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Guiltless Gold: Scientists Stumble Upon a Greener Way of Isolating the Metal

May 15, 2013 11:04 AM EDT
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Cyanide’s long-held role in the process of isolating gold from raw materials may be about to be replaced with the much cheaper and less deadly corn starch, based on a new study published in the journal Nature Communications.

“The elimination of cyanide from the gold industry is of the utmost importance environmentally,” Fraser Stoddart, one of the study’s researchers and a professor at Weinberg College of Arts and Sciences, said in a press release. “We have replaced nasty reagents with a cheap, biologically friendly material derived from starch.”

The discovery, like so many before it, happened by accident when Zhichang Liu, a postdoctoral fellow in Stoddart’s lab, mixed two test tubes containing aqueous solutions, one with the starch-derived alpha-cyclodextrin and the other a dissolved gold salt, in hopes of creating a cubic structure that could be used to store gases and small molecules.

However, rather than cubes, the experiment yielded needles, which formed quickly upon mixing the two solutions.

Though disappointed at first, Liu became determined to discover the composition of the needles, which, in the end, turned out to be alpha-cyclodextrin, a cyclic starch fragment capable of isolating gold “best of all.”

“Alpha-cyclodextrin is a gold medal winner,” Stoddart said. “Zchicang stumbled on a piece of magic for isolating gold from anything in a green way.”

Besides being safer for the environment, the new process is also more efficient than current commercial processes, according to the researchers.

The way it works is the supramolecular nanowires assemble spontaneously in a straw-like manner and in each wire the gold is held together in the middle of four bromine atoms. Meanwhile, the potassium ion is surrounded by six water molecules. These ions are sandwiched in an alternating fashion by alpha-cyclodextrin rings and approximately 4,000 wires are bundled parallel to each other in order to form the needles visible under an electron microscope.

“There is a lot of chemistry packed into these nanowires,” Stoddart said. “The elegance of the composition of single nanowires was revealed by atomic force microscopy, which throws light on the stacking of the individual donut-shaped alpha-cyclodextrin rings.”

Funding for the study was provided by the Defense Threat Reduction Agency and the National Science Foundation.

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