Researchers have developed a technique for detecting and measuring the concentration of many rare-earth elements in plants, without destroying the plant. The technique can be used to optimize "plant mining" efforts, in which plants take up and concentrate these critical materials so that they can be harvested for practical use.
"Rare-earth metals are essential for many technologies," says Colleen Doherty, co-corresponding author of a paper on the work. "These are not actually rare, it's just that they are rarely found in high concentrations in the environment in their pure form. Right now, the U.S. obtains most of the rare-earth materials it needs from international sources, so there is a great deal of interest in identifying domestic sources of these critical materials."
One option is to harvest the rare-earth elements found in mine waste and other polluted soils. However, while these toxic soils have relatively high concentrations of rare-earth elements compared to other soils, those concentrations are still too low to make this an economically feasible strategy.
But there is a potential solution: plants.
"Some plant species are capable of taking rare-earth elements out of polluted soil and concentrating it in their tissue," says Doherty, who is an associate professor of molecular and structural biochemistry at North Carolina State University. "In order to maximize this 'plant mining' technique, we wanted to find a way to detect and measure the concentration of rare-earth materials in these plants. This can inform not only which plants we want to use for these mining projects, but when the optimal time would be for harvesting those plants to maximize yield of rare-earth elements."
To solve this challenge, the researchers used fluorescence spectroscopy. The technique makes use of the fact that some chemical compounds absorb light and then re-emit that absorbed energy as light at different wavelengths. By cataloging which chemical compounds absorb and emit specific wavelengths, and how long those emissions last, you can determine which chemical compounds are present. Generally, the more intense the light emitted, the higher the concentration of the chemical compound.
"Plant matter itself fluoresces across a broad range of wavelengths," Doherty says. "So one challenge has been distinguishing the autofluorescence of the plant itself from the fluorescence of rare-earth elements the plant has taken up."
For this project, the researchers focused on dysprosium, a rare-earth element that is critical for manufacturing everything from cell phones to wind turbines to electric vehicle motors.
"We focused on dysprosium, in part, because it fluoresces for a relatively long time," says Michael Kudenov, co-corresponding author of the paper and the John and Catherine Amein Family Distinguished Professor of Electrical and Computer Engineering at NC State. "This means dysprosium will still be emitting light after the plant's autofluorescence has died down. That allows us to detect it, measure its intensity, and then calculate the concentration of dysprosium in the plant tissue."
The researchers demonstrated the technique using two species of pokeweed. The plants took up dysprosium from a substrate. The plant tissue was then treated externally with sodium tungstate, which interacts with the dysprosium to intensify the light being emitted by the dysprosium during fluorescence. The researchers then triggered fluorescence using a deep ultraviolet laser and measured the wavelengths and intensity of light emitted by the plant samples.
"The sodium tungstate makes it easier to detect the dysprosium," Doherty says. "But because it intensifies the light in a predictable way, we can still account for its presence and get an accurate reading on the concentration of dysprosium in the plant."
The researchers found their technique was accurate at both detecting the presence of dysprosium and measuring the concentration of dysprosium in the plant tissue.
"This technique can be done very quickly and we're excited that we can conduct the testing without destroying the plant, which allows us to test the same plant repeatedly," Doherty says. "This is critical for helping us determine the best time to harvest these plants in order to get the optimal concentration of rare-earth elements in the plants' tissue."
"We've also done enough preliminary work to be confident that this technique will work for the rare-earth elements terbium and europium," Kudenov says. "And we're fairly confident the technique will work for erbium and neodymium, with minor changes to the experimental setup. It's much too early to speak to other rare-earth elements, but we're interested in exploring those as well."
This new technique was developed as part of a larger project being led by Doherty and Kudenov that focuses on supplementing the U.S.'s domestic rare-earth metal needs while offsetting the cost of environmental remediation at fly ash ponds, areas contaminated by acid mine drainage, and other toxic sites.
"We're optimistic that this can make a real difference for both our manufacturing sector and the environment," Doherty says. "It could be an important part of our rare-earth supply chain moving forward."
The paper, "Detection and Quantification of Dysprosium in Plant Tissues," is published open access in the journal Plant Direct. First author of the paper is Edmaritz Hernández-Pagán, a Ph.D. student at NC State. The paper was co-authored by Kanjana Laosuntisuk and Cyprian Rajabu, former postdoctoral researchers at NC State; Anisa Guidira, a Ph.D. student at NC State; Allison Haynes, an undergraduate at NC State; Alex Harris, a former undergraduate at NC State; and David Buitrago, a former master's student at NC State.
This work was done with support from the Defense Advanced Research Projects Agency under DARPA Young Investigator Award D19AP00026.
