A Biochemical Approach to Plastic Waste

BY YALE HUANG
July 18, 2023 | | 9 min read

As the planet buckles under the weight of its own waste, recycling has always been upheld as our last bastion of hope. “Recycle your plastic bottles!” headlines cry. “Don’t forget to reuse your utensils and sort your waste!” 

However, calls for individual responsibility fail to consider that an individual’s carbon footprint pales in comparison to waste generated by various commercial industries.1 Even when recycling is plausible, the US Department of Energy’s National Renewable Energy Laboratory (NREL) reports that only 5% of the 44 million metric tons of plastic waste deposited in landfills in the United States is recycled.2 Recycling as it exists currently is both constrained by monetary goals—manufacturing new plastics is far cheaper than sorting, reprocessing, and reusing old materials3—as well as chemistry. Plastics are polymers, a wide class of substances composed of small unique chemical building blocks called monomers.4 Polymers encompass both synthetic and natural materials and are durable, chemically stable, and useful in a variety of applications, including Teflon (non-stick cookware), polystyrene (for all your midnight takeout cravings), and even DNA (the instruction booklet for your cell).

Figure 1. Polytetrafluoroethylene, polystyrene, and DNA polymers.

Polymers owe their creativity and malleability to the vast array of different monomers, linkages, and coupling combinations forming their distinct structural and physical properties. However, it’s this chemical diversity that makes polymer recycling costly, arduous, and inefficient. 

Current recycling technology only works effectively if plastics are cleaned, separated by type, then finally subjected to extreme conditions such as high temperatures, mechanical stress, or highly reactive reagents to break long chains of polymers into smaller pieces.3,4 Even so, mechanical breakdown compromises the material’s toughness, flexibility, and strength, requiring new polymer material to be added to reinforce its structural properties—which is entirely counterintuitive!3 Chemical breakdown involving pyrolysis (literally heating up the plastic until it melts) or biological degradation with enzymes can fragment plastics into smaller materials to be used as chemical feedstocks, fuels, and lubricants, but these smaller materials rarely return into the synthesis cycle as new polymers.3 Ironically, what the recycling industry needs most is true recycling, where used plastics are directly converted into new, useful materials without the need for complex sorting infrastructure. 

To address this issue, scientists are striving to create a circular polymer economy, an alternative to mechanical and chemical degradation pathways where polymers are designed to be degradable and reusable. Since polymers are made of composite elements, researchers can explore combinations of various monomers and bond linkages to create something that breaks down into useful constituents, allowing them to remain in the production chain and be recycled ad infinitum.4 This is also known as “polymer upcycling,” and is currently being pursued as a pathway to addressing the waste issue.3

In October 2022, researchers from NREL, Massachusetts Institute of Technology, University of Wisconsin-Madison, Oak Ridge National Laboratory, Oregon State University, and more took an important step forward to achieve a circular polymer economy.5 In a process they call “mixed plastics waste valorization,” researchers used catalytic metal to oxidize a mixture of different plastics into smaller, more biologically-friendly intermediates, which were then fed to an engineered soil microbe that converted them into a single target product: either polyhydroxyalkanoates, an emerging form of biodegradable bioplastics, or β-ketoadipate, a monomer used to manufacture performance-advantaged polymers.5 In other words, their process reduces the need for plastic sorting and greatly increases the efficacy of recycling!

Figure 2. Mixed plastic valorization through metal-catalyzed oxidation and biological funneling.

Researchers started with three common types of plastics: polystyrene (PS), polyethylene terephthalate (PET), and high-density polyethylene (HDPE). Walk into any supermarket and you’ll see these specific polymers everywhere: in milk jugs, carpets, polyester clothing, and even single-use beverage bottles like Pepsi or Coca-Cola.3 In addition, their chemical composition makes for easy analytical characterization of breakdown products, as PS and PET both produce one major product, benzoic acid and terephthalic acid respectively, while HDPE generated a variety of dicarboxylic acid compounds.6

After testing various reaction conditions, researchers determined optimal conditions to effectively oxidize the mixture of plastics in a process called “autoxidation.”

