Processing plastics for recycling
Current technology in plastic recycling can’t handle most of the plastic waste we generate. Biological plastic recycling is on the horizon. Scientists are looking into plastic-eating bacteria. It’s the enzymes in bacteria that do the work.
Plastics are kinds of polymers, chains of monomers. Remember that “mono” means “one” and “poly” means “many.” Think of “mer” as meaning “of something. A monomer is one of something. A polymer is at least ten of something. Enzyme plastic recycling must reduce polymers to monomers.
Scientists have been exploring enzymes made by bacteria, fungi, and other creatures hardly anyone else thinks about. Enzymes break down polymers naturally. Plant cellulose is a polymer, for example. What works for natural polymers ought to work on plastics (man-made polymers). But first, we need the right enzymes.
Unfortunately, the press often makes it look like microbes will be eating plastic before we know it. They may start chomping on one kind soon enough. Otherwise, we’ll have to wait for a long time. Here are some developments over the past few years.
A new enzyme for recycling PET plastic
In 2016, scientists working in Japan discovered a new bacterium in near a plastic recycling plant. It had started to use PET plastic as a food source. It degraded low-quality PET in six weeks. The grade of PET used in plastic bottles took longer. It was not the first plastic eating organism ever discovered. But it was by far the fastest acting.
John McGeehan of the University of Portsmouth led the study. His team looked at an enzyme the bacterium produced. Its structure resembles another enzyme. Many bacteria use that one to digest cutin, a polymer many plants make.
They altered the plastic-eating enzyme to study the connection. And unintentionally enhanced it to make it work about 20% faster. That’s only a small improvement, but it may be possible to enhance it even more. It also showed the possibility of improving the action of other enzymes.
McGeehan expressed the hope of making an enzyme to turn the plastic back into basic monomers.
Current technology merely chops and washes various kinds of plastic. Plastic bottles recycled that way can only be made into opaque PET. It’s suitable for making fabric, but not new bottles. If the new enzyme can reduce it to monomers, they will be suitable for making bottles. That, in turn, will make it possible to make less virgin PET. What’s more, it will reduce the need for new crude oil.
Recycling PET at industrial scale
Plastic-eating bacteria
Carbios, a French startup, has announced it has plastic-eating enzymes that can depolymerize pulverized PET waste in a matter of hours. Even the high-quality PET used to make bottles.
In fact, Carbios claims its biorecycling process can use any PET. It reduced a mixture of old polyester and PET food trays to monomers. Current recycling methods can’t remove dye from PET, but the enzymes do. So these monomers can make clear bottles. They are as good as the ones made from oil.
A consortium of large corporations, including Nestlé and PepsiCo, support Carbios with both money and expertise. Nestlé has pledged that half of its packaging in the US will come from recycled plastic by 2025.
As things stand now, the supply of recycled plastic can’t meet demand from all the companies that want to use it. The enzyme process, if successfully scaled up, will make more of the waste stream useful for recycling.
Enzyme recycling of polyester fabric
Researchers at Queensland University of Technology have made a related announcement. They found an enzyme that separates wool from wool-polyester blends. This enzyme works on the wool, not the polyester. That means it doesn’t damage the polyester. The recovered polyester can make new fabric or anything else that uses polyester.
Just as with PET, manufacturers need more recycled polyester than the market supplies. For example, Adidas has made a bold promise. It intends to use only recycled polyester and other plastics by 2024.
More plastic-eating organisms
Since the discovery in Japan, at least two more plastic-eating organisms have been discovered. A new fungus in a Pakistani dump and a new bacterium on polluted land in Houston. They make new, unknown enzymes. The engineering company Cambridge Consultants has begun to look for ways to manufacture similar plastic-eating enzymes.
They are not alone. Research projects all over the world seek enzymes that will work not only with PET, but other plastics. Perhaps at some time in the future, it will be possible to use enzymes outside of recycling facilities. That is, apply them directly to litter, or even in the ocean.
Of course, using new enzymes in nature will also require proof that they will do no harm.
It’s too early to cheer for plastic-eating bacteria
Bacteria: Scanning electron micrograph of two staph germs
It’s too soon to cheer for biological plastic recycling. So some microbes have evolved to add PET to their diet. That doesn’t mean that biological recycling of any other kinds of plastic will happen any time soon. Enzymes naturally degrade the kind of chemical bond PET has.
Consider two other common plastic wastes. Polyethylene is used for plastic bags and milk jugs. Polystyrene is the plastic in Styrofoam
. These plastics have an all-carbon backbone that’s harder for enzymes to break. Even with PET, films degrade faster than the more crystalline bottles.
Here’s what’s happening now. Researchers may think at first that they have found a plastic-eating organism that degrades non-PET plastic. On closer inspection, it doesn’t change the chemistry. It simply breaks it down into microscopic particles.
For example, in 2017, the press eagerly reported a new study. It found that wax worm caterpillars had broken down polyethylene. The team tested the hypothesis by making a paste of polyethylene and mashed caterpillars. Its analysis indicated the breakdown of the product.
Four months later, however, another lab disputed the findings. Its analysis pointed to proteins in the caterpillars, not ethylene. The matter remains controversial.
In another case, some tests indicated that a bacterium in the gut of mealworms consumed polystyrene. Then another lab tested the research. That study suggested that it converted about half of the polystyrene into carbon dioxide. It didn’t identify any styrene in the remainder.
No monomer, no plastic-eating enzyme. If it exists, some lab will find it, and another will verify it. Scientific agreement that the enzyme works only starts the journey to biorecycling.
The slow pace of scientific progress
A little over a year ago, I wrote about an enzyme found in gribbles, a marine pest that eats wood. One research team had published papers in 2009, 2013, and 2018. Each one announced a new breakthrough in the attempt to use the enzyme to make biofuel. But each one involves understanding the chemistry better, not making and using the enzyme.
Biofuel comes from the cellulose in plants. Cellulose is a naturally-occurring polymer. Enzymes have the same task whether digesting a natural or synthetic polymer. Chemists have similar problems in either case.
First, they must find something that works. Then they need to make sure it doesn’t have unintended consequences that do more harm than good. Once they find a workable process, some technology company must find a way to scale it up for commercial use. Then it has to make the process economically competitive. That’s often the hardest part.
The scientists studying gribble enzymes have a head start on scientists studying biological plastic recycling. So far, they have learned a lot. But they’re a long way from finding a process they can pass on to the next step.
Someday, I’m sure, we’ll have both biofuel and enzyme plastic recycling. Just not anytime soon. In the meantime, the press will report each breakthrough as if success is just around the corner. Believe it when you see some company making good money from plastic-eating bacteria.
Sources:
Could these plastic-eating enzymes be the miracle solution to our plastic problem? / Adele Peters, Fast Company. October 3, 2019
Enzyme discovery could keep tonnes of polyester from landfill / Niki Widdowson, Phys.org. November 29, 2019
In this “biorecycling” factory, enzymes perfectly break down plastic so it can be used again / Adele Peters, Fast Company. October 17, 2019
Plastics recycling with microbes and worms is further away than people think / Carmen Drahl, Chemistry & Engineering News. June 15, 2018
Photo credits:
Plastic recycling process. Some rights reserved by Argonne National Laboratory
Plastic eating bacteria. Public domain from Wikimedia Commons
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