Golden surprise

Just by accident, a cornstarch method has been discovered as a new, eco-friendly way to extract gold. Simple test-tube chemistry lies behind the discovery that can turn the mining business a lot greener.

When chemistry postdoctoral fellow Zhichang Liu started trying to make three-dimensional cubic structures suitable for storing gases and small molecules, he had no idea he was about to discover a potentially green and cheaper solution to recovering gold.

As he carefully mixed together dissolved gold-bromide salts and cyclodextrin in his team’s laboratory at Northwestern University in the US state of Illinois, Liu expected to see extended cube structures with large pores take shape. Instead, to his astonishment, he got needles, which had formed rapidly within a minute upon mixing the two solutions.

“Initially, I was disappointed when my experiment didn’t produce the cubes I was expecting, but rather a shiny pale-brown precipitate,” Liu says. “However, when I saw the needles forming, I got really excited. I wanted to learn more about the composition of these needles.”

A winning concept

Northwestern University described the research as a prime example of serendipity at work, brought to fruition by contemporary fundamental science.
The industrial application of the discovery would be good news for both the environment and the mining industry, since the waste product from cyanidation poses a major environmental hazard that is costly to protect against.

After discovering the needles, using simple test-tube chemistry, Liu went on to screen six different cyclodextrin complexes. He found that it was alpha-cyclodextrin, a cyclic molecule composed of six glucose units derived from cornstarch, that isolated gold best of all.

“Alpha-cyclodextrin is the gold medal winner,” says Sir Fraser Stoddart, the chemistry professor leading Zhichang Liu’s research team. “Zhichang stumbled on a piece of magic for isolating gold from anything in a green way.

“The elimination of cyanide from the gold industry is of the utmost importance environmentally,” Stoddart says. “We have replaced nasty reagents with a cheap, biologically friendly material derived from cornstarch.”

The Northwestern method extracts gold from crude sources and leaves behind other metals that are often found mixed together with the gold. According to Liu, the process can also be used to extract gold from consumer electronic waste. The cyclodextrin method is now poised to find a technological application.