Researchers at King’s College London have found a way to break down plastics into their building blocks for recycling. The process takes less than 40 hours and uses an enzyme commonly found in bio-based laundry detergents.
The advance reported in the journal Cell Reports Physical Science could lead to an efficient way to recycle the single-use plastic called polylactic acid (PLA). “We have chosen polylactic acid as this plastic does not have any real way of properly recycling it,” says chemistry professor Alex Brogan. “Our development changes this, as we have been able to convert the plastic to its constituent building blocks in less than 40 hours at 90°C.”
Although PLA is made from renewable sources such as corn starch and sugar cane, it is just as big an environmental problem as petroleum-derived plastics. It is the most common commercial bioplastic—the world produced 457,000 tons in 2021—and is used mainly to make disposable food containers, cups, and utensils.
Most of these items end up in landfills, where they do not biodegrade. PLA can only biodegrade in industrial compost settings, where it takes 12 weeks to break down at 60°C. This makes industrial composting “a fairly energy intensive and time-consuming process of converting the plastic to fertilizer or mulch,” Brogan says.
Using enzymes for any kind of industrial processing, including plastic recycling, is attractive because they can be more efficient than other chemicals. But they tend to be unstable, especially at the relatively high temperatures needed to break down plastics.
So the researchers set out to find the right enzyme for the job. They turned to published literature on medical implants to find “problem” enzymes that degrade PLA, which is also commonly used in implants, he says. “So we turned this ‘problem’ into an advantage.”
The enzyme they chose is Candida antarctica lipase B (CaLB), a ubiquitous enzyme commonly found in laundry detergent. The researchers chemically modified it to make it soluble and stable in a class of solvents called ionic liquids. Ionic liquids have the ability to convert common plastics into soluble fragments, which enzymes can then degrade. “Our strategy is a simple chemical transformation that in theory could be used to upgrade any enzyme to be used on any kind of plastic,” says Brogan.
The researchers dissolved the modified enzyme in the ionic liquid, and soaked pieces of a PLA plastic cup in the solution. The plastic pieces dissolved fully into the solution within 24 hours. In another 16 hours at 90°C, the fragments completely degraded into the plastic’s building block, lactic acid. The lactic acid could be used to make more plastic or other chemicals.
“We are now working with engineers to see how we can improve this process by including more precise pre-processing such as shredding, which would enable us to work on a larger scale,” Brogan says. The team is also trying to figure out the most efficient way to separate the lactic acid from the solution. “The main improvement that we need to show is that you can actually make the plastic again with the degraded plastic, and close the loop.”
Source: Susana M. Meza Huaman, Jake H. Nicholson, Alex P.S. Brogan. A general route to retooling hydrolytic enzymes toward plastic degradation. Cell Reports Physical Science, 2024.
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