2012-10-30

Co.Exist

Our New Miners: Plants

"Phytomining"—using plants to extract precious metals from the ground—is becoming a reality. And it might be a valuable way to clean up contaminated soil around old mines.

Mining the world’s minerals is often a nasty business for the environment. But it’s also a big one, and getting bigger: The value of minerals and metals exported over the last 50 years has soared from $23 billion to $671 billion. While more attention is focused on minimizing this damage, open pit mining and similar practices lay waste to vast tracts of land every year.

Scientists are now exploring ways to recruit plants that will mine our metals and minerals for us. Photosynthesis isn’t about to replace machinery anytime soon. Instead, phytomining transports a few billion atoms of nickel, mercury, platinum or other valuable atoms into plant cells that, at a large scale, can enable economic extraction of those elements. Plants are also helping clean up hopelessly contaminated soils for food production. The land for such phytomining is often mine waste itself, the tons of tailings left over after mechanical or chemical extraction of the ores. The low mineral concentrations in these rock piles promise offer a new, if small, source of trace metals, while the plants help make the contaminated soils grow less toxic over time.

“The ability of plants to extract [platinum metals] from soil and redeposit the metal as nanoparticles in cells is remarkable," says biotechnology professor Neil Bruce at the Centre of the University of York, one of the universities in a $1.4 million PHYTOCAT project exploring phytomining for certain catalysts. "This project will allow us to investigate the mechanisms behind this process and provide a green method for extracting metals from mine tailings that are currently uneconomical to recover.”

The initiative is investigating plants known as hyperaccumulators—about 400 species from more than 40 plant families such as willow, corn, and mustard—that soak up metals in the soil through their roots and then accumulate them in their tissues. Independent research in California has shown plants can yield as much as 100 kilograms of nickel per hectare, and enough titanium to return $1,200 per hectare, twice the return from a crop of wheat, if harvested at scale.

So far, the technology has been slow to develop. The process of isolating, testing, and optimizing the right hyperaccumulating plants has taken years, if not decades. But there is momentum. Chinese researchers publishing last month in Pedosphere have shown trace elements such as arsenic, cadmium, nickel, and zinc can be extracted for industrial use through phytomining techniques, while crops excluding toxic metals (or cleaning them up) are possible on those some soils. Eventually, say the researchers, cropping systems for "safe and sustainable agriculture" on contaminated lands are possible and the miners of the future may be of the botanical variety.

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