates that just 9% of the total plastic waste has been recycled, whilst a further 12% has been burned. This leaves 79% in landfills, and in our oceans — from where it is passing into our food chain.
In 2015, Dutch researchers found that the number of species threatened by plastic waste had risen from 267 to 557 since 1997. Since that research, the number has soared to more than 2000 species. However, the number of species threatened by ingestion and contamination of the food chain is much more difficult to calculate.
There is no doubt that plastic has entered the food chain, with microplastics being found within the systems of a wide variety of species and systems. When these animals are eaten and ingested, the microplastics pass through the food chain through a process called trophic transfer. The plastic itself could be harmful enough — particularly as chemicals added to plastics during manufacture able to leak from them — but when in the environment, plastic tends to bind with pollutants. These attached toxins are carried with the plastics and accumulate in animal fat.
Therefore, methods to break down plastic beyond just recycling are a key research area. That is where a multi-institutional team of scientists led by the U.S. Department of Energy’s Ames Laboratory comes in. They have developed a first-of-its-kind artificial catalyst that is able to process polyolefin plastics such as polyethylene and polypropylene — commonly used in items such as plastic grocery bags, milk jugs, shampoo bottles, toys, and food containers. The end product is also re-usable.
Taking a Cue from Nature
Methods of breaking down plastics are not exactly new, but the difference between existing methods and the one unveiled by this team in a paper published in the journal Nature Catalysis is this that this new technique could turn waste plastics into a resource.
The method developed by the team breaks plastics down into useable, uniform, and high-quality components. These components can then be taken and used to make high-value fuels, solvents, and lubricating products. This method that could be very beneficial for nature and industry borrows its inspiration from the former.
The team began their research with an aim to mimic enzymes that break down macromolecules — large molecules, necessary for life, that are built from smaller organic molecules such as proteins and cellulose.
The process they developed depends heavily on nanoparticles, particularly a mesoporous silica nanoparticle consisting of a core of platinum with a catalytic active site. This is surrounded by long silica channels, through which the long polymer chains snake towards the catalyst.
The design above allows the catalyst to grip long polymer chains and snip them into smaller uniform pieces. These pieces then can be upcycled into useful and valuable end products.
There are already enzymes in nature that can break down plastics. In 2016, a plastic-eating microbe was discovered at a Japanese waste disposal site. Two years later, researchers used that bug to synthesize an enzyme that broke down the plastic used in bottles — polyethylene terephthalate (PET), which is one of the world’s most used plastics.
Improving on this development, earlier this year a team at the center for enzyme innovation, at the University of Portsmouth created a ‘mutant enzyme’ that could break down PETs much more quickly. The enzyme snips the bonds between PET’s two building blocks: terephthalate and ethylene glycol.
However, this new research could top even that development as it represents the first time that a controlled catalysis process has been designed with the use of inorganic materials. The team was able to show that the catalysis process driven by their nanoparticles could perform multiple identical deconstruction steps on the same molecule before releasing it.
In this way, the long polymer chains move through the pores of the catalyst in a way that closely resembles the action of the enzyme the team set out to mimic.
The Age of the Plastic Eaters
The team will now research the process further attempting to improve the catalytic process. Undoubtedly, should this be successful, the focus will then turn to the question of whether the nanoparticle plastic cleaving technique can be scaled up.
That is the stage of the research and development where the ‘mutant enzyme’ plastic eater described above is currently at. Industrial application is very much looming on the horizon for the technique.
The researchers involved in the testing of the mutant enzyme used a small reactor designed to test its capabilities, finding that it could break down 90% of 200 grams of PET in 10 hours. They now want to discover if they can alter their mutant so it can consume other forms of plastics. In addition to this, future steps will include introducing a mechanism to deal with dyes and chemicals added to the PET.
Despite needing further research, the green chemistry company Carbois is clearly enthused by the project's potential. The company, which brings an industrial approach to tackling plastic waste and deals primarily with enzymes, is currently building a demonstration plant that will operate using the mutant enzymes. It is expected to recycle hundreds of tons of PETs a year.
However, with 70 million tons of PETs produced annually, there is room in the plastics downgrading business for another modality. Time will tell if the fact that the nanoparticle alternative from Ames Labs can turn plastic into a valuable resource will lead to it dominating this new and vital marketplace.
References and Further Reading
Tennakoon. A., Wu. X., Paterson. A. L., et al.  ‘Catalytic upcycling of high-density polyethylene via a processive mechanism,’ Nature Catalysis. https://www.researchgate.net/deref/https%3A%2F%2Fwww.nature.com%2Farticles%2Fs41929-020-00519-4
Carrington. D.,  ‘New super-enzyme eats plastic bottles six times faster,’ The Guardian. https://www.theguardian.com/environment/2020/sep/28/new-super-enzyme-eats-plastic-bottles-six-times-faster
Service. R. F.,  ‘A huge step forward.’ Mutant enzyme could vastly improve recycling of plastic bottles. Science. https://www.sciencemag.org/news/2020/04/huge-step-forward-mutant-enzyme-could-vastly-improve-recycling-plastic-bottles