Enzymatic PET recycling of clothing and carpets reduces pre-treatment or is cheaper than petroleum manufacturing- Jiaxing Fuda Chemical Fiber Factory

Senior Researcher at the National Renewable Energy Laboratory (NREL) in the United States The reality is that most PET products, especially PET clothing and carpets, are not being recycled using traditional recycling techniques today. The research community is developing promising alternatives, including enzymes aimed at depolymerizing PET, but even these choices often rely on high energy consumption and high cost pre-treatment steps to be effective

Researchers Jaffes Gardo (left), Erica Erickson (right), and colleagues have discovered and characterized enzymes that degrade crystalline PET, a plastic used in disposable beverage bottles, carpets, clothing, and food packaging.

Therefore, most of the PET produced today ultimately enters landfills or the environment - even PET products that actually enter recycling stations.

Nevertheless, Beckham said that things are rapidly changing, and advanced methods in machine learning and synthetic biology have given scientists an unprecedented understanding of the fundamental biology of PET deconstructive enzymes. Recently, Beckham and his colleagues at the University of Portsmouth and Montana State University utilized these methods to discover new enzyme variants, which are expected to deconstruct the most challenging PET without additional pre-treatment.

This not only means that we are at the forefront of enzyme recycling for all forms of PET, including carpets and clothing - it also means that recycling PET may soon be cheaper than manufacturing PET from scratch with oil.

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Enzymes hidden in soil

The concept of enzyme recovery in PET has been known since 2005, but after Japanese scientists made astonishing discoveries, it made its debut on the world stage in 2016. Buried in the soil outside a recycling facility in Japan, an enzyme they call Ideonella sakaiensis quietly secretes to break down scattered old plastic beverage bottles.

Nature provides a great solution for breaking the chemical bonds of PET. For some reason, nature is demonstrating how to reduce PET bottles to their basic components: terephthalic acid and ethylene glycol.

A series of studies followed. Scientists are attempting to enhance the enzymes used in industrial technology to process millions of tons of PET produced annually. They assume that if improved, the enzyme recycling platform can completely change today's underperforming recycling systems, reduce energy and greenhouse gas emissions, and promote a circular economy for all PET products - even carpets and fabrics that cannot be recycled using traditional technologies.

As researchers realize the potential of using enzymes to break down plastics, new papers from around the world have lit up scientific literature, "said John McGeehan, a scientist at the University of Portsmouth (UoP) team in the UK. Experts from different fields such as pharmaceuticals and biofuels can reuse decades of research experience to modify enzymes

The enzyme recovery platform of NREL/UoP company effectively decomposes PET plastic raw material (left) into its chemical structural units. The PET sample on the right decreased in mass by 97.7% after being hydrolyzed by enzymes from NREL/UoP company.

In fact, in a series of frequently cited papers published between 2018 and 2021, Beckham, McGihan, and their colleagues characterized enzymes that increased their efficiency by a factor of six and analyzed the environmental and economic impacts of industrial-scale PET recycling. The corresponding 2022 paper evaluates the impact of life cycle on future enzymatic PET recovery systems.

These studies have yielded remarkable results. By processing PET in a bioreactor for 48 hours, they showed that it is possible to convert nearly 98 percent of the plastic into terephthalic acid and ethylene glycol – high-quality recycled building materials used to make new PET bottles, and even advanced plastics that are easier to recycle.

"Working in a field that is growing so rapidly is an incredible experience," McGihan added. "We are reaching a point where collaborative science has tremendous potential to accelerate the large-scale development and roll-out of enzyme-based solutions."

Despite rapid progress, a key barrier to industrial-scale enzyme recycling looms.

This enzyme is only effective against a small percentage of PET products — those made from "amorphous" PET. Without first softening them with high temperatures and extra energy, they worked hard to deconstruct the very common, durable varieties of PET "crystals." Crystalline PET accounts for nearly 90% of all PET products, including polyester fibers in clothing and some single-use beverage bottles.

Scientists need enzymes that are better at breaking down crystalline PET, and hopefully they can find other plastic-eating enzymes hiding in the dirt.

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Use machine learning to "mine" new enzymes

Fortunately, the team doesn't need a shovel to mine for new enzyme varieties, and advances in bioinformatics and machine learning have made it possible to look for various enzymes active on crystalline PET in a vast database of existing enzyme sequences.

