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Electron ‘bullets’ and bacteria to turn unrecyclable plastic into useful materials

Professional Engineering

Iowa State's Xianglan Bai is leading the two projects that aim to convert waste plastic into useful materials (Credit: Christopher Gannon/ Iowa State University)
Iowa State's Xianglan Bai is leading the two projects that aim to convert waste plastic into useful materials (Credit: Christopher Gannon/ Iowa State University)

Electron ‘bullets’ and bacteria will turn previously unrecyclable plastics into useful materials, according to researchers behind two projects.

Aiming to produce useful materials for construction, or simply to make the low-quality plastic waste biodegradable, the projects are led by Xianglan Bai, an Iowa State University associate professor of mechanical engineering. They are funded by $5m in grants from the US Department of Energy (DOE).

“We’re learning about new technologies and helping to develop new ideas for more interdisciplinary approaches to solving these complex problems,” said Bai.

The newest of the two three-year projects aims to convert the lowest quality of waste plastic, which has been rejected for recycling, into materials for construction industries.

The first step is to feed the biodegradable fraction of the mixed waste into an anaerobic digester, where bacteria break it down to produce biogas, a low-cost fuel that is mostly methane.

Leftovers from the digester would then be combined with the nonbiodegradable waste and heated without oxygen in a process called pyrolysis. That would produce bio-oils for asphalt binding products, as well as charcoal-like biochar for carbon-sequestering concrete additives and reinforced biocomposites.

The big problem, Bai said, is the rejected plastic waste is “very complex – there are too many things in there. It’s very heterogeneous.”

To tackle that issue, the waste must first be analysed to understand what is there. Then the researchers need to find technologies that can efficiently and economically convert the waste, before gaining a full understanding of the resulting material.

Bai has assembled a large team of researchers with the necessary expertise to tackle all sides of the project, to make the most of the rejected waste and keep it out of landfill.

The second three-year project will develop new plasma technology to convert single-use plastics into biodegradable plastics.

“The project is all about trying a hybrid of plasma and biological technologies to break down and upcycle single-use plastic films such as plastic bags, product packaging and food containers,” the researchers said.

Bai has developed novel, low-temperature plasma conversion technologies, creating plasma by applying a strong electric field to gas. Doing so accelerates electrons and creates charged particles.

“The free electrons are like bullets,” Bai said. “They speed up and hit other things. These ‘attacks’ can break down chemical bonds.”

By using the plasma with carbon dioxide, the researchers will break down decontaminated plastic films, creating a fermentable liquid. That liquid can then be fermented to produce biodegradable polymers. The fermentation creates carbon dioxide, which can be looped back into the plasma reactor.

The process could lower energy use, carbon emissions and production costs for biodegradable polymers, the researchers said.

Bai has assembled another big team of researchers to develop the idea. “These two projects are using very different approaches,” she said. “Both are using an interdisciplinary approach to solve the problems and understand what is happening at the molecular level… We’re going to learn and figure this out.”

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.

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