How plants could mine metals from the soil
Nickel may not grow on trees-but there's a chance it could someday be mined using plants. Many plant species naturally soak up metal and concentrate it in their tissues, and new funding will support research on how to use that trait for plant-based mining, or phytomining.
Seven phytomining projects just received $9.9 million in funding from the US Department of Energy's Advanced Research Projects Agency for Energy (ARPA-E). The goal is to better understand which plants could help with mining and determine how researchers can tweak them to get our hands on all the critical metals we'll need in the future.
Metals like nickel, crucial for the lithium-ion batteries used in electric vehicles, are in high demand. But building new mines to meet that demand can be difficult because the mining industry has historically faced community backlash, often over environmental concerns. New mining technologies could help diversify the supply of crucial metals and potentially offer alternatives to traditional mines.
Everyone wants to talk about opening a new gigafactory, but no one wants to talk about opening a new mine," says Philseok Kim, program director at ARPA-E for the phytomining project. The agency saw a need for sustainable, responsible new mining technologies, even if they're a major departure from what's currently used in the industry. Phytomining is a prime example. It's a crazy idea," Kim says.
Roughly 750 species of plants are known to be hyperaccumulators, meaning they soak up large amounts of metals and hold them within their tissues, Kim says. The plants, which tend to absorb these metals along with other nutrients in the soil, have adapted to tolerate them.
Of the species known to take in and concentrate metals, more than two-thirds do so with nickel. While nickel is generally toxic to plants at high concentrations, these species have evolved to thrive in nickel-rich soils, which are common in some parts of the world where geologic processes have brought the metal to the surface.
Even in hyperaccumulators, the overall level of nickel in a plant's tissues would still be relatively small-something like one milligram of metal for every gram of dried plant material. But burning a dried plant (which largely removes the organic material) can result in ash that's roughly 25% nickel or even higher.
The sheer number of nickel-tolerant plants, plus the metal's importance for energy technologies, made it the natural focus for early research, Kim says.
But while plants already have a head start on nickel mining, it wouldn't be feasible to start commercial operations with them today. The most efficient known hyperaccumulators might be able to produce 50 to 100 kilograms of nickel per hectare of land each year, Kim says. That would yield enough of the metal for just two to four EV batteries, on average, and require more land than a typical soccer field. The research program will aim to boost that yield to at least 250 kilograms per hectare in an attempt to improve the prospects for economical mining.
The seven projects being funded will aim to increase production in several ways. Some of the researchers are hunting for species that accumulate nickel even more efficiently than known species. One candidate is vetiver, a perennial grass that grows deep roots. It's known to accumulate metals like lead and is often used in cleanup projects, so it could be a good prospect for soaking up other metals like nickel, says Rupali Datta, a biology researcher at Michigan Technological University and head of one of the projects.
Another awardee will examine over 100,000 herbarium samples-preserved and catalogued plant specimens. Using a technique called x-ray fluorescence scanning, the researchers will look for nickel in those plants' tissues in the hopes of identifying new hyperaccumulator species.
Other researchers are looking to boost the mining talents of known nickel hyperaccumulators. One problem with many of the established options is that they don't have very high biomass-in other words, they're small. So even if the plant has a relatively high concentration of nickel in its tissues, each plant will collect only a small amount of the metal. Researchers want to tweak the known hyperaccumulators to plump them up-for example, by giving them bigger root systems that would allow them to reach deeper into the soil for metal.
Another potential way to improve nickel uptake is to change the plants' growth cycle. Most perennial plants will basically stop growing once they flower, says Richard Amasino, a biochemistry researcher at the University of Wisconsin-Madison. So one of his goals for the project is figuring out a way to delay flowering in Odontarrhena, a family of plants with bright yellow flowers, so they have more time to soak up nickel before they quit growing for the season.
Researchers are also working with these known target species to make sure they won't become invasive in the places they're planted. For example, Odontarrhena are native to Europe, and researchers want to make sure they wouldn't run wild and disrupt natural ecosystems if they're brought to the US or other climates where they'd grow well.
Hyperaccumulating plants are already used in mineral exploration, but they likely won't be able to produce the high volumes of nickel we mine today, Simon Jowitt, director of the Center for Research in Economic Geology at the University of Nevada, Reno, said in an email. But plants might be a feasible solution for dealing with mine waste, he said.
There's also the question of what will happen once plants suck up the metals from a given area of soil. According to Jowitt, that layer may need to be removed to access more metal from the lower layers after a crop is planted and harvested.
In addition to identifying and altering target species, researchers on all these projects need to gain a better understanding where plants might be grown and whether and how natural processes like groundwater movement might replenish target metals in the soil, Kim says. Also, scientists will need to analyze the environmental sustainability of phytomining, he adds. For example, burning plants to produce nickel-rich ash will lead to greenhouse-gas emissions.
Even so, addressing climate change is all about making and installing things, Kim adds, and we need lots of materials to do that. Phytomining may be able to help in the future. This is something we believe is possible," Kim says, but it's extremely hard."