A team of researchers have used bacteria to break down the molecular building blocks of polystyrene waste - and convert them into useful chemicals.
Polystyrene is widely used in packaging and single-use goods, with around 20 million tonnes produced every year, of which only about 10% is recycled. Researchers estimate that about 30% of landfill waste is from polystyrene, as well as significant amounts of the waste found in the world's oceans.
While there have been attempts to depolymerise polystyrene into aromatic monomers for recovery and reuse, routes that convert the plastic into drop-in polymer precursors are limited.
Now a team working with Christoph Wittmann, professor of biotechnology at Saarland University, Germany, have been able to use the bacteria Pseudomonas putida to metabolise substrates to yield useful products. The research was published in Chemical Engineering Journal.
The process involves the photolytic degradation of polystyrene into benzoic acid, followed by microbial conversion to muconic acid, then the chemical synthesis of the final products.
“This study presents a strategy to upcycle [polystyrene] waste into valuable chemical products, including adipic acid, hexanediol, hexamethylenediamine, and nylon-6,6, using metabolically engineered Pseudomonas putida KT2440,” the paper said.
"Pseudomonas putida is a well-known xenobiotic degrader, and our strain KT2440 is a safe, industrial-ready variant. It’s part of a group of over 200 potent Pseudomonads, known for their strength and versatility. The strain can tolerate and break down toxic chemicals, with a flexible metabolism that adapts well to tough process conditions. Plus, it’s easily tunable through genome editing, making it a strong candidate for this kind of application," Wittmann told C&I.
“By transforming hard-to-recycle polystyrene into a sought-after feedstock for high-quality technical and high-performance polymers, the biological 'upcycling' that has been developed in Saarbrücken offers a clear advantage over conventional recycling,” the university said.
Wittmann said the researchers were able to demonstrate that the products yielded by the method have identical properties to those made from virgin petroleum-based sources.
“The conversion of [polystyrene]-derived benzoate into muconic acid, followed by chemical transformation into nylon-6,6, establishes an effective microbial–chemical route for plastic waste valorization. These results confirm the feasibility of hybrid strategies for transforming polystyrene into industrially relevant products, complementing recent studies for plastic waste upcycling,” the paper said.
Nylon-6,6 is a widely used synthetic polymer across textiles, automotive, and engineering plastics, but production relies heavily on fossil-based resources and significantly contributes to carbon emissions. This new approach may serve as a sustainable ‘drop-in’ alternative for the nylon industry.
“This finding is crucial since it demonstrates that sustainable alternatives can seamlessly integrate into existing industrial supply chains as drop-in solutions to ensure compatibility with conventional manufacturing infrastructures,” the paper added.
"The next step is scaling up," Wittmann said. "We’re focused on refining the initial breakdown of polystyrene and designing a low-cost, efficient process. Life Cycle Analysis and Techno-Economic Assessment will play a big role in optimizing this."
Further reading on recycling and sustainability
- A new route to polystyrene waste recycling
- Can we recycle plastics forever? [C&I premium content]
- Solar catalysts: a path to green feedstocks from plastic waste ? [C&I premium content]
- PFAS to biomass: Sustainable chemicals initiative seeks big ideas for industry
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