Bacteria and catalysts recycle waste plastic into useful chemicals

A combination of chemical catalysts and engineered bacteria has been used to convert a mix of common plastic rubbish into a useful product. The technique could be adapted for other plastics or to make different materials.

Processes that convert plastic waste into useful chemicals tend to focus only on a single plastic, so it is difficult to design facilities that can cope with a mixture of plastic waste – which would be needed for a truly circular economy.

Gregg Beckham at the National Renewable Energy Laboratory in Colorado and his colleagues have 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.

The group managed to transform polystyrene, polyethylene terephthalate (PET) and high-density polyethylene (HDPE) into a family of biodegradable compounds called polyhydroxyalkanoates, which are frequently used in biomedical applications such as sutures or in repairing tendons.

The first step of the process borrows from a common industrial method to make terephthalic acid, one component of PET. This uses oxygen and chemical catalysts to break down carbon bonds in the mixed plastic waste, which makes the resultant compounds more digestible for the bacterium.

“Step one 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 funnel those 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 get it to produce other more widely used products instead, such as the building blocks for easily recyclable and environmentally friendly plastics. They also hope to extend the method to cope with a greater diversity of plastics.

“The cool thing about synthetic biology, metabolic engineering and this idea of biological funnelling… is that as long as the organism can eat or consume the oxygenated intermediates, then potentially one could make 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 at the University of Manchester, UK.

“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,” 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.