There is a new innovative way to break down mixed plastic without sorting. The new approach of two step process that combines chemical and biological processes could simplify the process of recycling mixed plastics.
First step is to break down mixed plastic polymers with a metal catalytic autoxidation reaction and then second step is to feed the oxygenated products to a genetically engineered bacteria(Pseudomonas putida which is a modified soil bacterium).
It’s a synthetic biology, metabolic engineering and this idea of biological funnelling. The idea that you can catalytically pretreat those polymers to get a diverse set of feedstocks that are then brought together into something that is more economically viable is really important.
New way for plastic problem by NREL in Colorado, US
The National Renewable Energy Laboratory(NREL) is responsible for this new innovation. Gregg Beckham and his colleagues designed the two step process, using readily available metal catalysts and Pseudomonas putida which is a modified soil bacterium.
First step is to use a catalysed oxygenation reaction, with a cobalt or manganese based catalyst, to break down the tough polymer chains into oxygen containing organic acid molecules.
This process was inspired by Walter Partenheimer, chemist at DuPon 2003 who used it to break down single plastics into chemicals such as benzoic acid and acetone.
https://patents.google.com/patent/US6958373B2/en
But Beckham wanted to turn the organic acid molecules into something more easily commoditized. So the team turned to microbes, specifically the Pseudomonas putida, which can be engineered to use different small organic molecules as a source of carbon.
The bacteria produced two chemical ingredients that are each used to make high quality performance-enhanced polyners or biopolymers.
Biology can take multiple carbon sources and funnel them into a single product, in this case a molecule which can be used to make a highly biodegradable polymer.
They have managed to transform several types of plastic compounds into biodegradable compound called polyhydroxyalkanoates which can be used un a multitude of biomedical uses and applications or for surgical sutures.
So far the process has only been successful in a laboratory type of environment because of different temperature limitations for the autoxidation reaction, and will need to be shown to be viable in a real production environment before it is fully implemented.
It’s also possible that this same process could be adapted so that it can be used to break down a number of other plastics. Traditionally only one type of plastic is the focus of such processes.
But if the realisation behind the theory is true, then it could be a game changer in the plastics recycling industry and could mean plastics become more economically and environmentally friendly at the same time.
First step: Using metal catalysts to break down mixed plastic waste
Sorting of mixed plastic waste is quite expensive. In many cases plastic waste is actually physically bonded together. To avoid such tedious sorting, scientists have found an interesting new way to break down mixed plastics into desirable compounds using the two step process, bacteria and catalysts.
It’s also thought that this new technique could be used to recycle mixed three common plastic garbage into reusable compound and products. The process uses polystyrene(styrofoam), high-density polyethylene(HDPE, soft plastic) and PET(polyethelene terephthalate) as feedstocks.
The three are converted them into either polyhydroxyalkanoates or β-ketoadipates, which can be used to make performance-enhanced nylons. The first step is made by existing chemistry already used on megaton scale in the petrochemical industry that uses oxidation.
Such oxidation in the field create the building blocks that are incorporated into polyesters and other valuable materials and this process is directly from mid century Amoco process.
Using the cobalt manganese bromide catalytic system from the Amoco process with the plastic feedstocks break down the polymers and produces a mixture of carboxylic acids. These are then fed to an engineered soil bacterium for further refinement.
Beckham’s team uses this biological funnelling process for converting mixed oxygenated intermediates derived from lignin into a single product. Now the team is applying the same concept to compounds derived from plastic waste.
Second step: Funnelling mixed plastic waste with microbes into useful molecule
The next biological step of the process relies on the pseudomonas putida bacterium, which by nature consumes benzoic acid and alkyldicarboxylates that are the oxygenated breakdown products(terephthalic acid) of polystyrene, PET and HDPE.
Then they took transporters and catabolic enzymes that convert terephthalic acid into the same intermediates that you can derived from polystyrene.
With further metabolic engineering the team tunes the chemical products generated by the bacteria to either of the desired β-ketoadipate or polyhydroxyalkanoate materials.
Beckham explains that a metabolic engineering approach allowed the researchers to expand the range of carbon sources consumed by the bacterium.
Current plastic recycling methods require sorting by chemical composition, a method that is expensive and results in products that are of lower quality and value than the starting plastic.
If plastic waste could instead be converted to valuable chemical intermediates, then economical use of mixed waste as a feedstock might be feasible. Sullivan et al. developed a two stage oxidation and biological funnelling approach that can break down and reform mixtures of common consumer plastics.
The end products can be adjusted by metabolic engineering of the microbes in the second step, which should enable tailored conversation into various platform or specialty chemicals.
In this work, we show that metal- catalysed autoxidation depolymerises co mingled polymers into a mixture of oxygenated small molecules that are advantaged substrates for biological conversion.
This hybrid process establishes a strategy for the selective conversion of mixed plastics waste into useful chemical products.
https://www.science.org/doi/10.1126/science.abo4626
The research was published October 2022 in journal science. Journal reference: science, DOI: 10.1126/science.abo4626
Source: Nature, science
Plastic waste is flooding around the world. It’s awesome there is a new microbes bio method to break down mixed plastic waste without sorting in an economical way.
The National Renewable Energy Laboratory in Colorado, US engineers metal-catalysed autoxidation depolymerises to funnel compounds into a single chemical product.
