Mixtures of plastics, which are often a headache to recycle, have been broken down into smaller, more useful chemical ingredients in a two-step process, reported in Science on October 13.
The plastics problem facing the planet is compounded by the difficulty of recycling these robust materials. Although there are chemical methods to cut their long polymer chains, these techniques have been difficult to implement at scale, in part because recycling must deal with mixtures of plastics.
The processes that convert plastic waste into useful chemicals tend to focus only on a single plastic, making it difficult to design facilities that can cope with a mix of plastic waste, which would be necessary for a truly circular economy.
Gregg Beckham of the National Renewable Energy Laboratory in Colorado and colleagues designed a two-step process that uses readily available catalysts and a modified soil bacterium, Pseudomonas putida, to treat mixtures of some of the most common plastic waste materials.
A team led by Gregg BeckhamChemical Engineer in the US, National Renewable Energy Laboratory (NREL) Golden, ColoradoThe two-step process uses chemistry and biology to break down a mixture of common plastics that reach recycling plants. Includes high-density plasticethylene (HDPE), which is often used for food packaging; polystyrene including polystyrene foam; as well as polyethylene terephthalate (PET), which is a strong, lightweight plastic used to create beverage bottles.
“Only a few papers have reported chemical recycling of plastic mixtures before”, ” Ning YanA chemist at the National University This is Singapore One of the few researchers who has developed a system that can do this. “The combination of chemical and biological pathways to convert the plastic mix is even rarer,” he continues.
The first step of the process is based on a common industrial method for producing terephthalic acid, a component of PET. This uses oxygen and chemical catalysts to break down the carbon bonds in the mixed plastic debris, making the resulting compounds more digestible for bacteria.
“The first step is like a big hammer: You just take oxygen and simple chemical catalysts to make oxygenated bioavailable intermediates, and then we engineer an organism to channel them into a single product,” says Beckham.
Although Beckham and his team engineered the bacterium to produce polyhydroxyalkanoates in this study, it should be feasible to make it produce other more widely used products, such as the building blocks for environmentally friendly and easily recyclable plastics. They also hope to extend the method to deal with a greater diversity of plastics.
“The nice thing about synthetic biology, metabolic engineering, and this biological funnel idea… is that as long as the organism can eat or consume the oxygenated intermediates, you could potentially do anything,” says Beckham.
The concept of combining chemical breakdown and biological conversion is novel and could form part of a new recycling chain for mixed plastic waste, says Mike Shaver of the University of Manchester, UK.
“The idea that you can catalytically pretreat those polymers to get a diverse set of feedstocks that are then combined into something that’s more economically viable is really important,” he says.
However, this process has only been demonstrated in the lab so far and will need to be shown to make economic sense in the real world, he adds.
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