Plucking gold metal out of ores and electronic waste could get more environmentally friendly, thanks to some sweet supramolecular chemistry. Chemists have discovered that α-cyclodextrin, an inexpensive cyclic compound composed of six sugar moieties, can pull gold tetrabromide salts out of solution, offering a greener method for gold recovery (Nat. Commun. 2013, DOI: 10.1038/ncomms2891).
Currently, the most popular way to extract gold involves converting the metal to its water-soluble cyanide salt, followed by reduction, but this process can result in environmental contamination with toxic cyanide. “Judging from the responses we received within hours of the publication of our article, the gold-mining industry is very seriously on the lookout for alternatives to the currently toxic and caustic process of extraction,” says J. Fraser Stoddart of Northwestern University, who led the research effort.
To extract the precious metal, Stoddart’s group converts the gold to soluble HAuBr4, using concentrated hydrobromic and nitric acids. They use potassium hydroxide to adjust the solution’s pH, producing KAuBr4. When α-cyclodextrin is added to the solution, the oligosaccharide forms a complex with AuBr4– and K(OH2)6+ counterions. This supramolecular structure quickly precipitates out of solution and can then be reduced to give gold metal. Using this process, Stoddart’s team extracted gold from two small samples of scrap metal, achieving recoveries on par with the cyanide method.
Stoddart says his team is working to make the process more amenable to large-scale operation. “We’re looking at more benign methods,” he says. The new approach removes cyanide from the leaching process but brings in HBr and HNO3, strong acids that can be corrosive. “That’s not perfect either,” he says, so the team is looking for alternatives.
“The work details a simple, seemingly cost-effective method to recover gold, whose isolation has traditionally come at huge environmental cost,” comments Jonathan L. Sessler, a supramolecular chemistry expert at the University of Texas, Austin. The complexes may also teach chemists more about ion pair recognition and how it can be used to pull specific ions from solutions, he adds.