• 07.28.14

Cleaning Up Polluting Mines With Plants–Plants That Then Turn Into Precious Metals

One enterprising scientist thinks we’re close to creating a whole new, much greener mining industry.

Nothing grows here at Walker Ridge. Oaks, pines, and wildflowers stop abruptly at the edges of a huge swath of bare earth. The dead zone–tinged an uneasy shade of green–stretches almost as far as the eye can see in one direction, down a slope that feeds directly into a watershed. Piles of dirt, scraps of rust-eaten metal, and a few crumbling bricks seem the only signs left of what was once a Gold Rush-era mercury mine. They’re not.


Downriver, fish have 20 times more mercury in their flesh than the EPA says is safe for consumption. Two hours south, mercury concentrations spike in San Francisco Bay during big floods. Geologists and hydrologists estimate that this abandoned mine–and at least 5,200 others like it in the state–will continue to leak poison for the next 10,000 years. With the costs of “remediating” a single polluting mine falling somewhere between $.5 and $7 million, the solution often seems to be to just deal with the mercury and leave the mines as they are.

But what if there were a way to monetize that cleanup, to turn Superfund sites, abandoned mines, and other metal-contaminated dead zones into desirable (and healthier) real estate?

In Dylan Burge’s vision of the Walker Ridge site, mining operations are booming again. Thousands of rows of deeply green, compact plants are thriving in the toxic soil, reaching for sunlight that filters through fabric tarps stretched overhead. Downhill (just below glinting banks of solar panels), metal-contaminated effluent from the old mine is being captured and piped back up to the plants, watering some rows while filling hydroponics for others. The mercury problem is under control, trucks are rolling off the site, and no one’s spending $7 million. In fact, people are making money. That’s because, as Burge sees the future possibilities, the world’s first loads of truly “green,” sustainable metals–mostly nickel from this site, plus a little gold–are headed for market.

Burge, 34, is a botany curator at the California Academy of Sciences and an expert in hyperaccumulators–plants that attract and suck up huge quantities of metals by releasing ion-attracting compounds from their roots. Found generally in metal-rich serpentine soils (like the kind most hard rock mines, like Walker Ridge, sit on), each species has protein pathways that seem “tuned” for a particular type of metal. Gold, nickel, copper, zinc, cobalt, aluminum, manganese, even some rare earth elements, they’re are all on the menu.

The idea of “phytomining”–using these plants in commercial mining operations– isn’t new; mining companies actually funded much of the early research, a wave that gained momentum in the mid-1970s before petering out about 20 years ago. “Things got pretty quiet after that,” says Burge, “but not because phytomining didn’t work. It was because the yields weren’t profitable enough to be interesting. The technology wasn’t there, and the science wasn’t there.” He’s got a two-part plan to fix that.

Burge works with Streptanthus polygaloides, a small, flowering herb native to California that’s also the third most powerful nickel hyperaccumulator in the world, capable of sucking up as much as 2% of its dry bodyweight. In the Walker Ridge hypothetical, these plants are harvested up to six times a year and mixed into a live slurry. Microbes break the slurry down–creating sellable carbon-neutral ethanol as a byproduct–and metal production begins with the material that’s left. During the process, massive amounts of hydrolysis occur, allowing hydrogen to be captured, stored, and converted into electricity that helps power the plant. “And all this could be done right now,” says Burge. “No waiting. All you need is a botanist, an abandoned mine, and a tech startup that’s good at scalable solutions.”

Dylan refers to these mines as “point-source problems” (small sites with huge environmental impact), but monetizing their cleanup by creating a consumer market for sustainable metals could have benefits far beyond safer, healthier local ecosystems. Metals worldwide are cheap not because they’re unlimited or easy to get at, but because we pass on the vast environmental and social costs of mining them to other countries. If American consumers were to start asking where the metal in their devices, cars, and wedding rings come from–and paying for the kind that doesn’t leave destruction in its wake–it could pave the way for a new kind of mining.


Burge is already at work on one key to that future: unraveling the genetic secrets of hyperaccumulators. Last month, he became the first person known to have sequenced the full genome of a hyperaccumulator–of 24 of them, actually–and somewhere in the resulting terabytes of data, he expects to find the gene (or suite of genes) that gives Streptanthus its metal-mining abilities. With that discovery should come the Holy Grail of phytomining: the potential to create larger, more efficient hyperaccumulators.

“If you make it your goal as a scientist to affect the world in your lifetime,” says Burge, “you’re almost guaranteed to fail. But every once in a while,” he adds, “it’s possible to get lucky.”

Profits from a Streptanthus metal harvest will never be big enough to get the commercial mining industry excited about becoming farmers. But splice the gene for hyperaccumulation into something with significant biomass–something like corn, for example–and one of the dirtiest, most dangerous, most destructive industries in the world might start paying attention again.