Sex Pistols, Jugs, Bottles and Speed: NREL breaks through on a fast, cheap route to recycling plastic in one pot, at one time

October 16, 2022 |

News arrives from Colorado that BOTTLE consortia researchers led by NREL have 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. Combining chemical and biological processes is a promising new strategy for the valorization of mixed plastic waste — yes, so say the Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment (BOTTLE) group. My, they’ve got bottle.

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.

The backstory

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

Yep, cops, bottles, and milk jugs

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.

The newly enhanced critter

The BOTTLE team engineered 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.

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.

Has Johnny Rotten returned to delight us anew?

If you are wondering if you’ve heard of this supercritter before and in the back of your mind is rattling around the phrase “oil-eating bacteria,” yes this is the same guy, back for more. 

It’s Putida, the Johnny Rotten of bacteria. Putida being a word for rotten in Latin and the engineered forms of the bacteria being arguably as influential and groundbreaking as the Sex Pistols band that performer Johnny Rotten once fronted. You can almost hear Putida singing it, as the Sex Pistols intoned in Bodies, “Body! I’m not an animal / Body! I’m not an animal / I’m not a discharge / I’m not a throbbing squirm.” It may take a little imagination, but if you ask me, putida m

Ananda Mohan Chakrabarty engineered it back in the 1970s while at GE as a biological remedy for removing oil pollution especially during disastrous oil spills and leakages in marine ecosystems. As his recent obituary tells the story, he “produced a new stable bacterial species (now known as Pseudomonas putida)…which was capable of digesting two thirds of hydrocarbons found in typical oil spill and…about one or two orders of magnitudes) than previously existing strains of oil-eating microbes.” More on that discovery here.

I might add, little Putida became the first microbe to make it to a winning Supreme Court case, when Chakrabarty applied for a patent on his engineered critter and ended up fighting for his right in the Supreme Court, which ruled in his favor in the landmark Diamond vs Chakrabarty case,. and his success opened up the floodgates for patenting GMOs, and for that he was referred to as ‘Father of Patent Microbiology’. Time Magazine named the decision to its list of one of the 25 most important moments in American history.

Nature eventually wrote, “without Diamond v. Chakrabarty, commercial biotechnology based on recombinant DNA technologies would not exist today.” 

Eventually in a separate development stream, DuPont developed a process using chemical oxidation 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.

No, the critters do not eat the plastic

The authors emphasize that the engineered bacteria do not degrade plastics directly but rather up-cycle 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.

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.

An NREL mission to the International Space Station will test whether microgravity improves the bacterial upcycling process.

Reaction from the stakeholders

“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 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.

“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.”

“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 paper itself

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 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.

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