This article presents enzymatic recycling aiming to solve the plastic waste crisis, introduced here.
Enzymatic recycling: a solution for opaque PET(?) and the story of the plastic-eating caterpillars
In France, opaque PET is rapidly replacing HDPE in the manufacture of milk bottles due to its lighter mass and cheaper cost. Opaque PET is manufactured by the addition of an opacifying agent, such as titanium oxide. This additive, which turns the plastic opaque, also makes it impossible to recycle on its own! Opaque PET can be recycled mixed to other PET-based products (up to 15% in weight) but then reduces the overall recycling quality. Adding an extra sorting step to existing recycling plants cannot be achieved at an affordable cost.
However, a new concept to address these sorts of problems has arisen: enzymatic recycling. Is it a credible solution? To form an opinion, we need to first understand what it is, when it is used, and what kind of potential it has.
What is an enzyme?
An enzyme is a protein – a natural polymer made up of amino acids. Enzymes are present in abundance in all living creatures, and act as catalysts: they facilitate many of the chemical reactions that take place inside of us.
Enzymes are highly specialised. An enzyme will ‘recognise’ a single protein or molecule, termed a substrate and will only permit a single type of chemical reaction to occur. An enzyme placed in the presence of the wrong reactants would be completely inactive. It is possible to modulate enzyme activity by using certain molecules to either accelerate, or slow down, the turnover of the enzyme. Modulation can also be achieved by adjusting the conditions of the chemical reaction, such as temperature and acidity.
Reactions catalysed by enzymes typically fall into two categories:
- Either they catalyse the breaking of a chemical bond, where the enzyme acts as a “molecular scissors” (see to the left of the figure shown below);
- Or they catalyse the opposite reaction, catalysing the formation of a chemical bond, where the enzyme acts as “molecular glue” (see to the right of the figure shown below).
Enzymes have the useful property of being reusable, provided that they are used and kept in the correct conditions (which is dependent on the enzyme in question). If these conditions are preserved, an enzyme can carry out the same chemical reaction repeatedly (multiple millions and billions of times). Enzymes eventually stop catalysing reactions when they unfold and lose their activity. This unfolding is inevitable, and can be sped up, particularly if it is hot. However, it is possible to produce artificial enzymes that are more resistant to unfolding, and that therefore have a longer shelf life.
Enzymes can be designed to recognise and cut a given type of bond in a polymer, resulting in extremely efficient depolymerisation. The resultant monomeric products from the depolymerisation can then be separated thanks to a classic technique frequently employed in chemistry called chromatography. This technique uses a column containing a porous material to filter the contents. The porous material allows the smallest molecules to pass through faster than the larger molecules, enabling the products to be separated by size. The porous material can also be combined with a specific chemical glue, which will strongly retain certain molecules over others (chemical affinity).
What are the limits?
The selectivity that enzymes display for chemical bonds is also an inconvenience, since one single type of enzyme cannot exist that is uniquely able to degrade all plastics. Each type of plastic requires the custom design of an enzyme and, if required, reinforcement to enhance its resistance against unfolding.
If we consider that enzymatic recycling does indeed provide a clean and robust solution to plastic pollution, then to reach a point where all the parameters have been optimised – from design to production of the enzyme – would require a substantial investment in research and development. What’s more, there is no guarantee that an efficient enzyme for each type of plastic will be found.
Where are we practically speaking, with regards to enzymatic recycling of plastics?
The French enterprise Carbios is trying to pioneer the first industrial enzymatic recycling product on the market. In collaboration with TechnipFMP, the enterprise is working on an enzymatic solution to recycle PET, regardless of type and additive contained, and has announced that it will reach the market by 2020. It therefore presents a potential solution to the problem posed by opaque PET.
Thanks to the specificity of enzymes, this technique could also allow the extraction of PET in multilayered polymers and thus make it possible to recycle at least part of the multilayer. In the long term, this could result in a reduction in the need to sort plastic waste.
Source : Carbios
Carbios specialises in harnessing enzymes for ecological purposes. The company has also developed a biological method to produce polylactic acid (PLA), with the aim of replacing the more expensive traditional methods to manufacture the plastic, as well as an enzyme enriched PLA that biodegrades in a programmed manner.
Follow the trail of the plastic-eating caterpillars
Earlier this year, reports were published documenting the discovery of a plastic-eating caterpillar. Indeed, the larvae of Galleria mellonella made the front page of scientific journals when its ability to “eat” PE bags was discovered during a collaboration between the University of Cambridge (UK) and the University of Cantabria (Spain).
Photo credit: Federica Bertocchini, Paolo Bombelli, and Chris Howe
One hundred of these caterpillars were shown to be able to consume about 92 mg of polyethylene (PE) plastic within 12 hours. While this rate may seem a little on the low side (when compared to the quantities of plastic waste produced daily), it is the fastest known way of degrading PE. In 2016, a bacterium able to breakdown PET at a rate of 0.13 mg per day was discovered; the larvae of Galleria mellonella can do this at a rate of around a thousand times faster. It’s also worth keeping in mind that most plastics will persist in landfill for several hundred years.
The fact that this caterpillar can eat PE is perhaps not too surprising: in the wild, this caterpillar will find its way into beehives and feed on the beeswax. This feeding habit is why Galleria mellonella is also known as the “Honeycomb Moth”. The ability of the caterpillars to ‘eat’ PE can be explained by the similarity in chemical structure between PE and beeswax.
But how are these caterpillars relevant to this article, you ask? Well, the structural similarity has led the scientists involved in the research to hypothesise that an enzyme must be responsible for the larvae’s ability to degrade PE plastic.
Practically speaking, it is not possible to breed these caterpillars in sufficient quantities to release them into landfills – they would then spread in an uncontrolled way, posing a significant risk for bees, an already endangered creature. The only way that this discovery could be used on an industrial scale for the biorecycling of plastic waste is by identifying, extracting, and understanding the enzyme responsible for the degradation of PE.