How Metal-Absorbing Plants Could Make Mining Greener

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, we’re going to take a trip underwater to listen to the sounds of the ocean, and believe me, it’s a noisier place than you might think. Plus, we’re going to talk to a Nobelist who switched from studying DNA to RNA and why he’s excited about the future of this powerful molecule.

But first, what if you could mine metal from the earth using plants? Talk about a green energy solution. Typically if soil has high levels of metal, plants will either die or do everything they can to avoid it, but there is another option, evolving to be able to safely absorb high amounts of the metals. And these special plants are called hyperaccumulators, and their ability to suck metals like nickel from the earth is called phytomining.

Joining me now to talk about these fascinating flora and their promise as a greener option to metal mining is Dr. David McNear, professor of plant and soil sciences at the University of Kentucky in Lexington. Welcome to Science Friday.

DAVID MCNEAR: Hi, Ira. Thanks for having me.

IRA FLATOW: You’re welcome. Can you explain how plants absorb metal from the soil without causing harm to the plant?

DAVID MCNEAR: That’s a good question, Ira, and actually something that we’re still trying to figure out. So you mentioned these plants grow in soils, and they have a couple strategies that they’ve evolved. One of those is to exclude the metal, so don’t take it up at all, and that happens at the plant-soil root interface. But the other option is to take it up and take it up in large quantities. So the mechanisms of that process we’re still trying to figure out.

IRA FLATOW: Yeah, yeah. And where do they store it when they take it up?

DAVID MCNEAR: So mostly they store it in the leaves, really in the skin of the leaf, if you will. The cells on the outside of the leaf, they have these compartments like a closet that’s called a vacuole. And they take that metal up, and they store it in those compartments in the leaf.

IRA FLATOW: Wow. And about how many plant species are actually able to do this?

DAVID MCNEAR: There are probably upwards of 500 species that have been identified and counting that hyperaccumulate metals. There’s about 400 of those that are described mainly for nickel hyperaccumulation.

IRA FLATOW: And what other kinds of metals?

DAVID MCNEAR: Yeah, so you have plants that take up zinc and cobalt and arsenic, selenium. So there are a variety of plants out there that take up a variety of metals.

IRA FLATOW: And how big are these plants? Are they giant trees? What do they look like?

DAVID MCNEAR: So, generally, the plants that I’m normally working on are fairly small. They might get as high as knee or waist high. So they’re not massive plants. They inhabit an environment that’s pretty harsh. Many of those are found in dry or Mediterranean climates. So they have to be drought tolerant. So they’re not huge plants. They’re not corn. They’re not sorghum or some of these grasses.

IRA FLATOW: Yeah, are they all related species?

DAVID MCNEAR: So there’s probably. 42 different plant families that these hyperaccumulators come from. The main ones are brassica-type plants or mustard or arabidopsis might be a variety that you’ve heard in a lot of research that people do.

IRA FLATOW: And so the plants accumulate the nickel or the other metals. They store it in their leaves. Then how do you go about getting the metal out for using it for other purposes?

DAVID MCNEAR: So the agronomy– and you mentioned phytomining. I think that the common term now, at least, was coined in 2013 is agromining. So this is the process where you grow plants that hyperaccumulate metals, and there’s agronomy involved or the production. You have to grow it, you have to harvest it, and then you have to extract that metal from the plant.

IRA FLATOW: So once it’s grown and harvested, you have to send it out, so to speak, to get the metal removed?

DAVID MCNEAR: Yeah, it’s a pretty neat process. There’s a couple ways in which the plant is beneficial in that process. So a farmer can go out and bale this crop of nickel into a bale, a classic hay bale, but this is a nickel bale. And they can burn that for energy. And then they take that ash, that ash that contains now about 20% nickel, and they can, using refining processes that have been developed for rock with metal in it, they can then extract the nickel from the ash.

IRA FLATOW: Did you say that the plant is actually 20% nickel?

DAVID MCNEAR: So the plant can take up, ideally for a phytomining or agromining operation, you would like that plant to take up 2%, and some plants take up more. But after it’s been baled and burnt, the ash that comes from that plant can have upwards of 20% or more nickel in it.

IRA FLATOW: And this is a significant amount?

DAVID MCNEAR: This is a significant amount, and the beauty of that ash is it’s a plant. It’s a carbon-based life form. So there aren’t many other impurities in there. Like when you go and mine rock, you have silica and all these other elements that you have to try to get rid of. But when you burn this plant, it’s just carbon and nickel, essentially.

IRA FLATOW: Right. How much energy does it take to do mining in the conventional way than do it with a plant?

DAVID MCNEAR: Yeah, that’s a great question. That may be a little bit out of my wheelhouse, but I think that is actually what has sort of raised the interest in phytomining, particularly from the Department of Energy in the US, is that current mining and extraction processes for mainly low-grade ores is a pretty energy-intensive process. So compared to– I think nickel is the fourth most CO2 emitted for unit of nickel extracted in the mining process. That’s followed by platinum, gold, and then steel, and then it’s nickel. So they’re pretty energy-intensive processes.

