Nanotechnology

The future of recycling could one day mean dissolving plastic with electricity (w/video)

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July 05, 2023

(Nanowerk News) Chemists at the University of Colorado Boulder have developed a new way to recycle a type of plastic commonly found in soda bottles and other packaging. The team’s method relies on electricity and some nifty chemical reactions, and is simple enough that you can see plastic breaking before your eyes.

The researchers describe their new approach to chemical recycling in the journal Chemical Catalysis (“Electrically driven recycling of ester plastics using one-electron electro-organocatalysis”).

This study tackles the problem of increasing plastic waste worldwide. According to the Environmental Protection Agency, the United States alone produced nearly 36 million tons of plastic products in 2018. Most of the waste ended up in landfills, said study co-author Oana Luca.

“We pat ourselves on the back when we throw something in the recycling bin, but most of that recyclable plastic never ends up being recycled,” said Luca, an assistant professor in the Department of Chemistry. “We wanted to see how we could recover the molecular materials, the building blocks of plastic, so we could reuse them.”

In new research, he and his colleagues are one step closer to doing just that.

The group is focusing on a type of plastic called polyethylene terephthalate (PET), which consumers encounter every day in water bottles, blister packs and even some polyester fabrics. In small-scale laboratory experiments, the researchers mixed the pieces of plastic with a special type of molecule and then applied a small electrical voltage. Within minutes, the PET began to disintegrate.

The team has more work to do before its recycling tool can realistically address the world’s plastic waste problem. But it’s still fun to see trash, which can linger in trash heaps for centuries, disappear in a matter of hours or days, said lead study author Phuc Pham.

“It’s really great to observe the reaction progress in real time,” said Pham, a doctoral student in chemistry. “The solution first turns a deep pink color, then becomes clear as the polymer breaks down.”

(embed)https://www.youtube.com/watch?v=PTvwtrNOPYA(/embed)

Solutions containing tiny polyethylene terephthalate (PET) beads change color when electricity breaks down the plastic in new research led by chemists at CU Boulder.

One man’s trash

Luca says it’s a whole new way of thinking about the possibility of trash. Recycling bins, he said, might look like a good solution to the world’s plastic problem. But most cities around the world have struggled to collect and sort the tiny mountains of trash that people generate every day. The result: Less than a third of all PET plastic in the US is nearly recyclable (other types of plastic fall even farther behind). However, methods such as melting plastic waste or dissolving it in acid can change the properties of the material in the process.

“You end up changing the material mechanically,” says Luca. “Using current recycling methods, if you melt plastic bottles, you can produce, for example, one of the single-use plastic bags that we now have to pay for in grocery stores.”

Instead, he and his team wanted to find a way to use the basic ingredients of used plastic bottles to make new plastic bottles. It’s like destroying your Lego castle so you can take back the blocks to create new buildings.

Other people’s property

To accomplish the feat, the group turned to a process called electrolysis — or using electricity to break down molecules. Chemists, for example, have long known that they could apply tension to a beaker of water and salt to break the water molecules into hydrogen and oxygen gases.

But PET plastic is much more difficult to separate than water. In the new study, Pham grinded a plastic bottle then mixed the powder into a solution. Next, he and his colleagues added an additional ingredient, a molecule known as the salt (N-DMBI)+, to the solution. Pham explains that in the presence of electricity, these molecules form “reactive mediators” that can donate extra electrons to PET, causing the plastic granules to dislodge. Think of it like the chemical equivalent of sending a karate chop to a wooden plank.

Researchers are still trying to understand how exactly this reaction occurs, but they can break PET down into its basic building blocks — which the group can then recover and, potentially, use to make something new.

Deploying only tabletop equipment in their lab, the researchers report that they can break down about 40 milligrams (a small pinch) of PET over a few hours.

“While this is a good start, we believe there is still a lot of work to be done to optimize the process and improve it so that it can eventually be applied on an industrial scale,” said Pham.

Luca, at least, has some big picture ideas for the technology.

“If I wanted to be a mad scientist, I would use this electrochemical method to break down many types of plastic at once,” says Luca. “That way you can, for example, go to a big pile of garbage in the ocean, pull all that waste into a reactor and get a lot of useful molecules back.”



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