Heat and bacteria recycle mixed plastics into useful chemicals

Mixtures of plastics, usually a headache to recycle, were broken down into useful, smaller chemical ingredients in a two-step process, reported in Science October 13¹.

The plastics problem facing the planet is exacerbated by the difficulty of recycling these sturdy materials. Although chemical methods exist to chop their long polymer chains, these techniques have been difficult to implement on a large scale, in part because recycling must 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 mixture of the most common plastics that make in recycling plants: high-density polyethylene (HDPE), a flexible plastic often found in food packaging; polystyrene, which includes polystyrene foam; and polyethylene terephthalate (PET), a strong, lightweight plastic used to make beverage 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 this.². “Combining chemical and biological pathways to convert a plastic mixture is even rarer,” he adds.

Two-step process

The team first used a catalyzed 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 the DuPont Chemical Company in Wilmington, Delaware, who used it to break down simple plastics into chemicals such as benzoic acid and acetone.

But Beckham wanted to turn the organic acid molecules into something more easily trivialized. To do this, the team turned to microbes, specifically the bacteria Pseudomonas putida, which can be engineered to use different small organic molecules as a carbon source. “It’s a pretty interesting organization,” says Beckham. The team engineered the microorganisms to consume the oxygenated organic molecules that the researchers made from the various plastics using their “auto-oxidation” 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 that 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 mixture of pure polymer pellets, but also tested it on mixed plastics found in everyday products. “We bought HDPE in the form of milk containers, PET from the vending machine outside my office in single-use drink bottles. And then polystyrene or polystyrene cups,” says Beckham.

Temperature limits

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 at which the auto-oxidation reaction is carried out. At the moment, each plastic reacts best to a different temperature, and the one the team uses for mixing is the most recalcitrant of reactions. More fundamental chemistry is needed to understand exactly how this reaction works and improve reaction yields, Stahl says.

But he adds that many companies are already working with auto-oxidation processes, to transform xylene into teraphthalic acid, a precursor molecule of PET. “There’s a lot of internal knowledge built in, and if one or more of these companies chose to explore that, I think they could offer a lot of technical know-how,” Stahl says. Beckham says the team is working on an economic analysis and lifecycle assessment of its process.

Another problem will be selling the smaller molecules produced by the bacteria, because the demand for these products is much lower than the amount of plastic waste, says Yan. “Scaling up the process will depend on economic competitiveness,” he says.