Many have seen the haunting images of wildlife — including sea turtles, dolphins and seals — tangled in abandoned fishing nets. According to the United Nations: “Abandoned, lost and discarded fishing gear is the deadliest form of marine plastic [are] threatening 66 per cent of marine animals, including all sea turtle species and 50 per cent of seabirds.”
In another example, a 2019 study, which focused on the Maldives in the Indian Ocean documented 752 ‘ghost nets’ that had entangled 131 turtles over a 51-month period.
The main issue behind Nylon-6, the plastic inside these nets, is that it’s too strong and durable to break down on its own. Therefore, once it is in the environment, it lingers for thousands of years, littering waterways, breaking corals and strangling birds and sea life.
Catalyst solution
To address this problem, Northwestern University chemists have developed a new catalyst that quickly, cleanly and completely breaks down Nylon-6 in a matter of minutes — without generating harmful byproducts. Even better: The process does not require toxic solvents, expensive materials or extreme conditions, making it practical for everyday applications.
Not only could this new catalyst play an important role in environmental remediation, it also could perform the first step in upcycling Nylon-6 wastes into higher-value products.
A deadly difficulty
Nylon-6 is found in a variety of materials that most people use every day. However, when people are done with these materials, they end up in landfills or worse: loose in the environment, including the ocean. According to the World Wildlife Fund for Nature (WWF), up to 1 million pounds of fishing gear is abandoned in the ocean each year, with fishing nets composed of Nylon-6 making up at least 46% of the Great Pacific Garbage Patch.
The greenest solvent is no solvent
Current methods to dispose of Nylon-6 are limited to simply burying it in landfills. When Nylon-6 is burned, it emits toxic pollutants such as nitrogen oxides, which are linked to various health complications including premature death, or carbon dioxide, an infamously potent greenhouse gas.
Although other laboratories have explored catalysts to degrade Nylon-6, those catalysts require extreme conditions (such as temperatures as high as 350 degrees Celsius), high-pressure steam (which is energetically expensive and inefficient) and/or toxic solvents that only contribute to more pollution.
Recovering building blocks for upcycling
To bypass these issues, the researchers looked to a novel catalyst was selected. The catalyst harnesses yttrium (an inexpensive Earth-abundant metal) and lanthanide ions. When the team heated Nylon-6 samples to melting temperatures and applied the catalyst without a solvent, the plastic fell apart — reverting to its original building blocks without leaving byproducts behind.
In experiments, researchers were able to recover 99% of plastics’ original monomers. The monomers could then be upcycled into higher-value products, which are currently in high demand for their strength and durability.
Efficiently targeting Nylon-6
In addition to recovering a high yield of monomers, the catalyst is highly selective — acting only on the Nylon-6 polymers without disrupting surrounding materials. This means industry could apply the catalyst to large volumes of unsorted waste and selectively target Nylon-6.
“If you don’t have a catalyst that’s selective, then how do you separate the nylon from the rest of waste?” Marks said. “You would need to hire humans to sort through all the waste to remove the nylon. That’s enormously expensive and inefficient. But if the catalyst only degrades the nylon and leaves everything else behind, that’s incredibly efficient.”
Recycling these monomers also avoids the need to produce more plastics from scratch.
The study, “Catalyst metal-ligand design for rapid, selective and solventless depolymerization of Nylon-6 plastics,” appears in the journal Chem.
