The enzyme works record-breakingly fast and proves that biological recycling of PET can go much faster than was thought possible until recently.

In the search for enzymes that can break down plastic, German scientists have made a very special discovery. In a compost heap in a cemetery in Leipzig, they have found an enzyme that can break down polyethylene terephthalate (PET, a commonly used type of plastic) at record speed.

On the compost heap

At first glance it may seem a bit strange that researchers find such a plastic-breaking enzyme on a compost heap, but nothing could be further from the truth. “Bacteria and fungi use these types of enzymes to break down natural polyesters in plants, such as cutin,” explains researcher Christian Sonnendecker, of the University of Leipzig. “Such natural polyesters have a chemical structure similar to that of a synthetic polymer such as PET. And coincidentally, these enzymes can also break down PET plastic.”

PHL 7

And in the case of the enzyme that has now been found on a compost heap in Leipzig, it is moving quickly too. For example, the enzyme – which bears the name PHL7 – appears to be able to break down a plastic container with a hinged lid (which, for example, often contains strawberries) in less than 24 hours.

Hooks and eyes

And that offers possibilities. Because recycling plastic may sound very sustainable, but at the moment it does have some snags. For example, in today’s conventional ‘mechanical recycling’ plastic has to be heated to very high temperatures in order to be melted down. This results in high energy consumption and considerable emissions. In addition, the quality of the plastic deteriorates slightly every time it is recycled. No wonder scientists are desperately looking for alternative ways to recycle plastic. In this regard, particular attention is paid to enzymes. These are proteins produced by all organisms on Earth that can trigger or speed up chemical reactions in living things. For example, certain enzymes in bacteria are involved in breaking down plant remains.

But for some time now we have known that there are also enzymes that can break down plastic. A fairly well-known example of this is LCC, which was discovered in Japan in 2012 and knows how to handle PET. The advantage of such enzymes – compared to mechanical recycling – is that they need little to do their job; some water and a temperature of 65 to 70 degrees Celsius is all they ask. In return, they break down the plastic they are presented with into the components terephthalic acid and ethylene glycol, which can then be used to make new PET. And with this approach – in contrast to the conventional recycling method – a closed, circular plastic chain can be formed.

More efficient

But this biological form of PET recycling is difficult to get off the ground; at the moment there is only a factory in France that dares to do this. And that’s just a pilot project. “The process is not yet interesting from an economic point of view,” says Sonnendecker Scientias.nl† After all, it is still cheaper to extract the components of plastic from oil than from discarded plastic. “In addition, only amorphous PET (packaging material) can be broken down directly by enzymes.” Other forms of PET – such as the well-known PET bottles – must first be pre-treated before the enzymes can do anything with them. “And that also costs money. All in all, we need to make the entire process much more efficient. Starting with the development of more efficient methods to pre-treat plastic and faster and more stable enzymes.”

Faster

And with PHL7, researchers now have a considerably faster enzyme at their disposal. For example, it overpowers the previously discovered enzyme LCC – which until recently was regarded as the gold standard among plastic-eating enzymes – in terms of speed. While LCC broke down about 45 percent of the plastic presented in experiments in 16 hours, PHL7 broke down 90 percent of the plastic in the same time. “So our enzyme is twice as active,” Sonnendecker says. And that’s important. “Time is money. If you can break down a certain amount of PET in half the time, you can double the yield and make the process much more efficient, because you can now break down two lots of plastic in the time it took before to break down one lot of plastic. break.”

That is not to say that plastic recycling using enzymes is suddenly economically interesting; more needs to be done for that. But fast enzymes such as PHL7 can certainly help make biological recycling of PET plastic profitable in the future. “I am optimistic that with our current research we will be able to optimize the entire process,” says Sonnendecker. Incidentally, it is not only ‘his’ University of Leipzig that has charged itself with this task; Besides the university, several European universities and companies are working together within the European research projects ENZYCLE and MIPLACE on a solution to the problem of plastic recycling.

Researchers can therefore certainly not sit back with the discovery of PHL7. For example, the enzyme will be studied in more detail in the near future, also from an economic point of view. “It works in the lab, but now we need to scale the reaction in such a way that we can better estimate the costs it entails on a larger scale,” Sonnendecker says. “Only then will we know where we stand. Because ultimately it has to be economically interesting if the industry wants to use such a technology.” Meanwhile, the search for even faster and more efficient plastic-degrading enzymes continues. “We were already very surprised when we found PHL7, but there is still a chance that we will find something better.”