Money that grows on crops

He can't quite make money grow from trees, but a New Zealand scientist has devised a way to harvest gold from plants.

The idea: Use common crops to soak up contaminants in soil from gold-mining sites and return the areas to productive agriculture. The gold harvested from the process pays for the cleanup - with money left over for training in sustainable agriculture.

"We get the plants to do the hard work, and then we basically harvest the plants and extract the metal," says Christopher Anderson, an environmental geologist from Massey University in Palmerston North, New Zealand. "So we farm mercury and gold."

Aimed at small-scale mines, such a program could prove especially beneficial to Latin America, where some 1 million artisanal miners ply their trade, according to estimates by the International Labor Organization. The bulk of them are in Brazil.

Dr. Anderson has already run successful field tests last year in an Amazonian gold mine near Bahía, Brazil. In a few months, he plans to begin a larger project, most likely in Serra Pelada, about 1,800 miles northwest of Bahía.

Small gold mines are especially troublesome for the environment. In the fragile Amazon River basin, for example, there are hundreds of artisanal mines where workers pour mercury, cyanide, and other chemicals onto gold-rich areas to extract the metal. Once the mine is exhausted, they abandon it and move on, leaving behind a toxic soup of contaminants.

Mercury, for example, is one of the most toxic contaminants for humans and animals, and one of the most difficult and costly to clean up. But using regular corn and canola plants, Anderson has found that this can be done at almost no cost, and with benefits to the environment and local community.

The process is called phyto- remediation. First, he treats the contaminated soil with chemicals that break the gold down into water-soluble particles. Then he introduces the crops.

"Basically a plant will take up anything that's in the soil," he says. Corn and canola have a natural ability to take up huge amounts of metal.

Of course, the crops aren't eaten because they're full of toxic metals.

Instead, Anderson harvests them for their minerals as they begin to die. He estimates he can recover 1 kilogram of gold per hectare (14 ounces an acre) and about half as much mercury through this process. Then the gold is used to pay for the cleanup and to educate locals about sustainable agriculture.

During the metal-harvesting, his team trains local people in farming techniques, so once the land is clean, they can reclaim it and use it for subsistence farming. "It's turning waste into a resource," says Anderson. "We're looking to create an alternative lifestyle for these artisanal miners to help them escape the poverty."

Anderson's research has won attention from the international community. Iain Gillespie, director of the biotechnology unit for the OECD, says Anderson is using a proven process to clean mining sites, but adapting it to benefit communities. "I can think of few, if any, better examples of putting the triple bottom line of sustainability into practice - delivering environmental and economic benefit directly to local communities," says Mr. Gillespie.

Anderson's process also has more widespread applications. Plants could be used to stop contaminants from leaching out of mine sites and waste dumps.

Anderson's field trials also yielded an unexpected and potentially profitable byproduct. The plants he harvested had purple leaves because they contained gold nanoparticles, which are purple, not yellow. These nanoparticles melt at one tenth the temperature of regular gold - which makes them highly sought after for industrial processes, such as cleaning up carbon monoxide in fuel cells.

Anderson doesn't yet know how easily the nanoparticles can be processed for industrial use, but the potential is there.