Microbes, Microwaves, and Mixes: Eco-friendly Plastics Solutions
Amanda Jasi speaks to innovators striving to improve plastics sustainability and reduce their environmental impact
PLASTICS can be made to biodegrade, but it is unlikely they will end up in an environment such as an industrial composting facility that will allow them to do so, according to researcher Jon Pokorski. Based at the University of California (UC) San Diego in the US, Pokorski is leading work to ensure plastic can biodegrade regardless of the environment. For that he needs the help of Bacillus subtilis, a spore-forming bacterial organism widely used in biotechnology.
A spore is a metabolically dormant state that some bacteria enter to survive harsh conditions, including high temperatures. The state allows the microorganisms to survive for years, ready to begin growing and dividing when conditions permit.
The UC San Diego-led team took advantage of this state to create a thermoplastic urethane (TPU) that can biodegrade in months.1 Pokorski, who leads a biocomposites lab focused on using biological materials to impart new functions onto polymers, explained that they succeeded in this by incorporating bacteria to break down TPU directly in the material. However, this required them to develop spores that could survive at 135°C.
“Most thermoplastic polymers are manufactured using melt-based technology,” he said. This involves heating the polymer to above its melting point so it will flow like a liquid and using extrusion to shape it. He explained extrusion by saying: “You have a heated screw which melts the polymer. As that screw turns, it conveys the polymer into its final form. Once at its final form it cools down and solidifies.”
To create their “living” TPU, Pokorski’s team simply dry the spores into a powder, and then drop them into the extruder as an additive.
“It’s pretty darn easy actually, that’s the beauty,” he said. “The hard part was getting the spores to survive and degrade their target.”
To create the bacteria they needed, the group first selected a strain of B. subtilis able to degrade TPU. They then used adaptive laboratory evolution to enhance the heat-shock tolerance of B. subtilis spores, exposing them to boiling water for increasing amounts of time and selecting the survivors for further rounds.
With their evolved bacteria, Pokorski’s group produced a TPU that achieved more than 90% biodegradation in five months when placed in compost. In their experiments, TPU not containing their microorganism only achieved up to 40% degradation in that time.
With the concept of the technology proven, Pokorski said the group may expand its portfolio to make other types of plastic biodegradable.
Made better with biodegradability
In 2016, researchers at the University of Konstanz, Germany reported a polyacrylic acid-based polymer with properties that could make it a go-to for dentistry or an easily recycled shock-absorber for transporting large goods.2 It was mouldable, self-healing, fire-resistant, and, after fulfilling its purpose, could easily be reshaped for another use just by adding water.
Led by the late chemist Helmut Cölfen, the team looked to improve the polymer further by replacing fossil-based polyacrylic acid (PAA) with a renewable and biodegradable alternative. Ilesha Avasthi, a postdoctoral researcher who joined Cölfen’s team in 2021, explained that by using polyglutamic acid (PGA) instead of PAA she created a material that retains the “state-of-the-art” properties of the original polymer while bringing in biodegradability.3
Polyglutamic acid is a naturally occurring polymer that can be sustainably sourced from microorganisms. As the polymer is not foreign to environmental microbial biochemistry, microorganisms exist that are able to break it down.
But to further encourage degradation, Avasthi said the group incorporated elements into the polymer that microorganisms would “like”. The so-called mineral plastic includes calcium and nitrogen in its basic composition, but other minerals such as iron can be added.
Describing the lab-scale production of the biodegradable polymer, Avasthi said she mixes solutions of polyglutamic acid and calcium chloride in equimolar ratio, with the final solution remaining clear. Iron can be included in the calcium chloride mixture to incorporate the mineral into the polymer. She then adds a 1:1 ratio mixture of isopropanol and water to the clear solution, stirring all the while, and a soft, white solid gradually forms.
“Once the addition is complete, the solid glues together to form one little ball, for example. And then when I take out that ball and knead it for a while it becomes quite stretchable and basically you see the properties then.”
While the process seems simple, it needs to be precise. “Even a slight change in the ratio or the concentration…if that changes, then you lose all the properties altogether.”
What applications these interesting properties could allow is intriguing but yet to be explored, with further investigation into the polymer’s full potential halted by research leader Cölfen’s untimely death from cancer last November.
Reinforcing the renewable
At the Kobe University, Japan, Seiichi Taguchi is leading work to reduce the use of plastics from non-renewable feedstocks by improving polylactic acid, or PLA, which he called the most popular bio-based polyester. He aimed not only to make the material better and stronger, but also ensure it could biodegrade after use, limiting its environmental impact.
While it can be eco-friendly, its physical properties are lacking, exhibiting low impact resistance and thermal stability. “We would like to compensate this weakness of the PLA,” said Taguchi, whose work focuses on synthetic biology.
His group succeeded in this by combining PLA with another lactic acid-based polymer, poly(D-lactate-co-(R)-3-hydroxybutyrate), or LAHB. The researchers selected LAHB expecting that the lactic acid-based interactions between PLA and LAHB would improve blending. The group also focused on incorporating higher molecular weight LAHB into PLA, as higher molecular weight is associated with stronger polymeric materials.
