Microwave breakthrough accelerates plastic recycling

The world is drowning in plastic bottles, and the consequences are worrisome. An insatiable demand for plastic packaging, especially the persistent polyethylene terephthalate (PET) bottles, has resulted in mountains of plastic waste. Shockingly, much of this plastic goes unrecycled, polluting our oceans and endangering wildlife. A new method, combining microwave irradiation with specialised enzymes, has the potential to change the management of plastic waste.

“The key current strategy in this field is to generate thermally stable enzymes for plastic recycling, enabling degradation at high temperatures. We are taking a different approach – developing a method for recycling at room temperature, making this process much more energy-efficient,” explains Per-Olof Syrén, Associate Professor at the Division of Coating Technology, KTH Royal Institute of Technology, Stockholm, Sweden.

78% yield for conversion

Historically, plastic recycling primarily relied on mechanical processes, limiting its effectiveness to clean, single-type plastics. The advent of biocatalytic degradation, which uses enzymes to break down plastics, has shown promise. However, synthetic plastics such as PET present tough challenges because of their complex structure and high crystallinity.

“Pretreatment is essential in biocatalytic plastic recycling. We drew inspiration from nature’s solutions for tough biomass materials, such as lignocellulose from plants. We are using a more refined microwave pretreatment method to enhance accessibility and control material conformation. The microwave irradiation softens PET’s recalcitrant structure.”

In their experiments, PET bottles served as models. After subjecting the plastic to optimised microwave pretreatment, Per-Olof Syren and colleagues introduced engineered enzymes called PETases. This strategic combination resulted in a remarkable 1400-fold increase in the release of terephthalic acid, a critical PET breakdown product.

“This approach significantly enhances the accessibility of PET to the enzymes, setting the stage for efficient recycling. Microwave irradiation appears to create shorter, more manageable PET chains that align favourably for enzymatic degradation. After just 2 hours of microwave pretreatment followed by 1 hour of enzymatic reaction at a mild 30°C, we achieved 78% yield for converting PET.”

Exclusive conformations

The mechanism of microwave irradiation is still not completely understood. However, the hypothesis is that hydrolysis during pretreatment crystallises the plastic material, making it more accessible for enzymes to degrade or recycle.

“The pretreatment method actually works by hydrolysis, and it hydrolyses the amorphous parts first, which generates a very crystalline material in the end. So, the material actually recrystallises. What happens is that we end up with something that is still polymers but much shorter plastic polymers, and the enzymes can more easily degrade these shorter chains.”

To reuse the plastic, the long plastic polymer chains need to be broken down into single monomers, which, like LEGO blocks, can then be reassembled into new long plastic chains. There are, however, different types of blocks – trans and cis – and some enzymes can only pull apart or break down one type of block. Previous recycling methods often had a mix of these, making recycling challenging.

“We followed the degradation in real time by nuclear magnetic resonance (NMR) spectroscopy to analyse in detail the reaction outcome because this can help to identify the types and quantities of monomers produced.”

NMR turned out to be a powerful tool in assessing the efficiency of the process and guiding the optimisation of the new recycling methods.

“The microwave treatment leaves the monomers almost exclusively in the trans conformation. And that is very good because we created an enzyme that is highly active for trans.”

Difficult to get the industry to adopt

The new results suggest that microwave-based pretreatment might be a simple and effective method for enhancing enzyme-catalysed recycling of plastic since it efficiently shortens the polymer chains of PET and makes them adopt favourable conformations that are more accessible to the enzymatic hydrolysis reaction. Although the new research results might mark a fundamental breakthrough, significant challenges still remain.

“Microwave irradiation as a pretreatment step has the potential to fast-track biocatalytic plastic recycling, bringing us closer to a world in which plastic waste is a thing of the past. The future envisions scaling up this technology for industrial recycling processes. But scaling up microwave reactor technology is a challenge, and we need versatile enzymes to handle different types of plastic.”

Some start-ups are already mainly focusing on chemical recycling, which works well with some polyester plastics. So Per-Olof Syren is not worried about getting the industry to adopt that part of the process.

“I would say that recycling would probably be less conservative than the actual polymer synthesis. But if we want to make a new smart material based on the products from the recycling process or materials that are easier to break down, getting the industry to adopt this is very difficult because they are used to making specific products already and are making money from that. So why should they suddenly start making completely new molecules, meaning that they have to change their whole process?”