The gold-mining industry is one of the most environmentally destructive industries in the world. While there are a variety of reasons for this, one of the most prominent is the wide-scale use of cyanide as a means to extract gold from crude sources. But now, new research from Northwestern University may have addressed this, with the discovery of an effective alternative to cyanide — an inexpensive and environmentally benign method using simple cornstarch. The new process can also be used effectively to extract gold from consumer electronic waste.
The new ‘green’ method can effectively separate and extract gold from a large variety of crude sources, and the other metals with which they are often found bound and mixed together. This is the process that toxic cyanides are currently used for. Almost all of the major gold-mining companies in the world make use of cyanide for this purpose, leading to significant environmental contamination and often significant health problems for those that perform the mining.
“The elimination of cyanide from the gold industry is of the utmost importance environmentally,” said Sir Fraser Stoddart, the Board of Trustees Professor of Chemistry in the Weinberg College of Arts and Sciences. “We have replaced nasty reagents with a cheap, biologically friendly material derived from starch.”
The new material was actually discovered by accident, during experiments to create “an extended, three-dimensional cubic structure, which could be used to store gases and small molecules,” a Northwestern University press release notes.
“Zhichang Liu, a postdoctoral fellow in Stoddart’s lab and first author of the paper, took two test tubes containing aqueous solutions — one of the starch-derived alpha-cyclodextrin, the other of a dissolved gold (Au) salt (called aurate) — and mixed them together in a beaker at room temperature.”
Rather than the cubic structure being created though, ‘needles’ were forming rapidly upon the mixing of the two solutions.
“Initially, I was disappointed when my experiment didn’t produce cubes, but when I saw the needles, I got excited,” Liu said. “I wanted to learn more about the composition of these needles.”
“Nature decided otherwise,” said Stoddart, a senior author of the paper. “The needles, composed of straw-like bundles of supramolecular wires, emerged from the mixed solutions in less than a minute.”
The press release continues:
After discovering the needles, Liu screened six different complexes — cyclodextrins composed of rings of six (alpha), seven (beta) and eight (gamma) glucose units, each combined with aqueous solutions of potassium tetrabromoaurate (KAuBr4) or potassium tetrachloroaurate (KAuCl4).
He found that it was alpha-cyclodextrin, a cyclic starch fragment composed of six glucose units, that isolates gold best of all.
The supramolecular nanowires, each 1.3 nanometers in diameter, assemble spontaneously in a straw-like manner. In each wire, the gold ion is held together in the middle of four bromine atoms, while the potassium ion is surrounded by six water molecules; these ions are sandwiched in an alternating fashion by alpha-cyclodextrin rings. Around 4,000 wires are bundled parallel to each other and form individual needles that are visible under an electron microscope.
The best part of the new method/material is that the alkali metal salt wastes that are produced are relatively environmentally benign when compared to the cyanides that are currently used. According to the researchers, the new method is also a good bit more efficient than the currently used commercial processes.
The new research was published May 14 in the online journal Nature Communications.
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