Most people think of digging when they picture gold mining. That’s part of it, but there’s more to it than that. Gold isn’t usually found in veins of ore, but as tiny concentrations in otherwise nondescript rock. That rock is treated with compounds of cyanide, and it takes some 30 tons of rock to produce an ounce of gold. The result is a major environmental problem near the mines, which produce tons of highly toxic waste.
Now an accidental discovery from chemists at Northwestern University could make gold mining greener. The study, published in the current issue of Nature Communications, provides an environmentally benign method of extracting gold using cornstarch instead of cyanide.
Zhichang Liu, a postdoctoral fellow in chemistry and first author of the paper, took two test tubes, one full of starch-derived alpha-cyclodextrin, the other of dissolved gold salt called aurate — and mixed them together in a beaker at room temperature. He found that “needles” of gold formed because the cyclodextrin isolated the gold.
But Liu wasn’t trying to do that. He was actually looking to make cubic structures that could trap and store other molecules, and at first he thought he’d failed. But the gold changed his mind; he wanted to know more about how it happened.
Transmission electron image of the morphology of the nanostructures. (Nature Communications)
The needles form when gold atoms bind to bromine atoms in the cyclodextrin solution. Four bromine atoms link to the gold. The gold-bromine combination then links to an ion of potassium, which is surrounded by six water molecules forming a kind of snowflake-like structure. Cyclodextrin molecules then wrap around the whole lot, and link together like train cars. These long wire-like structures line up roughly parallel to each other and form the nanometer-sized needles.
There is still waste from this process — but it isn’t anywhere near as toxic as the cyanides gold mining usually makes.
The next question is whether this can be used on an industrial scale. The process has a lot of potential, though, as it could be used to extract gold from electronic waste, which has small amounts of the precious metal in it. It takes a lot less than 30 tons of e-waste to make an ounce of gold.
Top Image: A gold nugget in quartz. (Heritage Auctions)