Figure 3. Metal-catalyzed autoxidation of HDPE, PET, and PS polymers into oxygenated intermediates.

These oxygenated intermediates aren’t just for show. As carboxylic acids, they’re much more water-soluble than intermediates produced by normal pyrolysis recycling, which involves the thermal degradation of plastics at high temperatures. This hydrophilicity allows them to be shipped to the next stage of the process: biological funneling, the ability of microbes to generate a single product from mixtures of substrates. 

Researchers engineered two strains of a soil microbe called pseudomonas putida. One strain used carbon atoms from acetate, a variety of dicarboxylates, benzoate, and terephthalate as building blocks for polyhydroxyalkanoates. The other strain used benzoate and terephthalate as carbon sources to generate β-ketoadipate while consuming acetate and dicarboxylates through the Citric Acid Cycle to produce energy for the bacterial system. After the microbes completed the conversion, the cultivation was centrifuged, and the desired product was retrieved from the supernatant. 

The result was a high molar yield for all mixtures of original plastics. Researchers reported the production of polyhydroxyalkanoates from mixed PS and HDPE beads as well as mixed post-consumer EPS cups and HDPE bottles. In addition, they saw high molar yields for β-ketoadipate, indicating the successful conversion of carbon atoms sourced from mixed plastics into useful biological products. 

These results reveal how the coupling of chemical and biological processes can advance our knowledge and understanding of polymer upcycling. Although additional steps may be taken to optimize this process, such as improving catalyst recovery, separating autoxidation products prior to microbe digestion, and increasing the substrate scope of the microbe, researchers are eager to expand this method to other polymers susceptible to autoxidation, such as polypropylene and polyvinyl chloride.

As waste continues to stockpile in landfills and global consumption of plastic climbs daily, there grows a need for stronger, more efficient recycling infrastructure as well as the economic and scientific scaffolding to hold it up. Researchers at the NREL and various institutions have already shown that chemical and bioengineering strategies can be combined to great effect, allowing mixed plastic waste to skip the arduous sorting process and jump directly to conversion to useful biological products. 

Emerging scientific discoveries gradually move us closer to the goal of optimal polymer circularity, where plastic production relies on the reuse of old material rather than synthesis. The biochemical approach to plastic waste described in the paper is a step in the right direction to achieving optimized polymer upcycling. While the issue of plastic pollution remains prominent and requires systemic change to address, important steps are taken every day to ensure a waste-free future.

References
  1. Meredith, S. Just 20 companies are responsible for over half of “throwaway” plastic waste, study says. CNBC. https://www.cnbc.com/2021/05/18/20-companies-responsible-for-55percent-of-single-use-plastic-waste-study.html#:~:text=U.S.%20energy%20giant%20ExxonMobil%20tops.
  2. Laboratory, N. R. E. Researchers calculate lost value of landfilled plastic in US. phys.org. https://phys.org/news/2022-04-lost-landfilled-plastic.html (accessed 2023-06-27).
  3. Korley, L. T. J.; Epps, T. H.; Helms, B. A.; Ryan, A. J. Toward Polymer Upcycling—Adding Value and Tackling Circularity. Science 2021, 373 (6550), 66–69. https://doi.org/10.1126/science.abg4503.
  4. Britannica. Polymer. Encyclopædia Britannica; 2019.
  5. Sullivan, K. P.; Werner, A. Z.; Ramirez, K. J.; Ellis, L. D.; Bussard, J. R.; Black, B. A.; Brandner, D. G.; Bratti, F.; Buss, B. L.; Dong, X.; Haugen, S. J.; Ingraham, M. A.; Konev, M. O.; Michener, W. E.; Miscall, J.; Pardo, I.; Woodworth, S. P.; Guss, A. M.; Román-Leshkov, Y.; Stahl, S. S. Mixed Plastics Waste Valorization through Tandem Chemical Oxidation and Biological Funneling. Science 2022, 378 (6616), 207–211. https://doi.org/10.1126/science.abo4626.
  6. Sullivan, K. P.; Werner, A. Z. NREL’s Renewable Resources and Enabling Sciences Center, Golden, CO. Personal communication, July 2023.