Dr. Doris Ribitsch, Environmental Biologist at the University of Natural Resources and Applied Life Sciences Vienna, Austria (c) BOKU Wien
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At the beginning of the noughties, it was thought that certain polymers such as polyester and especially polyethylene terephthalate (PET) were not degradable at all, says environmental biotechnologist Dr. Doris Ribitsch from the University of Natural Resources and Applied Life Sciences in Vienna. She conducts research at the Institute for Environmental Biotechnology, which is headed by Professor Georg Gübitz. Together with her colleagues, she is part of one of the five research groups worldwide that believed in the potential of microorganisms and enzymes in nature even back then. Years later, they were able to prove it – and even make PET recyclable.

Here is Ribitsch in an interview with IO:

What is the problem with plastic recycling and how do you think you can solve it?

All existing recycling methods have serious disadvantages – including for the environment – and for certain types of plastic, there are no recycling methods at all yet. In environmental biotechnology, we try to use concepts that already exist in nature.

To what extent do existing methods in plastic recycling have a negative impact on the environment?

Chemical recycling methods are based on harsh chemicals and mechanical recycling methods damage the plastic so much that only downcycling is possible. More important, however, is that these methods have limitations when it comes to composite materials that consist of several polymers.

It must be remembered that most plastics are made from the fossil raw material crude oil, which is valuable and only available in limited quantities. If you use it to make a plastic bag, which often has a useful life of only a few minutes, you are wasting natural resources.

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Due to the lack of recycling methods, large amounts of plastic waste are still being incinerated. Although this produces usable energy, it is not recycling, but merely recovery.

We want to develop truly environmentally friendly recycling methods. To do that, we are gathering knowledge about how biodegradation occurs in nature and what is needed to make polymers more degradable in the future. We can also use this knowledge to research materials that are more biodegradable.

Polymer binding to enzymes (c) BOKU Vienna

How did you discover natural enzymes for recycling PET?

First, we had to find out where such concepts can be found in nature. That required a lot of patience. Then we recreated the models in the laboratory and applied them. That was a long process because nothing was known about it yet. It was actually assumed that plastic recycling with microorganisms and enzymes from nature was impossible.

We found the first microorganisms and enzymes that can break down PET in compost, where they are involved in breaking down organic material. Fruits like apples and tomatoes have skins made of a natural polyester, a natural polymer called cutin. For such a fruit to degrade, microorganisms must also be able to penetrate the peel. To do this, they build enzymes that act like scissors that can cut or break down the polymers in the peels. This is what we also want to achieve in plastic recycling.

Bacteria growing on polyester (c) BOKU Vienna

How does this concept apply to synthetic plastics?

Cutin is also polyester, but its chemical structure is of course very different from PET, for example. We have isolated these enzymes and produced them in the laboratory and found that they can also break down PET. However, they are less active on synthetic polyester than on natural polyester. In nature, enzymes are highly specific to certain molecules. Since PET is not a natural polyester, there is no enzyme tailored to it. That is why we are optimizing the natural enzymes in the laboratory for use in recycling processes. This means that we exchange certain amino acids and thus improve the interaction between the enzyme and the polyester so that this enzyme can also bind better and cut up the polyester. The reaction conditions under which the enzymes in recycling processes have to work are also different from natural conditions, so we have to adapt to them.

The interaction with and the binding of the enzyme to the plastic are our research focus. These are the aspects that determine whether the enzyme becomes active and carries out the degradation.

Microbes at work on plastics (c) BOKU Vienna

How far has your research progressed and what is the goal?

In the past, we have already conducted research on enzymes together with companies. These companies have meanwhile developed processes with which PET bottles can be almost completely decomposed in 24 hours. That’s already quite a good degradation rate, but we want to improve it further together with company partners.

There are many polymers and especially composite materials that are not yet degradable. That’s why we want to expand the knowledge we’ve been able to generate through our research so far. This leads us to other environments where we investigate how the different polymers are degraded. For example, we have found enzymes that degrade polymers in water. With this knowledge, we were able to show that enzymatic degradation also works under anaerobic conditions. This allows us, for example, to develop enzymes that enable plastic recycling in biogas plants.

In addition, we are also working on the degradation of bioplastics, because in one way or another they will also end up in nature and possibly produce microplastics there.

Is that because bioplastics are not necessarily degradable, either?

Correct, the definition of bioplastic is that it either comes from renewable raw materials or it is biodegradable. That means it doesn’t necessarily have to be degradable. Moreover, compostable does not always mean biodegradable in the environment. Not everything you throw into nature degrades there. That means we will also have to collect bioplastics and recycle them in our own recycling plants.

Thank you for the interview.

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