Chemistry

A pinch of polymer-munching enzymes could make biodegradable plastic packaging and forks truly compostable.

With moderate heat, enzyme-laced films of the plastic disintegrated in standard compost or plain tap water within days to weeks, Ting Xu and her colleagues report April 21 in Nature.

“Biodegradability does not equal compostability,” says Xu, a polymer scientist at the University of California, Berkeley and Lawrence Berkeley National Laboratory. She often finds bits of biodegradable plastic in the compost she picks up for her parents’ garden. Most biodegradable plastics go to landfills, where the conditions aren’t right for them to break down, so they degrade no faster than normal plastics.

Embedding polymer-chomping enzymes in biodegradable plastic should accelerate decomposition. But that process often inadvertently forms potentially harmful microplastics, which are showing up in ecosystems across the globe (SN: 11/20/20). The enzymes clump together and randomly snip plastics’ molecular chains, leading to an incomplete breakdown. “It’s worse than if you don’t degrade them in the first place,” Xu says.

Her team added individual enzymes into two biodegradable plastics, including polylactic acid, commonly used in food packaging. They inserted the enzymes along with another ingredient, a degradable additive Xu previously developed, which ensured the enzymes didn’t clump together and didn’t fall apart. The solitary enzymes grabbed the ends of the plastics’ molecular chains and ate as though they were slurping spaghetti, severing every chain link and preventing microplastic formation.

Filaments of a new plastic material degrade completely (right) when submerged in tap water for several days.Adam Lau/Berkeley Engineering

Adding enzymes usually makes plastic expensive and compromises its properties. However, Xu’s enzymes make up as little as 0.02 percent of the plastic’s weight, and her plastics are as strong and flexible as one typically used in grocery bags.

The technology doesn’t work on all plastics because their molecular structures vary, a limitation Xu’s team is working to overcome. She’s filed a patent application for the technology, and a coauthor founded a startup to commercialize it. “We want this to be in every grocery store,” she says. More

Today, airplanes pump a lot of climate-warming carbon dioxide into the atmosphere. But someday, carbon dioxide sucked from the atmosphere could be used to power airplanes.
A new iron-based catalyst converts carbon dioxide into jet fuel, researchers report online December 22 in Nature Communications. Unlike cars, planes can’t carry batteries big enough to run on electricity from wind or solar power. But if CO2, rather than oil, were used to make jet fuel, that could reduce the air travel industry’s carbon footprint — which currently makes up 12 percent of all transportation-related CO2 emissions.
Past attempts to convert carbon dioxide into fuel have relied on catalysts made of relatively expensive materials, like cobalt, and required multiple chemical processing steps. The new catalyst powder is made of inexpensive ingredients, including iron, and transforms CO2 in a single step.
When placed in a reaction chamber with carbon dioxide and hydrogen gas, the catalyst helps carbon from the CO2 molecules separate from oxygen and link up with hydrogen — forming the hydrocarbon molecules that make up jet fuel. The leftover oxygen atoms from the CO2 join up with other hydrogen atoms to form water.
Tiancun Xiao, a chemist at the University of Oxford, and colleagues tested their new catalyst on carbon dioxide in a small reaction chamber set to 300° Celsius and pressurized to about 10 times the air pressure at sea level. Over 20 hours, the catalyst converted 38 percent of the carbon dioxide in the chamber into new chemical products. About 48 percent of those products were jet fuel hydrocarbons. Other by-products included similar petrochemicals, such as ethylene and propylene, which can be used to make plastics. More