In recently published work4 they produced the high molecular weight LAHB they needed using Cupriavidus necator, a bacterium that naturally produces polymerised 3-hydroxybutyrate (3HB) as a carbon store.
The work involved genetically modifying the bacteria to produce lactic acid and incorporating it into the 3HB polymers to create LAHB. The team also tuned the microorganism to produce longer chain LAHB. They did this by engineering the bacteria to overproduce lactate-polymerising enzyme (LPE), which is needed to copolymerise lactic acid and 3HB, and deleting genes that produce enzymes to break down lactic acid.
The LAHB could then be extracted from C. necator by lysing the cells.
To create PLA/LAHB, the team then simply heated their LAHB and purchased PLA and blended them in a plastic container. They then dried the material in an oven and extruded it into sheets.
Testing proved the sheets had better mechanical properties than PLA alone, while they were also biodegradable. However, the group does not yet understand the mechanisms behind these improvements. Future work is planned to understand the changes, as well as to increase production of LAHB by C. necator.
The group also intends to work to exploit C. necator’s ability to use CO2 to grow, allowing the greenhouse gas to act as a feedstock for this greener plastic option.
Microwave acceleration
Using microwaves as a process intensification method is nothing new, but recycling technology company GR3N has developed a particular approach that it hopes could help address textile waste.
According to Franco Cavadini, chief technology officer at GR3N, “more than 60% of the global market for PET is towards textiles, and there is no technology in the market capable of recycling, at a large scale, textile waste”. GR3N’s technology offers a route for recycling PET (polyethylene terephthalate) and other polyester-based waste from textiles as well as packaging.
Cavadini explained the company’s process, which begins with pre-treating material to ensure it is in a processable state, including by shredding. The plastic material is then depolymerised by alkaline hydrolysis in GR3N’s microwave depolymerisation reactors (pictured right). Importantly, Cavadini highlighted that GR3N’s reactors overcome the limited penetration depth of microwave irradiation. Their patented technology includes an Archimedean screw that operates within the company’s reactors, helping to ensure that irradiation reaches all parts of the reactant mixture.
Hydrolysis in the reactor results in a slurry that includes water, contaminants, and the PET monomers monoethylene glycol (MEG) and terephthalic acid (TPA). The TPA is in the form of a disodium salt due to reaction with the sodium hydroxide used for hydrolysis.
The slurry is then separated into separate MEG and TPA streams for further processing. MEG is recovered from its stream via distillation approach. Meanwhile, the TPA stream is progressively cleaned using a more complex method to recover purified terephthalic acid (PTA). Cavadini said this is “where most of our know-how and also industrial secrets reside”.
In March, GR3N began operating a demonstration plant to showcase its technology and “learn as many operational lessons as possible” as the company moves towards full-scale plants.
Operating as a technology licensor, the company recently announced that it would work with Intecsa Industrial to build the industrial-scale plant of its technology. The 40,000 t/y plant is slated to start operating in 2027.
A single step
“Currently, wastewater treatment plants are being neglected as CO2 emitters,” said Mariana Paredinha Araújo, scientist at technology company Avantium. However, industry interest in capturing and valorising CO2 emitted from the biogas stream is increasing.
Avantium is one of 12 partners involved in the Horizon-funded EU project HICCUPS, working to develop a process to convert biogenic CO2 emissions from wastewater treatment plants into bio-based polymers for the packaging industry.
Avantium is responsible for developing technology to convert captured CO2 into the project’s target plastic – polylactic-co-glycolic acid (PLGA). Primarily used in biomedical applications, PLGA has excellent water and gas barrier properties, is fully biodegradable, and can be made using renewable resources, making it a promising candidate to replace fossil fuel-based polyethylene.
To produce PLGA, Avantium first produces glycolic acid in a two-step process. CO2 is converted into oxalic acid in an electrochemical conversion step, which is then converted to glycolic acid by catalytic hydrogenation.
The glycolic acid, produced from CO2, and lactic acid, produced from biomass, can then be polymerised to create PLGA.
Avantium scientist Reyes Menendez Gonzalez said: “Traditional methods used for PLGA production require prior steps that are costly to meet the necessary polymer quality. The novel route under development aims to avoid these costly extra steps while still achieving the required polymer properties/quality.”
She declined to disclose additional detail about the process but noted that the company can alter the ratio of the different monomers to tune the properties of the final polymer.
Avantium is currently developing its novel PLGA synthesis technology at “pre-pilot scale” and aims to scale up its production to pilot-plant scale.
Started in September 2023, the €5m (US$5.4m) HICCUPS project is set to finish in August 2027. Araújo said: “In the next three years, there will be a lot happening and hopefully in the last year of the project we will have end-user testing of PLGA.”
Plastics have many different types and applications and pose a wide range of environmental problems from fossil-reliant production to mismanaged waste. So, it makes sense that the solutions are just as diverse. There is no single “silver bullet” but in these technologies, innovators believe they have the ammunition to reduce the environmental impact of polluting plastics.
References
1. https://go.nature.com/3LeDgJD
2. https://bit.ly/3xQAPdb
3. https://bit.ly/3RUqqEk
4. https://bit.ly/3RWlWwE
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