Researchers at North Carolina State University say they developed a fast, non-destructive technique to measure and detect rare earth materials in plants.
Rare earth metals are essential for modern technology. They’re used in cars, cellphones, and batteries.
The United States gets the majority of these elements from abroad. China controls 80% of the world’s rare earth market.
Phytomining, a sustainable technique where plants extract rare earth metals from contaminated soil, is an alternative to mining. Additionally, it helps clean up contaminated lands, like fly ash, sludge, and acid mine drainage.
Testing a plant’s metal levels required sending samples to a specialized laboratory to go through an chemical analysis called Inductively Coupled Plasma Mass Spectrometry (ICP-MS). However, this method can be destructive and costly, as it kills the plants, according to Colleen Doherty, co-author of the study and associate professor at N.C. State.
“What we found is a way to measure how much rare earth elements the plants are taking up in a way that doesn't kill the plant,” Doherty said. “We can do a lot of plants at a time, and it's very cheap.”
For this study, published in Plant Direct, they focused on dysprosium, which is a valuable rare earth element that’s often used to build electric vehicles, wind turbines, and even smartphones.
They used fluorescent spectroscopy, in which some chemical compounds absorb light and re-emit that absorbed energy as light at different wavelengths. According to N.C. State, researchers are able to catalog which compound or mineral is present based on the light it gives off. Dysprosium has a unique optical trait.
“Long after the plant fluorescence has died away, we can still see the dysprosium, because it just has a really long fluorescence lifetime,” Doherty said.
Afterwards, Doherty says they still send the plants off to ICP-MS, but this new technique acts as a screening.
“We still validate everything by ICP-MS, but it lets us screen all the ones we're not interested in first, and then we can, you know, have a smaller set to go send to ICP-MS,” she said.
Additionally, Doherty said since they can keep the plants alive, they can continue to breed the plants that show high absorption rates.
Pokeweed: A prime candidate
In this case, Doherty found pokeweed worked very well at absorbing these materials.
Pokeweed is a native, weed-like plant that grows aggressively. Doherty said pokeweed is a prime candidate for multiple reasons — since it’s native, it can be placed safely without disrupting local ecosystems. It also grows fast. Doherty said it can grow up to ten feet in a single month.
Additionally, it takes up a ton of rare earth elements, especially high-value ones like dysprosium and terbium, and can filter out the low-value metals like cerium and lanthanum
“It preferentially takes up the heavy, which is like dysprosium and terbium, those really high valuable ones, over the low rare elements, which are cerium and lanthanum, and some of the other ones,” Doherty said.
Doherty said the sheer amount of metal the pokeweed pulled up into its leaves surprised the team.
“Our plants took up all the rare elements we could feed them,” she said. “I couldn't believe how much they took up.”
Doherty said one of the goals of this project is to extract these materials from contaminated industrial waste sites, like coal fly ash and acid mine drainage. The plants can extract the valuable materials from these sites while cleaning the toxins.
These pools can be toxic and spread out in rural areas. Building multi-million dollar chemical refineries at every location isn’t feasible, according to Doherty.
“We know the plants can take up enough rare earth elements, we know they can grow in a lot of these wastes.” Doherty said.
Doherty emphasized these plants will never replace mining. However, she said this could realistically supply 5% of the national supply chain, helping protect the U.S. from international supply cutoffs while offsetting the public costs of environmental cleanup.
“I think it's a game changer for being able to measure things quickly and easily, you know. It just changes how we do things in implant science for rare earth elements.” Doherty said.
Doherty said the research is being expanded into processing harvested crops. They’re also working with teams at Columbia University and Virginia Tech to find clean, carbon-neutral ways to extract the metals out of the leaves.
