Heat And Bacteria Recycle Blended Plastics Into Useful Chemicals
Mixtures of plastics, normally a headache to recycle, have been broken down into helpful, smaller chemical ingredients in one two-step process, reported in Science on 13th October.
The plastics problem facing the globe is exacerbated by the difficulty of recycling these robust products. Although chemical techniques exist to chop up their long polymer chains, these methods have been difficult to implement at scale, partially because recycling needs to deal with mixtures of plastics.
A group led by Gregg Beckham, a chemical engineer at the US National Renewable Energy Laboratory (NREL) in Golden, Colorado, has developed one two-step process that uses chemistry and then biology to break down a mix of the most typical plastics that make it into recycling plants: high-density polyethylene (HDPE), one soft plastic often discovered in food packaging; polystyrene, which includes styrofoam; and also polyethylene terephthalate (PET), strong, lightweight plastic utilized to make drink bottles.
“Only one few works have reported chemical recycling of plastic mixtures before,” states Ning Yan, one chemist at the National College of Singapore and one of the few scientists to have developed one system capable of that. “Combining chemical and also biological pathways to convert plastic mixture is even rarer,” he adds.
Two-step process
The group first used a catalyzed oxygenation reaction with a cobalt or manganese-based catalyst to break down the challenging polymer chains into oxygen-containing organic-acid particles. The process was inspired by one 2003 study led by Walter Partenheimer, a drug store at chemicals company DuPont in Wilmington, Delaware, who utilized it to break down single plastics into chemicals such as benzoic acid and acetone.
Yet Beckham wanted to turn the organic-acid molecules into something more easily commoditized. To do that, the group turned to microbes– specifically, the bacterium Pseudomonas putida, which could be engineered to utilize different small organic molecules as a carbon source. “It’s quite an interesting organism,” says Beckham. The group engineered the microorganisms to consume the oxygenated organic molecules that the researchers made from the different plastics utilizing their ‘autoxidation’ reaction: dicarboxylic acids from polyethylene, teraphthalic acid from PET and benzoic acid from polystyrene.
The bacteria produced 2 chemical ingredients that are each used to make high-quality performance-enhanced polymers or biopolymers. “Biology could take multiple carbon sources and funnel them into a single material, in this case, a molecule which can be utilized to make a highly biodegradable polymer,” says Susannah Scott, a chemist at the College of California, Santa Barbara.
The researchers developed their process utilizing a mix of pure polymer pellets; however, also tested it on mixed plastics discovered in everyday materials. “We purchased HDPE in the form of milk containers, PET from the vending machine outside my office in single-use beverage containers. And then polystyrene or styrofoam cups,” states Beckham.
Temperature limitations
However, scaling up the process will be challenging, says co-author Shannon Stahl, a chemist at the University of Wisconsin– Madison. One problem is the temperature that the autoxidation response is run at. At the moment, each plastic reacts best at one different temperature, and the one that the group uses for the mixture corresponds to the most recalcitrant of the reactions. More fundamental chemistry is required to work out exactly how this reaction works and improve the yields of the responses, says Stahl.
But he includes that many companies currently work with autoxidation processes to turn xylene into terapthalic acid, a PET precursor molecule. “There is a lot of in-house knowledge built in, and also, if one or more of these companies could choose to explore this, I think they could provide a lot of technical know-how,” Stahl states. Beckham says the group is working on an economic analysis and also a life-cycle assessment of its process.
Another issue will be to sell the smaller particles that the bacteria produce because demand for those products is much smaller than the number of waste plastics, states Yan. “Whether the procedure will be scaled up depends on economic competitiveness,” he states.
Read the original article on Nature.
