Golden, CO, Oct. 13, 2022 (GLOBE NEWSWIRE) -- Combining chemical and biological processes is a promising new strategy for the valorization of mixed plastic waste, according to researchers with the Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment (BOTTLE) Consortium.
Waste plastics have emerged as a global energy and pollution problem as ineffectively managed materials continue to accumulate in landfills and the environment. Only about 5% is recycled in the United States, with existing strategies requiring separated and clean plastic inputs to operate effectively.
Different plastics comprise different polymers, each with their own unique chemical building blocks. When polymer chemistries are mixed—either in a collection bin or formulated together in materials such as multilayer packaging—recycling becomes expensive and difficult because each polymer often must be separated prior to chemical deconstruction. The BOTTLE researchers developed a process that can convert mixed plastics to a single chemical product, working toward a solution that would allow recyclers to skip sorting plastic by type.
“This is a potential entry point into processing plastics that cannot be recycled at all today,” said Gregg Beckham, a senior research fellow at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and head of BOTTLE. Beckham is the senior author of a new paper published in the journal Science that details work creating a tandem chemical and biological process to produce single high-value products from waste plastic. The paper, “Mixed plastics waste valorization through tandem chemical oxidation and biological funneling,” was co-authored by NREL researchers and BOTTLE team members from the Massachusetts Institute of Technology, Oak Ridge National Laboratory, and the University of Wisconsin–Madison.
The process builds upon work pioneered a decade ago: Chemical oxidation can be used to break down a variety of plastic types, which was developed by a scientist from DuPont. The NREL researchers built on this chemistry, which uses oxygen and catalysts to break down the large polymer molecules into their smaller chemical building blocks.
“The chemical catalysis process we have used is just a way of accelerating that process that occurs naturally, so instead of degrading over several hundred years, you can break down these plastics in hours or minutes,” said Kevin Sullivan, a postdoctoral researcher at NREL and co-author of the paper.
They applied the process to a mixture of three common plastics: polystyrene (PS), used in disposable coffee cups; polyethylene terephthalate (PET), used in single-use beverage bottles, polyester clothing, and carpets; and high-density polyethylene (HDPE), used in many common consumer plastics, often associated with milk jugs. Although not part of the initial proof-of-concept work, the team noted that this method could be extended to include other plastics including polypropylene (PP) and polyvinyl chloride (PVC). This will be a focus of ongoing efforts for the group.
This oxidation process breaks down the PS, PET, and HDPE plastics into a complex mixture of chemical compounds—including benzoic acid, terephthalic acid, and dicarboxylic acids—that would require advanced and costly separations to yield pure products. For the BOTTLE researchers, that is where biology came into play.
The BOTTLE team engineered a robust soil microbe, Pseudomonas putida, to biologically “funnel” the mixture of intermediates to single products: either polyhydroxyalkanoates (PHAs), which are an emerging form of biodegradable bioplastics; or beta-ketoadipate, which can be used to make new performance-advantaged nylon materials.
“Biological funneling simply means we’ve engineered the metabolic network of a microbes to direct the carbon from a large number of substrates to a single product,” said Allison Werner, a co-author on the study. “To do this, we take DNA from nature—usually other microbes—and paste it into Pseudomonas putida’s genome. The DNA is transcribed into RNA, which in turn is translated into proteins that perform diverse biochemical transformations, forming a new metabolic network and ultimately enabling us to capture more carbon and to tune where it goes.”
The researchers have previously used Pseudomonas putida to valorize chemical mixtures from lignin—the hardy parts of cell walls in plants that are difficult to break down. After considerable success in that space, the researchers decided to turn it loose on the plastics problem.
“Moving from lignin to plastics, there were similarities but also new challenges,” said Kelsey Ramirez, a technician at NREL and co-author on the project. “We were able to adapt some of the analytical methods, but we know there is a lot of work to do to understand and quantify all the additives, dyes, and other unknowns present in postconsumer plastics today.”
The authors emphasize that the engineered bacteria do not degrade plastics directly but rather upcycle the deconstructed mixture of chemical oxygenates into a single product. “If you took the bacteria that we use right now, and you combine it with polyethylene, the bacteria will die, and the plastic will stay there,” Beckham said. The oxidation process, he said, converts the recalcitrant plastic polymers into small molecules the bacteria can be engineered to consume. “After some engineering, these compounds are excellent carbon and energy sources for microbes.” Genetic and metabolic engineering enabled the team to tune where the microbe funnels that carbon, in this case to PHAs or to beta-ketoadipate materials that can be used for new performance-advantaged plastics.
An NREL mission to the International Space Station will test whether microgravity improves the bacterial upcycling process.
The other co-authors from NREL are Lucas Ellis, Jeremy Bussard, Brenna Black, David Brandner, Felicia Bratti, Bonnie Buss, Xueming Dong, Stefan Haugen, Morgan Ingraham, Mikhail Konev, Joel Miscall, Isabel Pardo, and Sean Woodworth.
Funding was provided by the U.S. Department of Energy’s Advanced Manufacturing Office and Bioenergy Technologies Office. The work was performed as part of the BOTTLE Consortium.
NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by the Alliance for Sustainable Energy, LLC.
