Money doesn't grow on trees — but gold might. An international team of scientists has found a way to grow and harvest gold from crop plants.
Called phytomining, the technique of finding gold uses plants to extract particles of the precious metal from soil. Some plants have the natural ability to take up through their roots and concentrate metals such as nickel, cadmium and zinc in their leaves and shoots. For years, scientists have explored the use of such plants, dubbed hyperaccumulators, for pollution removal.
But there are no known gold hyperaccumulators, because gold doesn't easily dissolve in water so plants have no natural way of taking the particles in through their roots.
"Under certain chemical conditions, gold solubility can be forced," said Chris Anderson, an environmental geochemist and gold phytomining expert at Massey University in New Zealand.
Fifteen years ago, Anderson first showed it was possible to get mustard plants to suck up gold from chemically treated soil containing gold particles. [The Deepest Man-Made Gold Mines & Wells (Infographic)]
The technology works something like this: Find a fast-growing plant with a lot of aboveground leafy mass, such as mustard, sunflowers or tobacco. Plant the crop on soil that contains gold. The waste piles or tailings surrounding old gold mines are a good place to look. Conventional mining can't remove 100 percent of the gold from surrounding minerals so some gets wasted. Once the crops reach their full height, treat the soil with a chemical that makes gold soluble. When the plant transpires, pulling water up and out through tiny pores on its leaves, it will take up the gold water from the soil and accumulate it in its biomass. Then harvest.
Getting the gold into plants is the easy part. Getting the gold out has proved more difficult, Anderson explained.
"Gold behaves differently in plant material," Anderson told LiveScience. If the plants are burned, some of the gold will stay attached to the ash, but some will disappear. Processing the ash poses difficulties, too, and requires the use of huge amounts of strong acids, which can be dangerous to transport.
The gold found in plants are nanoparticles, so there may be great potential for the chemical industry, which uses gold nanoparticles as catalysts for chemical reactions, Anderson said.
Gold phytomining won't ever take the place of traditional gold mining, Anderson said. "The value of it is in the remediation of polluted mine sites," he added.
The chemicals involved in making gold soluble also induce the plants to take up other soil contaminants such as mercury, arsenic and copper — common pollutants found in mine waste that can pose a risk to humans and the environment.
"If we can generate revenue by cropping gold while remediating the soil, then that is a good outcome," said Anderson, who is currently working with researchers in Indonesia to develop a sustainable system for small-scale artisanal gold miners to use the technique to reduce the mercury pollution from their operations.
However, some scientists say the environmental risks associated with growing gold itself may be too high. Cyanide and thiocyanate, the same hazardous chemicals used by mining companies to get gold to leach out of rock, must be used to dissolve gold particles in soil water.
"The process itself could create environmental problems," said J. Scott Angle, an agronomist at the University of Georgia.