IRA FLATOW: Is this something that was found out recently, or have we known about this for years?

DAVID MCNEAR: So I think that the process of or the idea or the identification of plants that take up metals I think first occurred in 1945 where they identified a plant that had a whole bunch of nickel in it. The term phytomining or the concept of phytomining was really, I guess, brought about in 1983 by some researchers at the USDA here in the United States, and they proposed this idea of metal hyperaccumulator as a plant species to use for soil remediation in that case, but then the idea of extracting the metal from that plant and recovering it.

IRA FLATOW: I know that ARPA-E, the federal government program that invests in research, announced in March up to $10 million in funding for more phytomining research. To the average person, that sounds like a lot of money, but to researchers, it’s not a whole lot of money, is it?

DAVID MCNEAR: Spread across several folks, several researchers, it’s not, but it is nice. As someone who did their PhD research on metals and soils and hyperaccumulating plants and have spent my academic career trying to find funds to support that research, it’s nice to see this resurgence of interest in plants as a mechanism for extracting metals from soils. A lot of the impetus from the DOE was from the carbon footprint of conventional mining, but I think greater impetus for your listeners is batteries and the drive that we need more of these metals to produce the electric-powered vehicles and electric storage.

IRA FLATOW: That’s a really interesting point, and my question about that, are we thinking of whole fields of plant that get harvested for the metals in them, or do we plant them in areas where there’s a lot of nickel that we know of is in the ground and we want to get it out?

DAVID MCNEAR: I think people are thinking all of the above. So these soils that have a lot of nickel in them, if they have been farmed, they are typically very low-producing fields. They’re not very agronomically productive. They’re not producing much feed or fiber or fuel. If you could grow a crop of nickel on those soils, that would be great.

You have to think about, there are widely dispersed regions across the world that have soils that are naturally enriched in metals. Those are sensitive environments. They’re unique environments. I don’t think you’d want to go plowing over all of these soils and start growing nickel in them, but there are some places where, again, if they have been already put into production and they need an alternative thing to do on that land, that might be an option.

But also I will just add that there have been places where there have been contamination around smelters or historic mining operations where these plants could be employed to help remediate but also remove metals from those soils.

IRA FLATOW: And is it possible to tweak the genome of these plants to make them better miners at what they’re doing?

DAVID MCNEAR: I would say certainly, yes, there is a way. I mean, the science of gene editing, I think, could certainly play a role in this process. There’s obviously some regulation issues you have to deal with down the road, but so little focus has been put on metal hyperaccumulation as towards agromining. I mean, if you think about it, corn used to be a wild species that we domesticated and now we grow in mass populations. So if focus is put on developing either conventional breeding or, like you’re saying, gene editing to get a plant that’s bigger, that takes up more metal, that could be beneficial. But that’s part of probably what some of these folks who are getting this grant are going to look at.

IRA FLATOW: Well, what about the unexpected consequences? I’m sure there must be some ecological concerns, right, about planting fields of hyperaccumulators to mine out the metals.

DAVID MCNEAR: No, for sure. And the area of phytomining or as an industry has had some fits and starts and mistakes, really, where some folks have taken nonnative species and started planting them in places where there’s nickel-rich soils, and they’ve escaped, and so they’ve become invasive. So there are certainly ecological considerations.

You’re also introducing metal from the soil now into a plant. So what impact does that have then on transferring metal to the surrounding environment? So there are still a lot of questions that need to be explored.

IRA FLATOW: Good points. But you feel optimistic then, though?

DAVID MCNEAR: I’m a researcher, so I guess I would say I’m skeptically optimistic.

IRA FLATOW: I mean about the ability to make a dent and the need to mine so much of this metal from the earth, possibly.

DAVID MCNEAR: Yeah, I think it definitely could play a role. I mean, I think it should play a role where we’re already doing surface mining for some of these metals that you could have from the tailings. You could be growing this plant and also continuing the extraction process of nickel from those, or in regions where smelters or refineries have contaminated large areas of soil around those facilities, you could grow crops of nickel there. So yeah, there are places.

IRA FLATOW: And they talk about the rare-earth metals that are needed so much these days. And it’s not that they’re rare, but it’s very difficult to get them out of the ground and process them. Could this be one way to do that?

DAVID MCNEAR: Absolutely, yeah, and I think that’s some of the focus. So we’re starting at nickel, which maybe is the low-hanging fruit of metals because there are so many of them, and we have some that are well characterized and have been deployed. Honestly, there are places where they are currently phytomining, not in the United States. So I think that with an eye toward so what we could learn from this research, learning about the mechanisms of metal uptake from soils and whatnot could be applied to then trying to find plant species that are taking up and concentrating rare-earth elements, exactly.

IRA FLATOW: Well, wow. Fascinating, Dr. McNear. Thank you for taking time to be with us today.

DAVID MCNEAR: Of course. Thanks for having me, Ira.

IRA FLATOW: Dr. David McNear, professor of plant and soil sciences at the University of Kentucky in Lexington.

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