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Tired of tire waste

In 2021, 274 million tires were scrapped in the USA alone, with 17 percent of them ending up in landfill: That’s 1 billion pounds of tire material, wasting space and posing potential threats including chemical leaching and auto-ignition.

One third of scrapped tires are chemically recycled through pyrolysis, but this process also presents serious health and environmental concerns due to its byproducts. Researchers at University of North Carolina at Chapel Hill, however, have developed a new chemical process that deconstructs used rubber into useful building blocks for new materials without relying on harsh conditions or toxic byproducts.

Their recycling method works at lower temperatures and uses water-based conditions to break the material into smaller, soluble fragments. These can then be used to make epoxy resins for coatings, adhesives and even aerospace components. When tested, they were just as strong as their commercial counterparts.

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Building blocks of sustainability

Since their discovery in the early 20th century, polymeric materials have revolutionized many aspects of our lives. Perhaps the most recognizable polymers in our daily lives are plastics.

Despite their enormous value, we produce more plastic than we recycle, and this is becoming a major environmental challenge. The figures are staggering: Just 9 percent of the global supply of plastic is recycled. Most plastic produced is incinerated or placed in landfills, leading to pollution. A significant amount of plastic waste is also found in the seas, creating not just an eyesore but damage to aquatic life and marine ecosystems.

At Khalifa University, Sharmarke Mohamed and his team at the Advanced Materials Chemistry Center (AMCC) are developing a new method for recycling post-consumer plastic waste that uses a combination of mechanical force (as part of mechanochemistry), light and catalysts.

The value of this technology is that it uses no corrosive or harmful chemicals.

– Sharmarke Mohamed, Khalifa University

While mechanical methods are common as a means for reducing the size of plastics prior to recycling, it is not possible to apply this for the depolymerization of most plastic waste. Instead, the researchers are looking for ways to perform low-cost recycling using a range of stimuli.

“Despite the enormous environmental challenge posed by plastic waste, we felt a sense of duty to develop these new mechanochemical tools. Most researchers around the world are exploring mechanical force as a means to build new chemicals. In other words, building complexity from simple structures. We decided to use the same principles and use mechanical force as well as light and catalysts to break down complex polymer waste materials into smaller building blocks that can then either be recycled or upcycled,” he says.

“Solar energy is responsible for the photodegradation of plastics in the environment, particularly in the UV region of the electromagnetic spectrum. We also know that some biological catalysts (e.g. enzymes) are adapted to using organic macromolecules such as plastics as fuel sources. So in essence, we are learning from nature as we try to develop a lab-scale protocol that uses these tried-and-tested methods for turning plastic waste into high-value chemicals,” Mohamed says.

“As the UAE declares 2023 to be the Year of Sustainability, our research group is very much leading this effort in a challenging area. But we are motivated by solving the environmental challenges posed by plastic waste,” Mohamed says.

About 380 million metric tons of plastic are produced each year. Of that, only about 9 percent is recycled, Mohamed tells the KUST Review. Some plastics are treated with harsh chemicals, like acid. But most plastic is incinerated, he says.

“But the problem (with incineration) is that it releases carbon dioxide and adds to the global carbon footprint. The other problem is that if you burn the plastic you can’t reuse it. Our group is trying to take the end-user plastic and come up with new low-cost mechanical methods that are able to break down these polymers into their constituent parts.”

Those constituent parts might then be reused to make new plastic products or chemicals for other uses.

Mohamed’s team is working on a three-year project to investigate a three-part process for recycling plastics. This research is supported by AMCC and funded by ASPIRE, the technology program management pillar of Abu Dhabi’s Advanced Technology Research Council (ATRC), via the ASPIRE Award for Research Excellence.

The first part involves mechanochemistry: using mechanical energy to induce the chemical depolymerization of the plastic waste.

“Mainly we use ball mills to grind the polymers in the presence of proprietary chemicals we are developing in our lab. This leads to the polymer essentially breaking down and releasing its constituent building blocks, known as the monomers. Preliminary results in our lab suggest this process can be done under ambient conditions in the solid-state with yields of up to about 70 percent or higher,” he says.

We are trying to think outside the box and look at the problem from a non-conventional perspective using a mechanocatalytic approach.

– Zeinab Mohamed Saeed, Khalifa University

The value of this technology is that it uses no corrosive or harmful chemicals, which is important as it makes the entire process much more environmentally friendly than incineration or land-filling the plastic waste.

The next step is to examine the influence of light on the process, followed by experiments with inorganic catalysts (i.e. metal salts) or enzymes to break down the plastics.

“Once we understand each of these processes on their own, we can see how they can be stitched up together to create what we refer to as a photolytic and mechanoenzymatic degradation (PMED) protocol. We envisage the PMED process will be implemented serially as part of a batch process, much like a conveyor belt in a factory. Our long-term goal is to take post-consumer plastic waste and to efficiently produce the chemical building blocks of the plastic waste via our PMED process.”

Different forms of plastic break down in different ways under mechanical force, complicating the process, Mohamed says. But he says the initial work is promising.

Zeinab Mohamed Saeed, a Ph.D. candidate working on the project, says she’s excited by the non-conventional approach to a long-standing problem.

“The field of polymer degradation was there for decades,” she says. “People have been trying to come up with different ways to tackle the issue using their expertise, and now we are trying to think outside the box and look at the problem from a non-conventional perspective using a mechanocatalytic approach. I find this research challenging but exciting, and can’t wait to see what kind of results we will end up with.”

Among the challenges, however, is creating vessels that can hold the material but also allow in light of a certain wavelength. And the enzymes known to break down plastics are expensive.

