By tweaking an enzyme known for breaking down plants’ waxy coatings, scientists have come up with an environmentally friendly way to recycle poly(ethylene terephthalate) (PET)—the popular plastic found in soda bottles and many types of food packaging. The method efficiently cleaves PET into its constituent monomers, which can then be reused to build the plastic all over again.
More than 60 million metric tons of PET are produced each year, much of which is not recycled. The most common recycling technique, a thermomechanical process, produces material with mechanical properties that are inferior to those of virgin PET.
As an alternative, scientists have been looking to enzymes to pull PET apart. But these processes have been slow and low-yielding. Now, scientists are reporting success with an engineered version of an enzyme microbes use to break down plant material. Their tailored leaf-branch compost cutinase enzyme can break down 90% of PET in less than 10 h (Nature 2020, DOI: 10.1038/s41586-020-2149-4), report researchers led by Alain Marty of Carbios and Isabelle André and Sophie Duquesne of the Toulouse Biotechnology Institute.
The work “shows the potential use of PET hydrolases to treat the waste PET bottles piled on the globe,” says Kohei Oda, a scientist at the Kyoto Institute of Technology who reported a PET-eating bacterium in 2016. What’s more, he adds, it should work “on an industrial scale and establishes technology for a circular economy of PET bottles.”
Others previously reported that the leaf-branch compost cutinase enzyme could break down PET. The Carbios-Toulouse team wanted to make it better at the job. They started by modeling the enzyme with PET in its active site so that they could observe which amino acids were important for recognizing the polymer, André explains. They then located spots where this recognition could be improved. By randomly optimizing those amino acids, they were able to enhance PET binding, she says.
The researchers wanted the enzyme to work at 70 ⁰C—the temperature at which PET goes from a rigid state to a more flexible one. They reasoned that this would be the best temperature for depolymerizing the material. So the team boosted the enzyme’s thermal stability by replacing its calcium ion sites with disulfide bonds.
The enzyme only works with amorphous PET, not the crystalline PET that’s used in bottles and packaging, so the scientists have created a process for converting the crystalline plastic to its amorphous form, enabling it to be recycled. And they showed that the monomers generated by the enyzmatic process were a more valuable starting material than the material generated by the conventional thermomechanical recycling process: the team was able to make the monomers into high-quality crystalline PET bottles.
Gert Weber, an expert in plastic-degrading enzymes at the Helmholtz Center Berlin for Materials and Energy, notes that new engineered enzyme “outperforms all known enzymes employed for PET depolymerization by far.” This finding, he says, will enable enzymatic PET recycling on an industrial scale and suggests that it’s possible to use enzymes to recycle other synthetic polymers, like polyamides or polyurethanes.
At the moment, Marty says, the technology is being used in a pilot-scale plant, but there are plans to create a facility that can process as many as 90,000 metric tons of PET by 2025.