Two-step approach eliminates the need to sort plastics before they are recycled

  • EiC summary slide from Dissolvable batteries for use when teaching plastics and recycling

    Download this

    Use this story and the accompanying summary slide to add new context when teaching plastics and recycling to your 14–16 learners.

    Download the story as MS Word or PDF and the summary slide as MS PowerPoint or PDF

Scientists have developed a process for converting mixed plastic waste into two distinct, useful chemicals. Their method uses a combination of chemical and biological techniques to upcycle mixtures of three common plastics. It could offer a way to streamline recycling processes by eliminating the need to first sort the plastics.

Sorting is not required

Currently, mixed plastic waste, such as what goes into household recycling bins, must be sorted by type before it can be recycled. ‘The motivation was to avoid the expensive and quite tedious sorting of mixed plastic waste,’ says Gregg Beckham from the National Renewable Energy Laboratory in Colorado, US, who led this research. The process is also suitable for plastic waste containing different polymers bonded together, such as multilayer packaging and some textiles.

The three polymers this process is suitable for are:

  • high-density poly(ethylene) (HDPE), with uses including milk containers and bottles for personal care products
  • poly(ethylene terephthalate) (PET), commonly used for food and drink packaging, especially for single-used beverage bottles
  • poly(styrene), which is widely used as disposable food trays, cups and plates.

These three plastics make up the majority of post-consumer plastic waste.

Chemistry and biology unite

The team’s recycling process has two steps. First, the polymers are oxidised by air with the help of a cobalt–manganese–bromide catalytic system to produce a soup of carboxylic acids. Next, the mixture is fed to soil bacteria for further conversion. The bacteria used are Pseudomonas putida, which naturally consume benzoic acid and dicarboxylic acids (produced by the HDPE and polystyrene breakdown). The team genetically engineered the bacteria to consume terephthalic acid (from PET) as well.

Further genetic engineering produced two distinct bacteria strains that produce two different end products. One converts the carboxylic acid mixture into poly(hydroxyalkanoates) (PHAs), which are used in biodegradable food packaging and for biomedical applications such as dissolvable sutures. The second strain transforms the soup into β-ketoadipate, which can be used to make performance-enhanced nylons.

‘The study is one of the first to show how to get from a mixed waste stream to a pure product and uses a very interesting interdisciplinary approach,’ says Ina Vollmer, an expert on chemical recycling of plastic waste based at Utrecht University in the Netherlands, who was not involved in this project.

Having demonstrated that their concept can work, Gregg and his team now hope to improve their process and expand the range of plastics that can be recycled.

Put this in context

Discover how analytical technician, Celine, develops ways to alter the structure of plastics so they become biodegradable in the natural environment.

This article is adapted from Jamie Durrani’s in Chemistry World.

The motivation was to avoid the expensive and quite tedious sorting of plastic waste

Nina Notman


KP Sullivan et al, Science, 2022, 378, 207 (DOI: 10.1126/science.abo4626)

Download this

Summary slide with questions as well as a pdf of this article for use with your 14–16 classes on plastics and recycling: