New Ways to Break Down Problem Plastic
From catalysts that break plastic into more valuable raw chemicals, to enzymes in worm saliva that digest problem plastics, scientists are now close to bagging several new ways to tackle plastic pollution...
Alarmingly, just 9% of the millions of tonnes of plastic we produce every year is recycled and 12% gets incinerated. This means that most plastic waste ultimately ends up in landfill, or worse, so new solutions to the accumulating plastic waste problem are desperately needed.
One of the most popular plastics is polyethylene, used frequently in food and consumer goods packaging. Part of the attraction is the high level of stability and robustness of the polymer, characteristics that derive from the long chains of stable, carbon-carbon bonds that make up the material. Unfortunately, this same chemical resistance and durability is what makes polythene a disposal headache.
Giving New Life to Plastic Waste
But now US researchers have developed a process that can break down polythene's strong carbon-carbon chains into short, three-carbon-long molecules of the feedstock gas propene. Also sourced from petroleum refining, propene is a valuable raw material for the chemical industry used in the production of a range of polymers and other materials, pharmaceuticals, solvents, cosmetics and surface coatings.
The US team have developed a special metal catalyst that introduces a change in the stable carbon-carbon chain that, "provides a kind of Achilles heel where we can then cleave the chain," says John Hartwig, University of California, Berkeley's Henry Rapoport Chair in Organic Chemistry. "You can think about the polymer chain as a physical chain. It has links that are very stable and cannot be pulled apart. But if we make a chemical change to the polymer chain that transforms one of the linkages into a clasp, similar to one on a necklace which can be opened. Now we have a way to break apart the chain under milder conditions."
The process consists of 3 reactions. The first involves a platinum and zinc catalyst to pull hydrogen atoms off the polymer, replacing some of the links between the carbon atoms double rather than single bonds and creating Hartwig's notional "clasp" configuration. The subsequent two reactions use a palladium catalyst to repeatedly move the "clasp" in steps along the chain, cleaving off three-carbon-long propene molecules until the entire polyethylene polymer chain is used up.
The process results in a 90% conversion yield, and because it takes place at just 200°C and under milder conditions than the 500°C pyrolysis technique currently used to break down polyethylene, its more energy efficient and produces higher quality outputs. The conversion of waste polyethylene into a more valuable product such as propene is a form of upcycling and turns waste plastic trash into chemical treasure. By adding value to what is currently worthless waste, Hartwig hopes that this will incentivise people to clean up and recycle plastic rather than dumping it.
Taking Cues from Nature
Meanwhile, a surprise discovery for Federica Bertocchini came from paying close attention to the wax worm Galleria Mellonella, after they wormed their way through polyethylene plastic bags. At the time it was unclear if the escapist worms had made their way through by physically tearing the plastic, or if they were able to secrete chemicals capable of digesting it.
Now writing in Nature Communications, Bertocchini, a researcher at the Spanish National Research Council, has confirmed that enzymes present in wax worm saliva do indeed digest plastic.
Polyethylene is largely resistant to biodegradation; but wax worm salivary enzymes are nevertheless capable of breaking apart stable carbon-carbon chains by incorporating oxygen into the polymer's structure. This process, known as oxidation, results in the breakage of the large polymer chains into smaller molecules. Most surprisingly, these enzymes were capable of degrading polyethylene within hours at room temperature, without the need for plastic pre-treatment. This is in stark contrast to the decades needed for such an oxidation process to occur under environmental conditions. "No pre-treatment, plastic as it is, room temperature, neutral pH; that's it!" says Bertocchini.
Given that the salivary enzymes can be cloned in the laboratory, it presents a promising solution to degrading plastic waste without the need for the actual wax worms.
"It can potentially provide a system where plastic can be biodegraded in a controlled environment," said Bertocchini. Piles of waste plastic, she speculates, can be "treated with a watery solution of these enzymes". Potential applications in the future may include its use in plastic waste management facilities and even "household kits where everyone can degrade their waste plastic."