The United States is looking to boost domestic mining of rare earth elements to secure greater control over pricing and manufacturing of the technologies we rely on such as electric vehicles, MRI machines, military defense systems and the semiconductors powering AI.

To that end, the U.S. Department of Energy is backing phytomining — the method of using plants to absorb critical minerals from soil — and NC State University’s interdisciplinary research for optimizing the innovative process.

“Phytomining uses a plant as a kind of straw that concentrates the stuff we want from the Earth,” says Colleen Doherty, an associate professor of molecular and structural biochemistry at NC State and a lead investigator on the latest study, which was funded by a federal grant from the Defense Advanced Research Projects Agency (DARPA).

Using plants to mine metals, like nickel, isn’t new. And despite their name, rare earth elements aren’t actually rare; they’re simply hard to find in high concentrations of their pure form in nature.

“We’re focused on rare earth minerals, so that’s a fairly new aspect of phytomining,” says Doherty, a co-corresponding author of a recently published paper on the study.

Advances to phytomining for rare earths comes at a critical juncture. At a recent U.S. Chamber of Commerce summit, the message was stark: America faces an immediate need to increase domestic rare earth mining and extraction.

“Rare elements are critical for defense, but currently, the U.S. and the rest of the world are highly dependent on China,” Doherty says. China currently dominates global mining and supply chains of rare earths, which impacts America’s ability to manufacture essential technologies.

“The U.S. has been working to secure domestic sources for both defense and civilian uses,” says Doherty.

Lighting a Better Way For Phytomining

Rare earth elements have special properties essential to the functionality of our everyday technology. For example, smartphones contain a variety of rare metals with fluorescent chemical compounds that help power their vibrant displays.

The research team used fluorescence as well to measure the concentrations of rare earths in plants. They developed an ultraviolet (UV) spectroscopy technique to measure fluorescence — the light that rare-earth compounds emit at different wavelengths. Generally, the more intense the light emitted, the higher the concentration of a specific metal or element.

The team focused on dysprosium, an element highly prized for creating strong, high-temperature and lightweight magnets used in smartphones, wind turbines, electric vehicle motors and other battery-powered technology. 

“If you shine a light on it, [dysprosium] fluoresces in a specific way,” says Michael Kudenov, co-corresponding author of the recent paper and professor of electrical and computer engineering at NC State. “It has a very bright, long-lived fluorescence compared to other elements.”

For the study, researchers arranged an array of plants that accumulated dysprosium from a substrate. They treated the plant tissue with a sodium solution to intensify dysprosium’s fluorescence. The team then used a deep-UV laser to measure the specific wavelengths and intensity of light emitted by the plant samples.

“The technique is critical for helping us determine the best time to harvest these plants to get the optimal concentration of rare earth elements.”

The study found their technique was highly accurate: fluorescence was directly proportional to the amount of dysprosium present, allowing precise measurement. The technique also holds promise for tracking other critical minerals.

Typically, measuring the concentration of rare earths requires destroying the plants. Using the new spectroscopy technique allows monitoring the plants and tracking their uptake levels over time.

“We’re excited that we can conduct the testing without destroying the plant, which allows us to test the same plant repeatedly,” says Doherty. 

“[The technique] is critical for helping us determine the best time to harvest these plants to get the optimal concentration of rare earth elements in the plants’ tissue.”

Detoxing Industrial Waste Sites

Currently, phytomining can’t compete with large-scale, conventional rare earth mining efforts. 

“The stuff from phytomining is a drop in the bucket compared to actual mining,” Doherty explains.

The researchers found, however, that phytomining could play an important role in conventional mining. Extracting elements from bedrock creates significant waste streams — often a slurry of material left behind after extraction, called tailings.

“Plants could play a role in that they could make mining more efficient, because mining doesn’t get 100% of the rare earth elements out,” says Doherty. “There’s still a lot in the tailings, and so plants could be used to get the remainder out.”

The team explored phytomining’s additional advantage: the ability to suck up toxins and simultaneously clean up waste sites. 

The researchers focused on acid mine drainage sludge — a seepage of water containing heavy metal out of active or abandoned mining operations. All across Appalachia, acid mine drainage sludge ponds are managed by environmental agencies. Industrial waste areas like these pose significant environmental risks due to high concentrations of heavy metals, toxic trace elements and crystalline silica. When improperly managed, severe air pollution, groundwater contamination and widespread ecological damage can result. 

But phytoming can help by absorbing heavy metals in soil or sludge — a detoxification process known specifically as phytoremediation — in industrial waste areas.

“Those sites tend to be enriched with rare earth elements,” Doherty says. “If plants are used to recover valuable metals, that helps reduce the cost of remediation. And it would be one less worry for the state to deal with.” 

“Phytoremediation works — it’s one of the best ways of cleaning things up, though it has its costs,” says Doherty. 

The biggest advantage of using plants is that they don’t need electricity. 

“At these remote sites, it would be incredibly expensive to set up a conventional refinery, which would require water and electricity. Instead, we can put plants at those sites,” Doherty explains.

A Humble Hero: Pokeweed

The research team decided to use a weed you’ve more than likely seen before: Phytolacca, commonly known as pokeweed. Its superpower is the ability to grow practically anywhere, no matter how disturbed the environment may be. 

Doherty recalled a previous study exploring whether maize might work for phytomining and remediation in coal fly ash — a byproduct of burning coal in electric utility power plants. While the maize failed to grow, pokeweed sprang up without anyone planting it.

“We knew from the literature that pokeweed would probably grow in the waste,” says Doherty. “And we’ve since confirmed it grows really well in fly ash and acid mine drainage sludge.”

Doherty’s team also chose pokeweed because it’s native to the local region, offering ecological advantages. For example, pokeweed needs less water than non-native species that risk becoming invasive.

The study lays crucial groundwork that could continue to advance phytomining methods. With further development, the researchers think a drone equipped with a UV sensor could capture measurements of rare earth uptake across wider areas.

Further research is needed to determine whether plants naturally separate out rare earths in their tissues for a biological purpose. That understanding could potentially advance future phytomining and environmental cleanup efforts.

“We’re optimistic that this can make a real difference for both our manufacturing sector and the environment,” says Doherty. “It could be an important part of our rare earth supply chain moving forward.”