Plants that eat metal sound like a biological impossibility. But these hungry little guys exist, sucking tiny bits of toxic metal from the soil. They don't just clean the Earth, either — they can actually mine bits of gold and nickel for use by humans.
It's called phytomining, and it's the subject of a fascinating story in New Scientist this month. In it, we meet a team of researchers from the US and UK who have been studying metal-hungry plants since the 1970s. They have collected a number of plants that eat metals like gold and nickel — which is plentiful in post-industrial areas and very useful, because it's necessary for batteries and other consumer products.
Amazingly, one plant they discovered has sap that's actually nine per cent nickel.
Here's how the mining process works:
Locate soils rich in metal, grow plants that absorb those metals through their roots and concentrate them in the leaves, and then harvest them. To recover the metal, you burn the plants and process the ash in a smelter or refinery. The clever part... is finding species that take up economically viable amounts of metal, and treating the soil to maximise the uptake.
Oddly, the team has known for decades that this process works well. It can remediate old mines and brownfields, and produce significant amounts of metal for reuse. One early test burned 500 milligrams of plant ash in a furnace, which returned 100 milligrams of pure nickel.
Alyssum murale, a "hyperaccumulator" that sucks metal from the soil. Image: Wikipedia.
So what's holding up the phytomining boom? Oddly enough, it's the Houston-based company that actually sponsored their early research.
In 2004, while the research team was ramping up real-world testing, the company mysteriously refused to go any further with the research. And, unfortunately, they held the two patents that the scientists would need to go big with their idea. So those scientists have had to sit on their hands to avoid a patent dispute — all the while knowing that an inexpensive, organic solution to contaminated soil exists. Luckily, now that those patents are up next year, plans to relaunch operations anew are already brewing.
What's so interesting about the science of this is that it has a tangible benefit: In poor countries where metal-rich soil makes farming difficult, these plants could remediate agricultural lands. But it's also an economic boon: Mining companies could use these plants to trawl soil for metal too difficult to mine with conventional techniques. It's one of those rare symmetrical technologies that benefits two opposing interests. [New Scientist (paywall)]