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Reduce, Reuse, Recycle

The original article appears on the College of Engineering News website. 

Several CBE faculty members and their colleagues are studying the fundamental science behind how to address plastic waste in the oceans.

Plastic waste is found almost everywhere on Earth. Public service announcements about how to reduce plastic consumption — skip the straw, the reusable bag and water bottle movements — are also almost everywhere. These are small ways people can use fewer single-use plastics in their own lives. But big questions remain, like how to collect plastic waste that is creating environmental health problems? What’s the most effective way to recycle plastic? And how many different kinds of plastics are out there?

“Worldwide, it’s recognized that these are useful materials,” said Professor Phil Westmoreland. “We can do things with polymers that we can’t do with other materials. The idea of using plastic still makes a lot of sense. The idea of using it once and throwing it away has a naive sort of attitude: ‘I don’t know where it came from, I don’t care.’”

Ideally, each piece of plastic ever produced, even the microplastics contaminating the oceans, can be reprocessed and made into something else that’s useful — “the circular economy.” But behind this well-known concept, as well as the common solution “reduce, reuse, recycle,” is the complicated, fundamental science of studying these materials and their reactions to different collection and recycling methods, and the need to figure out how to scale up solutions to meet this growing problem.

Re-making Polymers

Plastics are made of polymers, and there are many different types that require different methods to be broken down. Plastic bags are made of polyethylene, while Styrofoam cups are made of polystyrene. Polypropylene is used for fibers and plastic bottles, and polyester is commonly used in clothing. These, and many others, are used daily. Each polymer has different chemical properties, meaning each requires slightly different recycling methods.

One common plastics recycling method is pyrolysis, the thermal degradation of plastics at temperatures ranging from 250 to 900 degrees Celsius in an oxygen-free environment. The plastic is converted to liquid fuel. The gases that come off the thermally decomposed polymers are used to make new polymers. Westmoreland has conducted research in this space for 30 years.

Mechanical recycling generally leads to products made from recycled plastics that are designed to be thrown away or recycled again.

“Re-making the materials that you make polymers from is much more appealing as a circular economy,” he said. “You make it, use it, reconstitute it fresh.”

Figuring out the best way to do this involves experimental and theoretical work. Westmoreland’s lab decomposes different polymers using fast pyrolysis and slower, thermogravimetric analysis, which continuously measures the mass of the polymer while it is thermally decomposing. They measure the products and the decomposition rates to see which reactions are taking place and at what temperature. In parallel, they have successfully predicted these decomposition rates and mechanisms using computational quantum chemistry and a reactive molecular dynamics (RMD) method they created.

To model reactions in a polymer melt, RMD predicts the movements of each of the individual atoms within each of the polymer molecules. The key is computing the forces between every atom and molecule at every time step, and then applying Newton’s second law of motion to predict their new positions and velocities.

There are other approaches and methodologies behind pyrolysis, and Westmoreland is looking at induction and microwave heating to break down the materials. These can be powered by greener energy sources, such as solar and nuclear. The project started at Savannah River National Lab, focused on plastic film used in packaging, and is continuing at NC State.

“You can take a polyethylene bag, do treatment of it, generate ethylene, which then can be reacted again to make new polyethylene,” he said. “These materials are so useful that it’s a waste to dispose of them if you can use them again.”

Open-Ocean Filtration

Another facet of the plastics problem is how to collect the pieces that are already in environments where they shouldn’t be. Plastic pollution in the world’s oceans is especially concerning. As larger plastics have broken down, they’ve become microplastics, which are plastic pieces less than 5 millimeters long. These are becoming a threat to human and wildlife health.

Carol Hall, Worley H. Clark, Jr. Distinguished University Professor in Engineering, is the principal investigator (PI) on a project focused on developing a self-sustaining, circular system of “microcleaners” that remove microplastics from the ocean. Orlin Velev, S. Frank and Doris Culberson Distinguished Professor, and Nathan Crook, assistant professor, are the co-PIs. The team is working with researchers from Cornell University on artificial intelligence (AI) capabilities to accelerate the process.

“It’s an important environmental and societal problem,” Velev said. “There is quite a bit of interest in doing research that highlights the fundamental aspects that would allow us to solve this problem.”

The fundamental questions that the researchers are trying to address include: What are the particle interactions between the plastic and microcleaners? What material is best and safest for creating microcleaners? How can we engineer microbes to consume the plastic? And how can AI scale up and optimize the process?

The work will be published in the open literature, and the researchers hope that by answering these fundamental questions, others will be able to advance their work on plastic waste removal. The project is funded by a four-year, 2 million dollar grant from the National Science Foundation Emerging Frontiers in Research and Innovation program under grant EFMA-2029327.

Velev’s lab has developed new materials called soft dendricolloids (unique materials with the ability to stick to just about any surface, similar to behaviors seen in the feet of gecko lizards and spider webs) which are fibrillar with a large surface area. They will collect the microplastics, ball up, and then float to the surface along with the microplastic waste.

These microcleaners will be functionalized to exhibit specific physical and chemical behaviors with peptides. Hall is studying the interactions between the plastic particles’ surfaces and peptides to design peptides that best bind to the plastics. 

The microcleaning particles and microplastics would then be fed into a bioreactor, which would break down the microplastics and use the resulting byproducts to be used as fuel or to be metabolized in a way that produces more microcleaners (and possibly other useful materials). While there is one species of bacteria known to consume plastic, it does so extremely slowly. Crook is trying to take genes from this organism and put them into a fast-growing bacterium to create an organism that could consume plastic faster.

Diagram of microcleaning particles
Diagram of microcleaning particles

To make this process sustainable, the researchers will make these microcleaners from naturally occurring polymers. They are exploring chitosan, which is derived from crab shells.

“The hope is that when you feed the microplastics to the microbes, the microbes would then shuttle that energy to construct chitosan or another polymer, which can be used in the same way,” Crook said. “Then you make more cleaners using the plastic you’ve collected.”

The circular economy concept behind using the waste to collect more waste will be supported by AI. But there is still a long way to go before these concepts can be applied, and challenges include the lack of a one-size-fits-all approach, as different types of plastic particle surfaces will have varying interactions with the microcleaners.

“Every step is a challenge,” Hall said. “We are now trying to assemble the pieces and make each of them work well. Down the road, we will weave the pieces together into a system that hopefully will make a difference.”