Huge piles of used plastic bottles at a bottle recycling facility

Mixed plastics are difficult to recycle, but a new process shows how it can be done.Credit: China Photos/Getty

Mixtures of plastics, usually a headache to recycle, have been broken down into useful, smaller chemical ingredients in a two-step process, reported in Science on 13 October1.

The plastics problem facing the planet is exacerbated by the difficulty of recycling these robust materials. Although chemical methods exist to chop up their long polymer chains, these techniques have been difficult to implement at scale, partly because recycling needs to deal with mixtures of plastics.

A team led by Gregg Beckham, a chemical engineer at the US National Renewable Energy Laboratory (NREL) in Golden, 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.

“Only a few works have reported chemical recycling of plastic mixtures before,” says Ning Yan, a chemist at the National University of Singapore and one of the few researchers to have developed a system capable of that2. “Combining chemical and biological pathways to convert plastic mixture is even more rare,” he adds.

Two-step process

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. The process was inspired by a 2003 study3 led by Walter Partenheimer, a chemist at chemicals company DuPont in Wilmington, Delaware, who used it to break down single plastics into chemicals such as benzoic acid and acetone.

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, which can be engineered to use different small organic molecules as a source of carbon. “It’s quite an interesting organism,” says Beckham. The team engineered the microoorganisms to consume the oxygenated organic molecules that the researchers made from the different plastics using their ‘autoxidation’ reaction: dicarboxylic acids from polyethylene, teraphthalic acid from PET and benzoic acid from polystyrene.

The bacteria produced two chemical ingredients that are each used to make high-quality performance-enhanced polymers 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,” says Susannah Scott, a chemist at the University of California, Santa Barbara.

The researchers developed their process using a mix of pure polymer pellets, but also tested it on mixed plastics found in everyday products. “We purchased HDPE in the form of milk containers, PET from the vending machine outside my office in single-use beverage bottles. And then polystyrene or styrofoam cups,” says Beckham.

Temperature limitations

But scaling up the process is going to be a challenge, says co-author Shannon Stahl, a chemist at the University of Wisconsin–Madison. One issue is the temperature that the autoxidation reaction is run at. At the moment, each plastic reacts best at a different temperature, and the one that the team uses for the mixture corresponds to the most recalcitrant of the reactions. More fundamental chemistry is needed to work out exactly how this reaction works and improve the yields of the reactions, says Stahl.

But he adds that many companies already work with autoxidation processes, to turn xylene into terapthalic acid, a PET precursor molecule. “There’s a lot of in-house knowledge built in, and if one or more of these companies would choose to explore this, I think they could offer a lot of technical know-how,” Stahl says. Beckham says the team is working on an economic analysis and life-cycle assessment of its process.

Another problem will be to sell the smaller molecules that the bacteria produce, because demand for those products is much smaller than the quantity of waste plastics, says Yan. “Whether the process will be scaled up depends on economic competitiveness,” he says.