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Oxidation and bacteria unite to recycle mixed plastics

A US research is working to expand the range of plastics that can be recycled. The findings, reported in Science on 13 October, are important because plastic waste is a massive problem both globally and in the United States, where less than 10% of used plastic is recycled, according to the US Government Accountability Office.

The problem of mixed plastics

Although chemical methods exist to chop up the long polymer chains, these techniques have been difficult to implement at scale, partly because recycling needs to deal with mixtures of plastics. In fact, plastic waste contains a mélange of materials, so it typically goes through a laborious sorting process to separate certain types of plastic for mechanical recycling.

The aim of the research

A team led by Gregg Beckham, a chemical engineer at the US National Renewable Energy Laboratory (NREL) in Colorado, has developed a two-step process that uses chemistry and then biology to break down a mix of the most common plastics that make it into recycling plants: high-density polyethylene (HDPE), a soft plastic often found in food packaging; polystyrene, which includes styrofoam; and polyethylene terephthalate (PET), a strong, lightweight plastic used to make drink bottles.

A chemical-biological system

The team first used a catalysed oxygenation reaction, with a cobalt or manganese-based catalyst, to break down the tough polymer chains into oxygen-containing organic-acid molecules. But Beckham wanted to turn the organic-acid molecules into something more easily commoditized. To do that, the team turned to microbes — specifically, the bacterium Pseudomonas putida. Pseudomonas putida bacteria get to work on the soup of carboxylic acids, digesting them through multiple biochemical pathways into a single product. One strain of bacteria makes polyhydroxyalkanoates (PHAs), which are used in biodegradable packaging; another produces β-ketoadipate, which can be transformed into easy-to-recycle, nylon-like materials.

Encouraging prospects

Tandem chemical-biological processes have long been used to convert biopolymers like cellulose into useful compounds, but this particular approach has not been used for plastics.
The two stages of the process were inspired by pre-existing technology, a detail that generates optimism about the possibility of industrialization. Since completing this work on proof of principle, the researchers have been testing their system with other polymers, with very promising results. 


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