Catalyst degrades microplastics

28 August 2019

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A reusable magnetic catalyst can break down microplastics in water...

A team from the University of Adelaide have created a catalyst that can break down the long polymer chains that make up microplastics. The result is small, harmless molecules that can dissolve in water, and the entire process happens within just a few hours. This is in stark contrast to the thousands of years that would normally be needed for plastic to break down.

The key to the new catalyst, developed by Shaobin Wang and his colleagues, is carbon nanotubes. These are very thin sheets of carbon atoms, rolled up into tubes and laced with nitrogen compounds. “When we think about degrading, this is not just cleaving it into smaller pieces, but really degrading the chemical structure,” says Ljiljana Fruk, a chemical engineer from the University of Cambridge, commenting on the results of this paper.

Microplastics are tiny pieces of plastic generally less than 5mm in diameter, although they can be much smaller. They are often added to cosmetics, such as exfoliators, or can simply be created when larger plastic particles break down. When these end up in oceans, they can damage small marine organisms by choking them and can also release toxic chemicals that accumulate on the particles owing to their water-repelling properties.

Using a catalyst to degrade microplastics has been attempted before, but previous catalysts were made from iron and cobalt, which can be pollutants themselves. This new method uses carbon, which is considered much safer and does not release potentially toxic chemicals in the process.

One of the products of the degradation is carbon dioxide. “This carbon dioxide could be used by marine organisms in photosynthesis, if you have plankton to produce biomaterials,” says Fruk. This means that microplastics that choke up small biological organisms could in fact be turned into carbon dioxide to help them grow instead.

Fruk goes on to explain that, by using this in a sewage water system in a big city, the waste carbon dioxide produced by the process could also potentially be harnessed to grow biofuels.  “If you have one reactor within the [water treatment] plant with micro-organisms -  then the products could be used to create biomass. You could have a circular system.”

A quirk of this catalyst is in its shape: rather than plain tubes, they are twisted into springs. This means that the surface area of the catalyst, where the reaction takes place, is maximised. “If you have a spring-like surface, you are increasing the curvature so that the molecules can fit in, and you are also increasing the surface area,” says Fruk.

Another advantage is that these catalysts are re-usable. Being magnetic, they can be removed from the water when their work is done using a giant magnet, and reused elsewhere. This is also important to ensure that they don’t end up in the water supply. As Fruk explains, “you recycle your catalyst, and you also ensure that the catalyst is not ending up in your drinking water.”

The work is still in early stages. Although this has been demonstrated in a lab setting, it would need to be scaled significantly to produce the amounts needed for water purification on the scale of a city. More work also needs to be done to confirm that the catalyst isn’t a health hazard in its own right. As Fruk points out, “there needs to be a certain amount of time you spend investigating the biocompatibility. You would not want something to leak into the water that is maybe more toxic than the plastic itself.”

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