How biological enzymes are fighting the plastic crisis

The work was carried out with the support of UK Research and Innovation.

Plastic is an amazing material that has revolutionised our world - but at a staggering cost. From food packaging to medical devices, it’s quite literally everywhere. Yet its durability means it also lingers long term in our environment, to the detriment of - as we heard last week - our health, and the health of animals and even plants. With global plastic production set to double by 2050, the need for sustainable solutions and, particularly, better ways to recycle and reuse the constituents of plastic is a high priority. One area showing considerable promise is the development of enzymes, originally sourced from nature, that can attack certain plastics and break them down into their constituent building blocks, or monomers. Scientists are in the process of evolving these enzymes to optimise their activity. Will Tingle has been along to see one such venture in action…

Will - The world is producing about 400 million tonnes of plastic every single year, and it's not going anywhere by itself. We can burn it, we can bury it, but A, those are not permanent, and B, we just end up needing to produce more plastic. So, how can we close that plastic loop in a way that's sustainable and useful? Well, I've come down to the University of Portsmouth to visit the Preventing Plastic Pollution with Engineering Biology, or P3EB, Mission Hub, to find out more.

Andy - My name's Professor Andy Pickford. I am the lead of the P3EB Mission Hub. The P3EB mission is to advance technology for biological recycling and upcycling of plastic waste, so to support this transition towards what we call a circular plastics economy.

Will - Now, the means of plastic breakdown that P3EB is heavily focused on here, it seems, is using enzymes, biological molecules, to break down the bonds in plastic polymers. This isn't a novel concept, though, is it? We have been doing it for years, so how is that idea being furthered here?

Andy - Yes, we've been breaking down plastic with enzymes, certainly in the laboratories for a decade or so. And for certain plastic, like polyethylene terephthalate, or PET, we are nearing industrial application of the technology, but we really need to continue to advance the technology, not just for PET, but for other types of plastic, and make it more economically viable, so that recycled plastic becomes desirable from an economic point of view.

Will - Why is plastic traditionally so difficult to break down? Is it a case of the bonds in those molecules, in those plastics, just we've designed them to be so tough we now don't know what to do with them?

Andy - Yeah, that's certainly part of it, and certain plastics have different strengths of bonds within them, so as I mentioned, we've really made great advances in the breaking down of PET. PET has ester bonds in it, and these are actually quite weak bonds, and so enzymes can target those bonds and break the polymer chain down into its constituent molecules.

Will - Is it this way? It was time to take a trip down to the lab to get the breakdown on the breaking down.

Brooke - I'm Brooke Wain, and I'm a final year PhD student at the Centre for Enzyme Innovation.

Will - Okay, I appreciate this is one of the most difficult and convoluted questions of our time. If we want to use biological enzymes to break down plastic, how do you even start looking for the right thing to use?

Brooke - Well, to start with, it's the enzymes themselves. So plastics are polymers, similar to natural polymers, they comprise of monomers connected together by bonds which produce the resulting polymer. Now, in nature, you have enzymes which target those bonds to break down the polymer into the monomers, and natural polymers are things such as cutin, which is the waxy cuticle layer on your leaves that you see, that shiny surface, and there's enzymes in the natural environment which break down that cutin layer, called cutinases. So one large part of our research into enzymes which break down plastic focuses on engineering and taking those cutinases to be able to break down the synthetic plastics such as polyester. Natural enzymes are typically not very fast and not very tough, so we can introduce changes to their structure to improve these characteristics. So here at Centre for Enzyme Innovation, we are blessed because on one floor, in one huge lab space, we have the whole process start to finish. We do the discovery of the enzymes, we do the engineering of them, various approaches to work out the optimal conditions for each enzyme. Once we have a really fast, efficient enzyme, we take it to the next lab, which is where we do all the bioprocessing. That's these noises you're hearing now, these are bioreactors. Now, we can use these in two ways. We can grow them up and use them to recombinantly express the protein we need in high yields, which is also important because if you want to break down a lot of plastic, we're going to need a lot of enzyme in the lab to be able to do that. The other aspect is we actually do the plastic digestions in these bioreactors. So because one of the monomers released after PET breakdown is an acid, we can then use a base to be added to the system to neutralise the pH, which keeps our enzymes happy, but also allows us to quantify and track that reaction.

Will - As you said, we've got sort of jars of ground down what was once plastic alongside spinning milky vats of fluid. Are we looking at this breakdown process in action?

Brooke - Yeah, you're seeing exactly what I was describing about how you have that enzymatic digestion in a bioreactor. If you see this bottle on the right, this has the sodium hydroxide, which as the pH changes inside the vessel, it will automatically register, detect that change in pH and start adding in the sodium hydroxide to neutralise the pH so we can quantify how much TPA that acid monomer is being produced. Once we finish an enzymatic digestion, we can filter off anything that's undigested and characterise that.

We can pass it through activated carbon to remove any organic contaminants, anything like that. We filter it and once you get this, what we call a monomer soup, we can drop the pH to crash out the TPA. We can filter that and we get almost quite pure terephthalic acid ready to use and ready to dry for a future reaction. The other part is ethylene glycol, which is a bit more tricky to get out. Now this is not ideal, but there's technologies that are going on at the to improve this monomer recovery aspect.

Will - When this process is finished, what are we going to be left with?

Brooke - Once this bioreactor digestion has finished, what you have inside that vessel is a combination of bits and bobs. So you have undigested PET plastic, you also have buffer, you have leftover monomers, but that's it. The highlight of this technology is that those monomers can be purified and then either up-cycled into alternative resources or repolymerised back together to make the same virgin quality PET along with the catalyst.

Will - We're back now with you Andy and I can't believe I'm saying this, but thank you for a fascinating tour of a chemistry lab. But the obvious question is, once you've broken down these polymers into their constituent monomers, their building blocks, what do you do with them? Do they go back into making plastics? Do they go into making something completely new? Or is it a bit of both?

Andy - We can simply stitch them back together in a very simple chemical process to remake plastic that has identical properties to a virgin plastic, as we call it, that you might make from oil and gas. Or we can take those chemical building blocks and we can turn them into something else. So the terephthalic acid that you get from PET could be converted into fragrances or flavourings such as vanillin, and the ethylene glycol could be fed to microbes to turn into all sorts of other chemicals.

Will - To take nothing away from this fascinating science, 400 million tonnes of plastic year is an almost incomprehensible amount. This does need scaling, doesn't it?

Andy - Yes, scaling of this process is actually ongoing. One of our project partners on the Mission Hub, Carbios, a French biotechnology company, have a pilot plant currently running with this technology and they are building an industrial plant which they aim to open in 2027, and that will handle 50,000 tonnes of PET waste per annum, so equivalent to about five Eiffel Towers. There's clearly a considerable way to go, but we are at the point now where this can be scaled up.