Real recycling –
The resulting chemicals can be used to make brand-new bottles.
described in 510 – Found in sediment near a plastic recycling facility, naturally.
While microbes like this could solve the plastic waste issue, they don’t make plastics any more sustainable, since the carbon backbone of PET ends up being broken down completely. That means we have to constantly supply new material to replace PET containers as they’re broken down — material that currently comes from petrochemicals. The French team was interested in creating a circular PET process, in which existing material gets broken down in a way that allows it to be immediately reused to make new PET products. PET is a long collection of carbon rings linked by oxygen and carbon atoms. To break it down in a way that allows recycling, these carbon-oxygen links haven’t been broken, releasing a large collection of rings that can then be re-linked. The microbes that currently digest PET break down that ring as well, making them unsuitable for recycling.
To get the enzyme to work better on PET, the researchers looked up the cutinase structure and ran chemical simulations to figure out where PET would interact with the enzyme. They found it fit into a groove on the enzyme’s surface that included the location where the PET would be cut. So, to improve PET’s fit into this groove, the researchers created a large panel of mutant versions of the enzyme that, in different combinations, changed every single amino acid on the inside of the groove. While most of these nearly eliminated the enzyme’s activity, a few actually improved it and were used for further studies.
By combining all these changes, the researchers created two versions that they then tested on PET obtained by shredding drink bottles.
Cheap and effective
By combining all these changes, the researchers created two versions that they then tested on PET obtained by shredding drink bottles.
Cheap and effective
Given this source of PET, the original enzyme could digest about half in 50 hours. The researchers’ best modified version only needed (hours to hit) percent digestion. Optimizing the conditions, they were able to hit (percent breakdown of PET in under) hours. While there was still some crystalline PET left over, they found that they could take 1, (0kg of PET waste and produce kg of raw materials from it. Put in different terms, their redesigned enzyme is more efficient at digesting PET than our digestive enzymes are at breaking down starches.
They then used this raw material to make new PET products using standard industrial reactions. The new product’s ability to withstand pressure was only 5 percent off from the value measured for PET made from standard chemical sources. Appearance wise, it was within 19 percent of the PET produced the regular way.
How much would using recycled PET cost compared to starting with petrochemical feedstocks? The authors estimate that, if the protein can be made for about $ a kilogram, then the cost of the process will end up being about 4 percent of what you can get with for the PET made from it. While that might not be as cheap as petrochemicals — especially now, after oil prices have collapsed — it’s going to be relatively immune to future price shocks and is far more sustainable.
Nature,
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