The hope, however, is to scale up the technology to levels required by industry. That’s still some time off, however.

“Now we can do up to a gram or two. This is fine for feasibility and patenting,” Mohamed says.

The Advanced Materials Chemistry Center (AMCC) was formed in 2022 and combines expertise from different disciplines to tackle major environmental problems. Its methods for treating plastic waste “align with the UAE’s ambitions to transition to a green circular economy and achieve its net-zero targets” Mohamed says.

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The bugs that eat plastic

By some estimates there are more than 8.3 billion tons of plastic on the planet – more than 6.3 billion tons of that is waste. Recycling isn’t an option for all of it. But scientists around the world are looking at organic solutions in the form of hungry bugs and the enzymes and bacteria they produce.

Among them: Dr. Chris Rinke and a team of researchers at Australia’s University of Queensland in 2022 published a study in Microbial Genomics about their work with the larvae of the darkling beetle Zophobas morio.

It found that the so-called “superworms,” which normally feed on such decaying material as dead leaves and animal carcasses, could survive on polystyrene alone. Most are able to complete their transition to adult beetles on just a diet of the synthetic resin commonly used for such items as disposable cups and surfboards.

“Our understanding is that superworms mechanically shred the polystyrene, ingest it, and then the bacteria in the worm’s gut further degrade the plastic. We found several encoded enzymes associated with polystyrene degradation in the gut bacteria,” Rinke tells KUST Review, adding that the team is also looking into the degradation of such other thermoplastics as polyethylene and polypropylene.

And, sure, your local waste-reclamation facility might set up a giant worm farm to decompose unwanted polystyrene, but Rinke tells NPR it  would be cheaper and easier to reproduce the enzymes that allow the larvae to digest, say, old dishwasher parts and packing material. A synthetic “enzyme cocktail” could be sprinkled over shredded waste. Add microbes to the material and you could create useful and more sustainable bioplastics.

Rinke cautions that it will take a while before the enzymes are available for industrial use.

“It will take sufficient research funding and several years of research to characterize the enzymes involved in polystyrene degradation, but once we have found the most efficient enzymes, we can offer a biological solution to degrade plastic waste,” he says.

In the meantime, he encourages consumers to avoid plastic, “especially single-use plastic packaging, whenever possible,” he tells KUST Review.

“If plastic needs to be used and eventually becomes waste, then one should recycle plastic waste as much as possible. Last but not least, it’s also important to ask local councils to increase the amount of plastic recycling,” he says.

ANOTHER HUNGRY, HUNGRY CATERPILLER

But the Zophobos morio isn’t the only insect bellying up to the plastics buffet.

Researchers in Poland published their results on a study of Tenebrio molitor in the journal Polymers.

The researchers fed the insect – commonly called a yellow mealworm and another species of darkling beetle – a diet of polystyrene foam (PS), two types of polyurethane (PU1 and PU2, like kitchen sponges and commercial insulation foam) and polyethylene foam (PE, commonly used in packing materials).

The researchers concluded that genetic variances among mealworm populations could account for different rates of consumption, but say 1 kilogram of PS, PU1, PU2 and PE could be consumed over 58 days by 40.5 kg, 46.0 kg, 36.5 kg and 30.9 kg of Z. morio, respectively.

FROM PEST TO PROMISE

The Polish researchers mention other plastivore species, including Galleria mellonella, a wax moth whose palate for plastics was discovered accidentally when a researcher put the caterpillars in a plastic bag and found later that they had eaten holes in it. The information that resulted was featured in a recent study from Brandon University in Canada.

The moth caterpillar larvae, which normally invade beehives and eat wax, can digest polyethylene – the kind of plastic found in shopping bags – and excrete ethylene glycol, a form of alcohol that can be used as antifreeze.

In the study, 60 waxworms consumed 30 square centimeters of the plastic in less than a week. The researchers published their results in Current Biology.

Although the waxworms can consume the plastics on their own, researchers also isolated an intestinal bacteria from the larvae that was able to survive on polyethylene as its sole source of nutrition for a year. Working together, the waxworms and the bacteria accelerate plastic biodegradation. Researchers caution, however, that the waxworms and their bacteria aren’t a solution to the plastics problem but point to possible future directions for waste management.

ENTER MICROBES

Different kinds of bugs – not insects but microbes – are also emerging as potential solutions to the world’s plastics-waste problem.

Researchers in 2016 discovered a bacterium in a Japanese garbage dump that had evolved naturally to eat plastic, and when they tweaked a promising enzyme to see how it evolved, they accidentally made the molecule even better at breaking down polyethylene terephthalate, the plastic used in soft-drink bottles.

But more recently, a group of scientists in Sweden has found that microbes around the world are evolving to eat the plastic trash that has found its way into mountain peaks, ocean depths and remote tropical beaches. They published the results of their study, the first to assess the global potential of plastic-eating microbes in mBio.

Scanning 200 million genes, the researchers found 30,000 enzymes that could degrade 10 kinds of plastics.

The number and type of enzymes they found corresponded to the amount and type of plastics in their locations. One in four organisms examined carried an enzyme that could break down plastics.

“We did not expect to find such a large number of enzymes across so many different microbes and environmental habitats. This is a surprising discovery that really illustrates the scale of the issue,” Chalmers University researcher Jan Zrimec says in the Guardian.

The remarkable thing about these microbes and insects is that plastics are man-made and, in evolutionary terms, quite recent, says Khalifa University’s David Sheehan. “Yet microbes clearly have evolved enzymes that can degrade them in a short period of evolutionary time. If we can identify a panel of these enzymes, we could use enzyme engineering approaches to improve their activity and substrate range and produce these commercially much as we do with biological detergents.”