"Traditional methods of finding new plastic-eating enzymes from databases may be ineffective because enzymes that are very similar in chemical composition do not necessarily retain plastic destructuring activity," said NREL computational scientist Japhaus Gado.

To address this challenge, Gado built a statistical model to learn the biological rules of known plastic breakdown enzymes. The model assigns probabilities to the unique composition of the enzymes studied so far. Gado also built a supporting machine learning model to predict the heat tolerance of enzymes, which is important for industrial applications.

DeepMind's 3D rendering revealed unexpected structural features, such as the enzyme 611 in the figure. Careful analysis of the structural signatures of proteins like enzyme 611 could help the team improve their performance.

Together, these two computational models allow Gado and his colleagues to project into uncharted territory. In less than an hour, they screened more than 2 million proteins, creating a short list of promising candidates. Further testing confirmed that 5 were able to deconstruct PET, 36 that had not previously been described in the scientific literature.

Importantly, some are even better at breaking down crystalline PET than amorphous PET.

"These new enzymes are not only genetically diverse," Gado explains. "They have different structures and different geometries of the active centers."

Gado can confidently talk about the structure of the 24 new enzymes because he's seen them look like — at least in the 3D renderings provided by researchers at DeepMind, an Alphabet subsidiary. Known for mapping the "whole protein universe," DeepMind characterized these enzymes with its deep learning tool, AlphaFold, so the team could compare the enzymes side-by-side and notice their differences.

All tools have the ability to deconstruct PET, but there are a few that look strikingly different. According to Gado, DeepMind's renderings provide valuable clues as to how plastic deconstructases act on PET.

"State-of-the-art AI methods help us find patterns in enzyme data, which will enhance our understanding of what makes good plastic edible enzymes," Gado added. "This will allow us to improve enzymes with protein engineering and find other enzymes in nature that perform similarly."

This is another step forward for an already prolific research team and another step towards large-scale PET recycling.

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Cheaper and more environmentally friendly

The analysis quantified the advantages of enzymatic PET recovery

According to Beckham, cleaning, shredding and heating – the steps required to prepare for the decomposition of PET – are among the most important sustainability drivers for industrial-scale enzyme recycling facilities.

"Minimizing these pretreatment steps is critical to making enzyme recovery costs competitive compared to producing PET resin from petroleum," he explains.

Scientists at NREL University and UoP have developed a cost-effective, environmentally friendly enzymatic platform that can quickly break down post-consumer PET into identical chemical building blocks, terephthalic acid (TPA) and ethylene glycol (EG).

In subsequent experiments, the team noticed that some enzymes labeled by their machine learning method were equally effective at breaking down crystalline and amorphous PET. These enzymes do not require pretreatment at all to help soften the binding of plastics.

"By eliminating pretreatment, the technology enables industrial-scale PET recycling, which is actually cheaper than using petroleum to produce virgin PET," he saidBeckham added. "Even better, it can reduce associated energy and greenhouse gas emissions."

In an earlier article published in Joule, in 2021, the team had quantified the economic and environmental advantages of using active enzymes on crystalline PET. In industrial-scale facilities, doing so can reduce supply chain energy demand by 45% and life-cycle greenhouse gas emissions by 38% compared to systems using pretreatment.

The economic advantages are equally impressive. When discarding PET carpet and clothing — which can't be recycled with conventional techniques — they can also produce terephthalic acid for less than $1 per kilogram. Petroleum-derived terephthalic acid has historically been sold for $1 to $1.50 per kilogram.

"Our enzyme platform creates an economic incentive to clean our oceans," said Erika Erickson, a former NREL postdoctoral researcher who conducted much of the experimental work behind these studies. "At such price points, PET contamination can be affordably recycled into new PET products or find new uses in wind turbine blades or carbon fiber bumpers."

Post-consumer PET products, often today's source of pollution, can be turned into valuable resources to support a more environmentally sustainable plastics economy.

It's not hard to imagine how this would change the story on plastic: PET recycling costs so low that economics favors throwing it in the recycling bin rather than the trash. A T-shirt, a rug, a soda bottle – everything is put in and as a building block, start their circular journey to create a cleaner, greener world.